Merge commit '6ed6f1e6a1a8f414ba7e6d9b8222e7e5a1686e42' into clippyup

src/tools/clippy/.cargo/config

@@ -1,7 +1,7 @@
 [alias]
 uitest = "test --test compile-test"
 dev = "run --target-dir clippy_dev/target --package clippy_dev --bin clippy_dev --manifest-path clippy_dev/Cargo.toml --"
-dev-lintcheck = "run --target-dir clippy_dev/target --package clippy_dev --bin clippy_dev --manifest-path clippy_dev/Cargo.toml --features lintcheck -- lintcheck"
+lintcheck = "run --target-dir lintcheck/target --package lintcheck --bin lintcheck --manifest-path lintcheck/Cargo.toml  -- "
 
 [build]
 rustflags = ["-Zunstable-options"]

src/tools/clippy/.github/workflows/clippy.yml

@@ -27,6 +27,7 @@ env:
 
 jobs:
   base:
+    # NOTE: If you modify this job, make sure you copy the changes to clippy_bors.yml
     runs-on: ubuntu-latest
 
     steps:
@@ -50,11 +51,16 @@ jobs:
     - name: Build
       run: cargo build --features deny-warnings,internal-lints
 
-    - name: Test "--fix -Zunstable-options"
-      run: cargo run --features deny-warnings,internal-lints --bin cargo-clippy -- clippy --fix -Zunstable-options
+    - name: Test
+      run: cargo test --features deny-warnings,internal-lints
 
-    - name: Test Workspace
-      run: cargo test --all --features deny-warnings,internal-lints
+    - name: Test clippy_lints
+      run: cargo test --features deny-warnings,internal-lints
+      working-directory: clippy_lints
+
+    - name: Test rustc_tools_util
+      run: cargo test --features deny-warnings
+      working-directory: rustc_tools_util
 
     - name: Test clippy_dev
       run: cargo test --features deny-warnings
@@ -64,6 +70,10 @@ jobs:
       run: ../target/debug/cargo-clippy
       working-directory: clippy_workspace_tests
 
+    - name: Test cargo-clippy --fix
+      run: ../target/debug/cargo-clippy clippy --fix -Zunstable-options
+      working-directory: clippy_workspace_tests
+
     - name: Test clippy-driver
       run: bash .github/driver.sh
       env:

src/tools/clippy/.github/workflows/clippy_bors.yml

@@ -72,6 +72,7 @@ jobs:
 
     runs-on: ${{ matrix.os }}
 
+    # NOTE: If you modify this job, make sure you copy the changes to clippy.yml
     steps:
     # Setup
     - uses: rust-lang/simpleinfra/github-actions/cancel-outdated-builds@master
@@ -112,8 +113,16 @@ jobs:
     - name: Build
       run: cargo build --features deny-warnings,internal-lints
 
-    - name: Test Workspace
-      run: cargo test --all --features deny-warnings,internal-lints
+    - name: Test
+      run: cargo test --features deny-warnings,internal-lints
+
+    - name: Test clippy_lints
+      run: cargo test --features deny-warnings,internal-lints
+      working-directory: clippy_lints
+
+    - name: Test rustc_tools_util
+      run: cargo test --features deny-warnings
+      working-directory: rustc_tools_util
 
     - name: Test clippy_dev
       run: cargo test --features deny-warnings
@@ -123,11 +132,22 @@ jobs:
       run: ../target/debug/cargo-clippy
       working-directory: clippy_workspace_tests
 
+    - name: Test cargo-clippy --fix
+      run: ../target/debug/cargo-clippy clippy --fix -Zunstable-options
+      working-directory: clippy_workspace_tests
+
     - name: Test clippy-driver
       run: bash .github/driver.sh
       env:
         OS: ${{ runner.os }}
 
+    - name: Test cargo dev new lint
+      run: |
+        cargo dev new_lint --name new_early_pass --pass early
+        cargo dev new_lint --name new_late_pass --pass late
+        cargo check
+        git reset --hard HEAD
+
   integration_build:
     needs: changelog
     runs-on: ubuntu-latest

src/tools/clippy/.gitignore

@@ -21,6 +21,7 @@ out
 /clippy_utils/target
 /clippy_workspace_tests/target
 /clippy_dev/target
+/lintcheck/target
 /rustc_tools_util/target
 
 # Generated by dogfood

src/tools/clippy/CHANGELOG.md

@@ -2104,6 +2104,7 @@ Released 2018-09-13
 [`if_not_else`]: https://rust-lang.github.io/rust-clippy/master/index.html#if_not_else
 [`if_same_then_else`]: https://rust-lang.github.io/rust-clippy/master/index.html#if_same_then_else
 [`ifs_same_cond`]: https://rust-lang.github.io/rust-clippy/master/index.html#ifs_same_cond
+[`implicit_clone`]: https://rust-lang.github.io/rust-clippy/master/index.html#implicit_clone
 [`implicit_hasher`]: https://rust-lang.github.io/rust-clippy/master/index.html#implicit_hasher
 [`implicit_return`]: https://rust-lang.github.io/rust-clippy/master/index.html#implicit_return
 [`implicit_saturating_sub`]: https://rust-lang.github.io/rust-clippy/master/index.html#implicit_saturating_sub
@@ -2134,6 +2135,7 @@ Released 2018-09-13
 [`invisible_characters`]: https://rust-lang.github.io/rust-clippy/master/index.html#invisible_characters
 [`items_after_statements`]: https://rust-lang.github.io/rust-clippy/master/index.html#items_after_statements
 [`iter_cloned_collect`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_cloned_collect
+[`iter_count`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_count
 [`iter_next_loop`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_next_loop
 [`iter_next_slice`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_next_slice
 [`iter_nth`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_nth

src/tools/clippy/Cargo.toml

@@ -1,13 +1,7 @@
 [package]
 name = "clippy"
 version = "0.1.52"
-authors = [
-	"Manish Goregaokar <manishsmail@gmail.com>",
-	"Andre Bogus <bogusandre@gmail.com>",
-	"Georg Brandl <georg@python.org>",
-	"Martin Carton <cartonmartin@gmail.com>",
-	"Oliver Schneider <clippy-iethah7aipeen8neex1a@oli-obk.de>"
-]
+authors = ["The Rust Clippy Developers"]
 description = "A bunch of helpful lints to avoid common pitfalls in Rust"
 repository = "https://github.com/rust-lang/rust-clippy"
 readme = "README.md"
@@ -42,6 +36,7 @@ tester = "0.9"
 clippy-mini-macro-test = { version = "0.2", path = "mini-macro" }
 serde = { version = "1.0", features = ["derive"] }
 derive-new = "0.5"
+regex = "1.4"
 
 # A noop dependency that changes in the Rust repository, it's a bit of a hack.
 # See the `src/tools/rustc-workspace-hack/README.md` file in `rust-lang/rust`
@@ -55,3 +50,7 @@ rustc_tools_util = { version = "0.2.0", path = "rustc_tools_util" }
 deny-warnings = []
 integration = ["tempfile"]
 internal-lints = ["clippy_lints/internal-lints"]
+
+[package.metadata.rust-analyzer]
+# This package uses #[feature(rustc_private)]
+rustc_private = true

src/tools/clippy/README.md

@@ -202,7 +202,6 @@ the lint(s) you are interested in:
 ```terminal
 cargo clippy -- -A clippy::all -W clippy::useless_format -W clippy::...
 ```
-Note that if you've run clippy before, this may only take effect after you've modified a file or ran `cargo clean`.
 
 ### Specifying the minimum supported Rust version
 

src/tools/clippy/clippy_dev/Cargo.toml

@@ -1,27 +1,17 @@
 [package]
 name = "clippy_dev"
 version = "0.0.1"
-authors = ["Philipp Hansch <dev@phansch.net>"]
+authors = ["The Rust Clippy Developers"]
 edition = "2018"
 
-
 [dependencies]
 bytecount = "0.6"
 clap = "2.33"
-flate2 = { version = "1.0.19", optional = true }
-fs_extra = { version = "1.2.0", optional = true }
 itertools = "0.9"
 opener = "0.4"
 regex = "1"
-serde = { version = "1.0", features = ["derive"], optional = true }
-serde_json = { version = "1.0", optional = true }
 shell-escape = "0.1"
-tar = { version = "0.4.30", optional = true }
-toml = { version = "0.5", optional = true }
-ureq = { version = "2.0.0-rc3", optional = true }
-rayon = { version = "1.5.0", optional = true }
 walkdir = "2"
 
 [features]
-lintcheck = ["flate2", "serde_json", "tar", "toml", "ureq", "serde", "fs_extra", "rayon"]
 deny-warnings = []

src/tools/clippy/clippy_dev/src/fmt.rs

@@ -54,6 +54,7 @@ pub fn run(check: bool, verbose: bool) {
         success &= cargo_fmt(context, project_root.as_path())?;
         success &= cargo_fmt(context, &project_root.join("clippy_dev"))?;
         success &= cargo_fmt(context, &project_root.join("rustc_tools_util"))?;
+        success &= cargo_fmt(context, &project_root.join("lintcheck"))?;
 
         for entry in WalkDir::new(project_root.join("tests")) {
             let entry = entry?;

src/tools/clippy/clippy_dev/src/lib.rs

@@ -12,7 +12,6 @@ use walkdir::WalkDir;
 
 pub mod bless;
 pub mod fmt;
-pub mod lintcheck;
 pub mod new_lint;
 pub mod ra_setup;
 pub mod serve;
@@ -530,7 +529,7 @@ fn test_gen_deprecated() {
 #[should_panic]
 fn test_gen_deprecated_fail() {
     let lints = vec![Lint::new("should_assert_eq2", "group2", "abc", None, "module_name")];
-    let _ = gen_deprecated(lints.iter());
+    let _deprecated_lints = gen_deprecated(lints.iter());
 }
 
 #[test]

src/tools/clippy/clippy_dev/src/main.rs

@@ -2,10 +2,6 @@
 
 use clap::{App, Arg, ArgMatches, SubCommand};
 use clippy_dev::{bless, fmt, new_lint, ra_setup, serve, stderr_length_check, update_lints};
-
-#[cfg(feature = "lintcheck")]
-use clippy_dev::lintcheck;
-
 fn main() {
     let matches = get_clap_config();
 
@@ -13,10 +9,6 @@ fn main() {
         ("bless", Some(matches)) => {
             bless::bless(matches.is_present("ignore-timestamp"));
         },
-        #[cfg(feature = "lintcheck")]
-        ("lintcheck", Some(matches)) => {
-            lintcheck::run(&matches);
-        },
         ("fmt", Some(matches)) => {
             fmt::run(matches.is_present("check"), matches.is_present("verbose"));
         },
@@ -53,33 +45,7 @@ fn main() {
 }
 
 fn get_clap_config<'a>() -> ArgMatches<'a> {
-    #[cfg(feature = "lintcheck")]
-    let lintcheck_sbcmd = SubCommand::with_name("lintcheck")
-        .about("run clippy on a set of crates and check output")
-        .arg(
-            Arg::with_name("only")
-                .takes_value(true)
-                .value_name("CRATE")
-                .long("only")
-                .help("only process a single crate of the list"),
-        )
-        .arg(
-            Arg::with_name("crates-toml")
-                .takes_value(true)
-                .value_name("CRATES-SOURCES-TOML-PATH")
-                .long("crates-toml")
-                .help("set the path for a crates.toml where lintcheck should read the sources from"),
-        )
-        .arg(
-            Arg::with_name("threads")
-                .takes_value(true)
-                .value_name("N")
-                .short("j")
-                .long("jobs")
-                .help("number of threads to use, 0 automatic choice"),
-        );
-
-    let app = App::new("Clippy developer tooling")
+    App::new("Clippy developer tooling")
         .subcommand(
             SubCommand::with_name("bless")
                 .about("bless the test output changes")
@@ -196,10 +162,6 @@ fn get_clap_config<'a>() -> ArgMatches<'a> {
                         .validator_os(serve::validate_port),
                 )
                 .arg(Arg::with_name("lint").help("Which lint's page to load initially (optional)")),
-        );
-
-    #[cfg(feature = "lintcheck")]
-    let app = app.subcommand(lintcheck_sbcmd);
-
-    app.get_matches()
+        )
+        .get_matches()
 }

src/tools/clippy/clippy_dummy/Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "clippy_dummy" # rename to clippy before publishing
 version = "0.0.303"
-authors = ["Manish Goregaokar <manishsmail@gmail.com>"]
+authors = ["The Rust Clippy Developers"]
 edition = "2018"
 readme = "crates-readme.md"
 description = "A bunch of helpful lints to avoid common pitfalls in Rust."

src/tools/clippy/clippy_lints/Cargo.toml

@@ -3,12 +3,7 @@ name = "clippy_lints"
 # begin automatic update
 version = "0.1.52"
 # end automatic update
-authors = [
-	"Manish Goregaokar <manishsmail@gmail.com>",
-	"Andre Bogus <bogusandre@gmail.com>",
-	"Georg Brandl <georg@python.org>",
-	"Martin Carton <cartonmartin@gmail.com>"
-]
+authors = ["The Rust Clippy Developers"]
 description = "A bunch of helpful lints to avoid common pitfalls in Rust"
 repository = "https://github.com/rust-lang/rust-clippy"
 readme = "README.md"
@@ -29,10 +24,10 @@ smallvec = { version = "1", features = ["union"] }
 toml = "0.5.3"
 unicode-normalization = "0.1"
 semver = "0.11"
-rustc-semver="1.1.0"
+rustc-semver = "1.1.0"
 # NOTE: cargo requires serde feat in its url dep
 # see <https://github.com/rust-lang/rust/pull/63587#issuecomment-522343864>
-url = { version =  "2.1.0", features = ["serde"] }
+url = { version = "2.1.0", features = ["serde"] }
 quote = "1"
 syn = { version = "1", features = ["full"] }
 
@@ -40,3 +35,7 @@ syn = { version = "1", features = ["full"] }
 deny-warnings = []
 # build clippy with internal lints enabled, off by default
 internal-lints = ["clippy_utils/internal-lints"]
+
+[package.metadata.rust-analyzer]
+# This crate uses #[feature(rustc_private)]
+rustc_private = true

src/tools/clippy/clippy_lints/src/assign_ops.rs

@@ -209,7 +209,7 @@ fn lint_misrefactored_assign_op(
                 diag.span_suggestion(
                     expr.span,
                     &format!(
-                        "Did you mean `{} = {} {} {}` or `{}`? Consider replacing it with",
+                        "did you mean `{} = {} {} {}` or `{}`? Consider replacing it with",
                         snip_a,
                         snip_a,
                         op.node.as_str(),

src/tools/clippy/clippy_lints/src/async_yields_async.rs

@@ -50,8 +50,7 @@ impl<'tcx> LateLintPass<'tcx> for AsyncYieldsAsync {
                 let body_id = BodyId {
                     hir_id: body.value.hir_id,
                 };
-                let def_id = cx.tcx.hir().body_owner_def_id(body_id);
-                let typeck_results = cx.tcx.typeck(def_id);
+                let typeck_results = cx.tcx.typeck_body(body_id);
                 let expr_ty = typeck_results.expr_ty(&body.value);
 
                 if implements_trait(cx, expr_ty, future_trait_def_id, &[]) {

src/tools/clippy/clippy_lints/src/attrs.rs

@@ -640,7 +640,7 @@ fn check_mismatched_target_os(cx: &EarlyContext<'_>, attr: &Attribute) {
                     diag.span_suggestion(span, "try", sugg, Applicability::MaybeIncorrect);
 
                     if !unix_suggested && is_unix(os) {
-                        diag.help("Did you mean `unix`?");
+                        diag.help("did you mean `unix`?");
                         unix_suggested = true;
                     }
                 }

src/tools/clippy/clippy_lints/src/await_holding_invalid.rs

@@ -97,8 +97,7 @@ impl LateLintPass<'_> for AwaitHolding {
             let body_id = BodyId {
                 hir_id: body.value.hir_id,
             };
-            let def_id = cx.tcx.hir().body_owner_def_id(body_id);
-            let typeck_results = cx.tcx.typeck(def_id);
+            let typeck_results = cx.tcx.typeck_body(body_id);
             check_interior_types(
                 cx,
                 &typeck_results.generator_interior_types.as_ref().skip_binder(),
@@ -116,20 +115,20 @@ fn check_interior_types(cx: &LateContext<'_>, ty_causes: &[GeneratorInteriorType
                     cx,
                     AWAIT_HOLDING_LOCK,
                     ty_cause.span,
-                    "this MutexGuard is held across an 'await' point. Consider using an async-aware Mutex type or ensuring the MutexGuard is dropped before calling await.",
+                    "this MutexGuard is held across an 'await' point. Consider using an async-aware Mutex type or ensuring the MutexGuard is dropped before calling await",
                     ty_cause.scope_span.or(Some(span)),
                     "these are all the await points this lock is held through",
                 );
             }
             if is_refcell_ref(cx, adt.did) {
                 span_lint_and_note(
-                        cx,
-                        AWAIT_HOLDING_REFCELL_REF,
-                        ty_cause.span,
-                        "this RefCell Ref is held across an 'await' point. Consider ensuring the Ref is dropped before calling await.",
-                        ty_cause.scope_span.or(Some(span)),
-                        "these are all the await points this ref is held through",
-                    );
+                    cx,
+                    AWAIT_HOLDING_REFCELL_REF,
+                    ty_cause.span,
+                    "this RefCell Ref is held across an 'await' point. Consider ensuring the Ref is dropped before calling await",
+                    ty_cause.scope_span.or(Some(span)),
+                    "these are all the await points this ref is held through",
+                );
             }
         }
     }

src/tools/clippy/clippy_lints/src/bytecount.rs

@@ -28,7 +28,7 @@ declare_clippy_lint! {
     /// &vec.iter().filter(|x| **x == 0u8).count(); // use bytecount::count instead
     /// ```
     pub NAIVE_BYTECOUNT,
-    perf,
+    pedantic,
     "use of naive `<slice>.filter(|&x| x == y).count()` to count byte values"
 }
 

src/tools/clippy/clippy_lints/src/case_sensitive_file_extension_comparisons.rs

@@ -1,12 +1,11 @@
-use crate::utils::paths::STRING;
-use crate::utils::{match_def_path, span_lint_and_help};
+use crate::utils::span_lint_and_help;
 use if_chain::if_chain;
 use rustc_ast::ast::LitKind;
 use rustc_hir::{Expr, ExprKind, PathSegment};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::ty;
 use rustc_session::{declare_lint_pass, declare_tool_lint};
-use rustc_span::{source_map::Spanned, Span};
+use rustc_span::{source_map::Spanned, symbol::sym, Span};
 
 declare_clippy_lint! {
     /// **What it does:**
@@ -59,7 +58,7 @@ fn check_case_sensitive_file_extension_comparison(ctx: &LateContext<'_>, expr: &
                     return Some(span);
                 },
                 ty::Adt(&ty::AdtDef { did, .. }, _) => {
-                    if match_def_path(ctx, did, &STRING) {
+                    if ctx.tcx.is_diagnostic_item(sym::string_type, did) {
                         return Some(span);
                     }
                 },

src/tools/clippy/clippy_lints/src/casts/cast_lossless.rs

@@ -0,0 +1,87 @@
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind};
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, FloatTy, Ty};
+
+use crate::utils::{in_constant, is_isize_or_usize, snippet_opt, span_lint_and_sugg};
+
+use super::{utils, CAST_LOSSLESS};
+
+pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_op: &Expr<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>) {
+    if !should_lint(cx, expr, cast_from, cast_to) {
+        return;
+    }
+
+    // The suggestion is to use a function call, so if the original expression
+    // has parens on the outside, they are no longer needed.
+    let mut applicability = Applicability::MachineApplicable;
+    let opt = snippet_opt(cx, cast_op.span);
+    let sugg = opt.as_ref().map_or_else(
+        || {
+            applicability = Applicability::HasPlaceholders;
+            ".."
+        },
+        |snip| {
+            if should_strip_parens(cast_op, snip) {
+                &snip[1..snip.len() - 1]
+            } else {
+                snip.as_str()
+            }
+        },
+    );
+
+    span_lint_and_sugg(
+        cx,
+        CAST_LOSSLESS,
+        expr.span,
+        &format!(
+            "casting `{}` to `{}` may become silently lossy if you later change the type",
+            cast_from, cast_to
+        ),
+        "try",
+        format!("{}::from({})", cast_to, sugg),
+        applicability,
+    );
+}
+
+fn should_lint(cx: &LateContext<'_>, expr: &Expr<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>) -> bool {
+    // Do not suggest using From in consts/statics until it is valid to do so (see #2267).
+    if in_constant(cx, expr.hir_id) {
+        return false;
+    }
+
+    match (cast_from.is_integral(), cast_to.is_integral()) {
+        (true, true) => {
+            let cast_signed_to_unsigned = cast_from.is_signed() && !cast_to.is_signed();
+            let from_nbits = utils::int_ty_to_nbits(cast_from, cx.tcx);
+            let to_nbits = utils::int_ty_to_nbits(cast_to, cx.tcx);
+            !is_isize_or_usize(cast_from)
+                && !is_isize_or_usize(cast_to)
+                && from_nbits < to_nbits
+                && !cast_signed_to_unsigned
+        },
+
+        (true, false) => {
+            let from_nbits = utils::int_ty_to_nbits(cast_from, cx.tcx);
+            let to_nbits = if let ty::Float(FloatTy::F32) = cast_to.kind() {
+                32
+            } else {
+                64
+            };
+            from_nbits < to_nbits
+        },
+
+        (_, _) => {
+            matches!(cast_from.kind(), ty::Float(FloatTy::F32)) && matches!(cast_to.kind(), ty::Float(FloatTy::F64))
+        },
+    }
+}
+
+fn should_strip_parens(cast_expr: &Expr<'_>, snip: &str) -> bool {
+    if let ExprKind::Binary(_, _, _) = cast_expr.kind {
+        if snip.starts_with('(') && snip.ends_with(')') {
+            return true;
+        }
+    }
+    false
+}

src/tools/clippy/clippy_lints/src/casts/cast_possible_truncation.rs

@@ -0,0 +1,54 @@
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, FloatTy, Ty};
+
+use crate::utils::{is_isize_or_usize, span_lint};
+
+use super::{utils, CAST_POSSIBLE_TRUNCATION};
+
+pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>) {
+    let msg = match (cast_from.is_integral(), cast_to.is_integral()) {
+        (true, true) => {
+            let from_nbits = utils::int_ty_to_nbits(cast_from, cx.tcx);
+            let to_nbits = utils::int_ty_to_nbits(cast_to, cx.tcx);
+
+            let (should_lint, suffix) = match (is_isize_or_usize(cast_from), is_isize_or_usize(cast_to)) {
+                (true, true) | (false, false) => (to_nbits < from_nbits, ""),
+                (true, false) => (
+                    to_nbits <= 32,
+                    if to_nbits == 32 {
+                        " on targets with 64-bit wide pointers"
+                    } else {
+                        ""
+                    },
+                ),
+                (false, true) => (from_nbits == 64, " on targets with 32-bit wide pointers"),
+            };
+
+            if !should_lint {
+                return;
+            }
+
+            format!(
+                "casting `{}` to `{}` may truncate the value{}",
+                cast_from, cast_to, suffix,
+            )
+        },
+
+        (false, true) => {
+            format!("casting `{}` to `{}` may truncate the value", cast_from, cast_to)
+        },
+
+        (_, _) => {
+            if matches!(cast_from.kind(), &ty::Float(FloatTy::F64))
+                && matches!(cast_to.kind(), &ty::Float(FloatTy::F32))
+            {
+                "casting `f64` to `f32` may truncate the value".to_string()
+            } else {
+                return;
+            }
+        },
+    };
+
+    span_lint(cx, CAST_POSSIBLE_TRUNCATION, expr.span, &msg);
+}

src/tools/clippy/clippy_lints/src/casts/cast_possible_wrap.rs

@@ -0,0 +1,44 @@
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::Ty;
+
+use crate::utils::{is_isize_or_usize, span_lint};
+
+use super::{utils, CAST_POSSIBLE_WRAP};
+
+pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>) {
+    if !(cast_from.is_integral() && cast_to.is_integral()) {
+        return;
+    }
+
+    let arch_64_suffix = " on targets with 64-bit wide pointers";
+    let arch_32_suffix = " on targets with 32-bit wide pointers";
+    let cast_unsigned_to_signed = !cast_from.is_signed() && cast_to.is_signed();
+    let from_nbits = utils::int_ty_to_nbits(cast_from, cx.tcx);
+    let to_nbits = utils::int_ty_to_nbits(cast_to, cx.tcx);
+
+    let (should_lint, suffix) = match (is_isize_or_usize(cast_from), is_isize_or_usize(cast_to)) {
+        (true, true) | (false, false) => (to_nbits == from_nbits && cast_unsigned_to_signed, ""),
+        (true, false) => (to_nbits <= 32 && cast_unsigned_to_signed, arch_32_suffix),
+        (false, true) => (
+            cast_unsigned_to_signed,
+            if from_nbits == 64 {
+                arch_64_suffix
+            } else {
+                arch_32_suffix
+            },
+        ),
+    };
+
+    if should_lint {
+        span_lint(
+            cx,
+            CAST_POSSIBLE_WRAP,
+            expr.span,
+            &format!(
+                "casting `{}` to `{}` may wrap around the value{}",
+                cast_from, cast_to, suffix,
+            ),
+        );
+    }
+}

src/tools/clippy/clippy_lints/src/casts/cast_precision_loss.rs

@@ -0,0 +1,51 @@
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, FloatTy, Ty};
+
+use crate::utils::{is_isize_or_usize, span_lint};
+
+use super::{utils, CAST_PRECISION_LOSS};
+
+pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>) {
+    if !cast_from.is_integral() || cast_to.is_integral() {
+        return;
+    }
+
+    let from_nbits = utils::int_ty_to_nbits(cast_from, cx.tcx);
+    let to_nbits = if let ty::Float(FloatTy::F32) = cast_to.kind() {
+        32
+    } else {
+        64
+    };
+
+    if !(is_isize_or_usize(cast_from) || from_nbits >= to_nbits) {
+        return;
+    }
+
+    let cast_to_f64 = to_nbits == 64;
+    let mantissa_nbits = if cast_to_f64 { 52 } else { 23 };
+    let arch_dependent = is_isize_or_usize(cast_from) && cast_to_f64;
+    let arch_dependent_str = "on targets with 64-bit wide pointers ";
+    let from_nbits_str = if arch_dependent {
+        "64".to_owned()
+    } else if is_isize_or_usize(cast_from) {
+        "32 or 64".to_owned()
+    } else {
+        utils::int_ty_to_nbits(cast_from, cx.tcx).to_string()
+    };
+
+    span_lint(
+        cx,
+        CAST_PRECISION_LOSS,
+        expr.span,
+        &format!(
+            "casting `{0}` to `{1}` causes a loss of precision {2}(`{0}` is {3} bits wide, \
+             but `{1}`'s mantissa is only {4} bits wide)",
+            cast_from,
+            if cast_to_f64 { "f64" } else { "f32" },
+            if arch_dependent { arch_dependent_str } else { "" },
+            from_nbits_str,
+            mantissa_nbits
+        ),
+    );
+}

src/tools/clippy/clippy_lints/src/casts/cast_ptr_alignment.rs

@@ -0,0 +1,81 @@
+use rustc_hir::{Expr, ExprKind, GenericArg};
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, Ty};
+use rustc_span::symbol::sym;
+use rustc_target::abi::LayoutOf;
+
+use if_chain::if_chain;
+
+use crate::utils::{is_hir_ty_cfg_dependant, span_lint};
+
+use super::CAST_PTR_ALIGNMENT;
+
+pub(super) fn check(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+    if let ExprKind::Cast(ref cast_expr, cast_to) = expr.kind {
+        if is_hir_ty_cfg_dependant(cx, cast_to) {
+            return;
+        }
+        let (cast_from, cast_to) = (
+            cx.typeck_results().expr_ty(cast_expr),
+            cx.typeck_results().expr_ty(expr),
+        );
+        lint_cast_ptr_alignment(cx, expr, cast_from, cast_to);
+    } else if let ExprKind::MethodCall(method_path, _, args, _) = expr.kind {
+        if_chain! {
+            if method_path.ident.name == sym!(cast);
+            if let Some(generic_args) = method_path.args;
+            if let [GenericArg::Type(cast_to)] = generic_args.args;
+            // There probably is no obvious reason to do this, just to be consistent with `as` cases.
+            if !is_hir_ty_cfg_dependant(cx, cast_to);
+            then {
+                let (cast_from, cast_to) =
+                    (cx.typeck_results().expr_ty(&args[0]), cx.typeck_results().expr_ty(expr));
+                lint_cast_ptr_alignment(cx, expr, cast_from, cast_to);
+            }
+        }
+    }
+}
+
+fn lint_cast_ptr_alignment<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'_>, cast_from: Ty<'tcx>, cast_to: Ty<'tcx>) {
+    if_chain! {
+        if let ty::RawPtr(from_ptr_ty) = &cast_from.kind();
+        if let ty::RawPtr(to_ptr_ty) = &cast_to.kind();
+        if let Ok(from_layout) = cx.layout_of(from_ptr_ty.ty);
+        if let Ok(to_layout) = cx.layout_of(to_ptr_ty.ty);
+        if from_layout.align.abi < to_layout.align.abi;
+        // with c_void, we inherently need to trust the user
+        if !is_c_void(cx, from_ptr_ty.ty);
+        // when casting from a ZST, we don't know enough to properly lint
+        if !from_layout.is_zst();
+        then {
+            span_lint(
+                cx,
+                CAST_PTR_ALIGNMENT,
+                expr.span,
+                &format!(
+                    "casting from `{}` to a more-strictly-aligned pointer (`{}`) ({} < {} bytes)",
+                    cast_from,
+                    cast_to,
+                    from_layout.align.abi.bytes(),
+                    to_layout.align.abi.bytes(),
+                ),
+            );
+        }
+    }
+}
+
+/// Check if the given type is either `core::ffi::c_void` or
+/// one of the platform specific `libc::<platform>::c_void` of libc.
+fn is_c_void(cx: &LateContext<'_>, ty: Ty<'_>) -> bool {
+    if let ty::Adt(adt, _) = ty.kind() {
+        let names = cx.get_def_path(adt.did);
+
+        if names.is_empty() {
+            return false;
+        }
+        if names[0] == sym::libc || names[0] == sym::core && *names.last().unwrap() == sym!(c_void) {
+            return true;
+        }
+    }
+    false
+}

src/tools/clippy/clippy_lints/src/casts/cast_ref_to_mut.rs

@@ -0,0 +1,28 @@
+use rustc_hir::{Expr, ExprKind, MutTy, Mutability, TyKind, UnOp};
+use rustc_lint::LateContext;
+use rustc_middle::ty;
+
+use if_chain::if_chain;
+
+use crate::utils::span_lint;
+
+use super::CAST_REF_TO_MUT;
+
+pub(super) fn check(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+    if_chain! {
+        if let ExprKind::Unary(UnOp::Deref, e) = &expr.kind;
+        if let ExprKind::Cast(e, t) = &e.kind;
+        if let TyKind::Ptr(MutTy { mutbl: Mutability::Mut, .. }) = t.kind;
+        if let ExprKind::Cast(e, t) = &e.kind;
+        if let TyKind::Ptr(MutTy { mutbl: Mutability::Not, .. }) = t.kind;
+        if let ty::Ref(..) = cx.typeck_results().node_type(e.hir_id).kind();
+        then {
+            span_lint(
+                cx,
+                CAST_REF_TO_MUT,
+                expr.span,
+                "casting `&T` to `&mut T` may cause undefined behavior, consider instead using an `UnsafeCell`",
+            );
+        }
+    }
+}

src/tools/clippy/clippy_lints/src/casts/cast_sign_loss.rs

@@ -0,0 +1,70 @@
+use rustc_hir::{Expr, ExprKind};
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, Ty};
+
+use if_chain::if_chain;
+
+use crate::consts::{constant, Constant};
+use crate::utils::{method_chain_args, sext, span_lint};
+
+use super::CAST_SIGN_LOSS;
+
+pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_op: &Expr<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>) {
+    if should_lint(cx, cast_op, cast_from, cast_to) {
+        span_lint(
+            cx,
+            CAST_SIGN_LOSS,
+            expr.span,
+            &format!(
+                "casting `{}` to `{}` may lose the sign of the value",
+                cast_from, cast_to
+            ),
+        );
+    }
+}
+
+fn should_lint(cx: &LateContext<'_>, cast_op: &Expr<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>) -> bool {
+    match (cast_from.is_integral(), cast_to.is_integral()) {
+        (true, true) => {
+            if !cast_from.is_signed() || cast_to.is_signed() {
+                return false;
+            }
+
+            // Don't lint for positive constants.
+            let const_val = constant(cx, &cx.typeck_results(), cast_op);
+            if_chain! {
+                if let Some((Constant::Int(n), _)) = const_val;
+                if let ty::Int(ity) = *cast_from.kind();
+                if sext(cx.tcx, n, ity) >= 0;
+                then {
+                    return false;
+                }
+            }
+
+            // Don't lint for the result of methods that always return non-negative values.
+            if let ExprKind::MethodCall(ref path, _, _, _) = cast_op.kind {
+                let mut method_name = path.ident.name.as_str();
+                let allowed_methods = ["abs", "checked_abs", "rem_euclid", "checked_rem_euclid"];
+
+                if_chain! {
+                    if method_name == "unwrap";
+                    if let Some(arglist) = method_chain_args(cast_op, &["unwrap"]);
+                    if let ExprKind::MethodCall(ref inner_path, _, _, _) = &arglist[0][0].kind;
+                    then {
+                        method_name = inner_path.ident.name.as_str();
+                    }
+                }
+
+                if allowed_methods.iter().any(|&name| method_name == name) {
+                    return false;
+                }
+            }
+
+            true
+        },
+
+        (false, true) => !cast_to.is_signed(),
+
+        (_, _) => false,
+    }
+}

src/tools/clippy/clippy_lints/src/casts/char_lit_as_u8.rs

@@ -0,0 +1,42 @@
+use rustc_ast::LitKind;
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind};
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, UintTy};
+
+use if_chain::if_chain;
+
+use crate::utils::{snippet_with_applicability, span_lint_and_then};
+
+use super::CHAR_LIT_AS_U8;
+
+pub(super) fn check(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+    if_chain! {
+        if let ExprKind::Cast(e, _) = &expr.kind;
+        if let ExprKind::Lit(l) = &e.kind;
+        if let LitKind::Char(c) = l.node;
+        if ty::Uint(UintTy::U8) == *cx.typeck_results().expr_ty(expr).kind();
+        then {
+            let mut applicability = Applicability::MachineApplicable;
+            let snippet = snippet_with_applicability(cx, e.span, "'x'", &mut applicability);
+
+            span_lint_and_then(
+                cx,
+                CHAR_LIT_AS_U8,
+                expr.span,
+                "casting a character literal to `u8` truncates",
+                |diag| {
+                    diag.note("`char` is four bytes wide, but `u8` is a single byte");
+
+                    if c.is_ascii() {
+                        diag.span_suggestion(
+                            expr.span,
+                            "use a byte literal instead",
+                            format!("b{}", snippet),
+                            applicability,
+                        );
+                    }
+            });
+        }
+    }
+}

src/tools/clippy/clippy_lints/src/casts/fn_to_numeric_cast.rs

@@ -0,0 +1,37 @@
+use rustc_errors::Applicability;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, Ty, UintTy};
+
+use crate::utils::{snippet_with_applicability, span_lint_and_sugg};
+
+use super::{utils, FN_TO_NUMERIC_CAST};
+
+pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_expr: &Expr<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>) {
+    // We only want to check casts to `ty::Uint` or `ty::Int`
+    match cast_to.kind() {
+        ty::Uint(_) | ty::Int(..) => { /* continue on */ },
+        _ => return,
+    }
+
+    match cast_from.kind() {
+        ty::FnDef(..) | ty::FnPtr(_) => {
+            let mut applicability = Applicability::MaybeIncorrect;
+            let from_snippet = snippet_with_applicability(cx, cast_expr.span, "x", &mut applicability);
+            let to_nbits = utils::int_ty_to_nbits(cast_to, cx.tcx);
+
+            if (to_nbits >= cx.tcx.data_layout.pointer_size.bits()) && (*cast_to.kind() != ty::Uint(UintTy::Usize)) {
+                span_lint_and_sugg(
+                    cx,
+                    FN_TO_NUMERIC_CAST,
+                    expr.span,
+                    &format!("casting function pointer `{}` to `{}`", from_snippet, cast_to),
+                    "try",
+                    format!("{} as usize", from_snippet),
+                    applicability,
+                );
+            }
+        },
+        _ => {},
+    }
+}

src/tools/clippy/clippy_lints/src/casts/fn_to_numeric_cast_with_truncation.rs

@@ -0,0 +1,39 @@
+use rustc_errors::Applicability;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, Ty};
+
+use crate::utils::{snippet_with_applicability, span_lint_and_sugg};
+
+use super::{utils, FN_TO_NUMERIC_CAST_WITH_TRUNCATION};
+
+pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_expr: &Expr<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>) {
+    // We only want to check casts to `ty::Uint` or `ty::Int`
+    match cast_to.kind() {
+        ty::Uint(_) | ty::Int(..) => { /* continue on */ },
+        _ => return,
+    }
+    match cast_from.kind() {
+        ty::FnDef(..) | ty::FnPtr(_) => {
+            let mut applicability = Applicability::MaybeIncorrect;
+            let from_snippet = snippet_with_applicability(cx, cast_expr.span, "x", &mut applicability);
+
+            let to_nbits = utils::int_ty_to_nbits(cast_to, cx.tcx);
+            if to_nbits < cx.tcx.data_layout.pointer_size.bits() {
+                span_lint_and_sugg(
+                    cx,
+                    FN_TO_NUMERIC_CAST_WITH_TRUNCATION,
+                    expr.span,
+                    &format!(
+                        "casting function pointer `{}` to `{}`, which truncates the value",
+                        from_snippet, cast_to
+                    ),
+                    "try",
+                    format!("{} as usize", from_snippet),
+                    applicability,
+                );
+            }
+        },
+        _ => {},
+    }
+}

src/tools/clippy/clippy_lints/src/casts/mod.rs

@@ -0,0 +1,407 @@
+mod cast_lossless;
+mod cast_possible_truncation;
+mod cast_possible_wrap;
+mod cast_precision_loss;
+mod cast_ptr_alignment;
+mod cast_ref_to_mut;
+mod cast_sign_loss;
+mod char_lit_as_u8;
+mod fn_to_numeric_cast;
+mod fn_to_numeric_cast_with_truncation;
+mod ptr_as_ptr;
+mod unnecessary_cast;
+mod utils;
+
+use rustc_hir::{Expr, ExprKind};
+use rustc_lint::{LateContext, LateLintPass, LintContext};
+use rustc_middle::lint::in_external_macro;
+use rustc_semver::RustcVersion;
+use rustc_session::{declare_tool_lint, impl_lint_pass};
+
+use crate::utils::is_hir_ty_cfg_dependant;
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for casts from any numerical to a float type where
+    /// the receiving type cannot store all values from the original type without
+    /// rounding errors. This possible rounding is to be expected, so this lint is
+    /// `Allow` by default.
+    ///
+    /// Basically, this warns on casting any integer with 32 or more bits to `f32`
+    /// or any 64-bit integer to `f64`.
+    ///
+    /// **Why is this bad?** It's not bad at all. But in some applications it can be
+    /// helpful to know where precision loss can take place. This lint can help find
+    /// those places in the code.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// let x = u64::MAX;
+    /// x as f64;
+    /// ```
+    pub CAST_PRECISION_LOSS,
+    pedantic,
+    "casts that cause loss of precision, e.g., `x as f32` where `x: u64`"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for casts from a signed to an unsigned numerical
+    /// type. In this case, negative values wrap around to large positive values,
+    /// which can be quite surprising in practice. However, as the cast works as
+    /// defined, this lint is `Allow` by default.
+    ///
+    /// **Why is this bad?** Possibly surprising results. You can activate this lint
+    /// as a one-time check to see where numerical wrapping can arise.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// let y: i8 = -1;
+    /// y as u128; // will return 18446744073709551615
+    /// ```
+    pub CAST_SIGN_LOSS,
+    pedantic,
+    "casts from signed types to unsigned types, e.g., `x as u32` where `x: i32`"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for casts between numerical types that may
+    /// truncate large values. This is expected behavior, so the cast is `Allow` by
+    /// default.
+    ///
+    /// **Why is this bad?** In some problem domains, it is good practice to avoid
+    /// truncation. This lint can be activated to help assess where additional
+    /// checks could be beneficial.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// fn as_u8(x: u64) -> u8 {
+    ///     x as u8
+    /// }
+    /// ```
+    pub CAST_POSSIBLE_TRUNCATION,
+    pedantic,
+    "casts that may cause truncation of the value, e.g., `x as u8` where `x: u32`, or `x as i32` where `x: f32`"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for casts from an unsigned type to a signed type of
+    /// the same size. Performing such a cast is a 'no-op' for the compiler,
+    /// i.e., nothing is changed at the bit level, and the binary representation of
+    /// the value is reinterpreted. This can cause wrapping if the value is too big
+    /// for the target signed type. However, the cast works as defined, so this lint
+    /// is `Allow` by default.
+    ///
+    /// **Why is this bad?** While such a cast is not bad in itself, the results can
+    /// be surprising when this is not the intended behavior, as demonstrated by the
+    /// example below.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// u32::MAX as i32; // will yield a value of `-1`
+    /// ```
+    pub CAST_POSSIBLE_WRAP,
+    pedantic,
+    "casts that may cause wrapping around the value, e.g., `x as i32` where `x: u32` and `x > i32::MAX`"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for casts between numerical types that may
+    /// be replaced by safe conversion functions.
+    ///
+    /// **Why is this bad?** Rust's `as` keyword will perform many kinds of
+    /// conversions, including silently lossy conversions. Conversion functions such
+    /// as `i32::from` will only perform lossless conversions. Using the conversion
+    /// functions prevents conversions from turning into silent lossy conversions if
+    /// the types of the input expressions ever change, and make it easier for
+    /// people reading the code to know that the conversion is lossless.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// fn as_u64(x: u8) -> u64 {
+    ///     x as u64
+    /// }
+    /// ```
+    ///
+    /// Using `::from` would look like this:
+    ///
+    /// ```rust
+    /// fn as_u64(x: u8) -> u64 {
+    ///     u64::from(x)
+    /// }
+    /// ```
+    pub CAST_LOSSLESS,
+    pedantic,
+    "casts using `as` that are known to be lossless, e.g., `x as u64` where `x: u8`"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for casts to the same type, casts of int literals to integer types
+    /// and casts of float literals to float types.
+    ///
+    /// **Why is this bad?** It's just unnecessary.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// let _ = 2i32 as i32;
+    /// let _ = 0.5 as f32;
+    /// ```
+    ///
+    /// Better:
+    ///
+    /// ```rust
+    /// let _ = 2_i32;
+    /// let _ = 0.5_f32;
+    /// ```
+    pub UNNECESSARY_CAST,
+    complexity,
+    "cast to the same type, e.g., `x as i32` where `x: i32`"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for casts, using `as` or `pointer::cast`,
+    /// from a less-strictly-aligned pointer to a more-strictly-aligned pointer
+    ///
+    /// **Why is this bad?** Dereferencing the resulting pointer may be undefined
+    /// behavior.
+    ///
+    /// **Known problems:** Using `std::ptr::read_unaligned` and `std::ptr::write_unaligned` or similar
+    /// on the resulting pointer is fine. Is over-zealous: Casts with manual alignment checks or casts like
+    /// u64-> u8 -> u16 can be fine. Miri is able to do a more in-depth analysis.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// let _ = (&1u8 as *const u8) as *const u16;
+    /// let _ = (&mut 1u8 as *mut u8) as *mut u16;
+    ///
+    /// (&1u8 as *const u8).cast::<u16>();
+    /// (&mut 1u8 as *mut u8).cast::<u16>();
+    /// ```
+    pub CAST_PTR_ALIGNMENT,
+    pedantic,
+    "cast from a pointer to a more-strictly-aligned pointer"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for casts of function pointers to something other than usize
+    ///
+    /// **Why is this bad?**
+    /// Casting a function pointer to anything other than usize/isize is not portable across
+    /// architectures, because you end up losing bits if the target type is too small or end up with a
+    /// bunch of extra bits that waste space and add more instructions to the final binary than
+    /// strictly necessary for the problem
+    ///
+    /// Casting to isize also doesn't make sense since there are no signed addresses.
+    ///
+    /// **Example**
+    ///
+    /// ```rust
+    /// // Bad
+    /// fn fun() -> i32 { 1 }
+    /// let a = fun as i64;
+    ///
+    /// // Good
+    /// fn fun2() -> i32 { 1 }
+    /// let a = fun2 as usize;
+    /// ```
+    pub FN_TO_NUMERIC_CAST,
+    style,
+    "casting a function pointer to a numeric type other than usize"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for casts of a function pointer to a numeric type not wide enough to
+    /// store address.
+    ///
+    /// **Why is this bad?**
+    /// Such a cast discards some bits of the function's address. If this is intended, it would be more
+    /// clearly expressed by casting to usize first, then casting the usize to the intended type (with
+    /// a comment) to perform the truncation.
+    ///
+    /// **Example**
+    ///
+    /// ```rust
+    /// // Bad
+    /// fn fn1() -> i16 {
+    ///     1
+    /// };
+    /// let _ = fn1 as i32;
+    ///
+    /// // Better: Cast to usize first, then comment with the reason for the truncation
+    /// fn fn2() -> i16 {
+    ///     1
+    /// };
+    /// let fn_ptr = fn2 as usize;
+    /// let fn_ptr_truncated = fn_ptr as i32;
+    /// ```
+    pub FN_TO_NUMERIC_CAST_WITH_TRUNCATION,
+    style,
+    "casting a function pointer to a numeric type not wide enough to store the address"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for casts of `&T` to `&mut T` anywhere in the code.
+    ///
+    /// **Why is this bad?** It’s basically guaranteed to be undefined behaviour.
+    /// `UnsafeCell` is the only way to obtain aliasable data that is considered
+    /// mutable.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust,ignore
+    /// fn x(r: &i32) {
+    ///     unsafe {
+    ///         *(r as *const _ as *mut _) += 1;
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// Instead consider using interior mutability types.
+    ///
+    /// ```rust
+    /// use std::cell::UnsafeCell;
+    ///
+    /// fn x(r: &UnsafeCell<i32>) {
+    ///     unsafe {
+    ///         *r.get() += 1;
+    ///     }
+    /// }
+    /// ```
+    pub CAST_REF_TO_MUT,
+    correctness,
+    "a cast of reference to a mutable pointer"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for expressions where a character literal is cast
+    /// to `u8` and suggests using a byte literal instead.
+    ///
+    /// **Why is this bad?** In general, casting values to smaller types is
+    /// error-prone and should be avoided where possible. In the particular case of
+    /// converting a character literal to u8, it is easy to avoid by just using a
+    /// byte literal instead. As an added bonus, `b'a'` is even slightly shorter
+    /// than `'a' as u8`.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust,ignore
+    /// 'x' as u8
+    /// ```
+    ///
+    /// A better version, using the byte literal:
+    ///
+    /// ```rust,ignore
+    /// b'x'
+    /// ```
+    pub CHAR_LIT_AS_U8,
+    complexity,
+    "casting a character literal to `u8` truncates"
+}
+
+declare_clippy_lint! {
+    /// **What it does:**
+    /// Checks for `as` casts between raw pointers without changing its mutability,
+    /// namely `*const T` to `*const U` and `*mut T` to `*mut U`.
+    ///
+    /// **Why is this bad?**
+    /// Though `as` casts between raw pointers is not terrible, `pointer::cast` is safer because
+    /// it cannot accidentally change the pointer's mutability nor cast the pointer to other types like `usize`.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    ///
+    /// ```rust
+    /// let ptr: *const u32 = &42_u32;
+    /// let mut_ptr: *mut u32 = &mut 42_u32;
+    /// let _ = ptr as *const i32;
+    /// let _ = mut_ptr as *mut i32;
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// let ptr: *const u32 = &42_u32;
+    /// let mut_ptr: *mut u32 = &mut 42_u32;
+    /// let _ = ptr.cast::<i32>();
+    /// let _ = mut_ptr.cast::<i32>();
+    /// ```
+    pub PTR_AS_PTR,
+    pedantic,
+    "casting using `as` from and to raw pointers that doesn't change its mutability, where `pointer::cast` could take the place of `as`"
+}
+
+pub struct Casts {
+    msrv: Option<RustcVersion>,
+}
+
+impl Casts {
+    #[must_use]
+    pub fn new(msrv: Option<RustcVersion>) -> Self {
+        Self { msrv }
+    }
+}
+
+impl_lint_pass!(Casts => [
+    CAST_PRECISION_LOSS,
+    CAST_SIGN_LOSS,
+    CAST_POSSIBLE_TRUNCATION,
+    CAST_POSSIBLE_WRAP,
+    CAST_LOSSLESS,
+    CAST_REF_TO_MUT,
+    CAST_PTR_ALIGNMENT,
+    UNNECESSARY_CAST,
+    FN_TO_NUMERIC_CAST,
+    FN_TO_NUMERIC_CAST_WITH_TRUNCATION,
+    CHAR_LIT_AS_U8,
+    PTR_AS_PTR,
+]);
+
+impl<'tcx> LateLintPass<'tcx> for Casts {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+        if expr.span.from_expansion() {
+            return;
+        }
+
+        if let ExprKind::Cast(ref cast_expr, cast_to) = expr.kind {
+            if is_hir_ty_cfg_dependant(cx, cast_to) {
+                return;
+            }
+            let (cast_from, cast_to) = (
+                cx.typeck_results().expr_ty(cast_expr),
+                cx.typeck_results().expr_ty(expr),
+            );
+
+            if unnecessary_cast::check(cx, expr, cast_expr, cast_from, cast_to) {
+                return;
+            }
+
+            fn_to_numeric_cast::check(cx, expr, cast_expr, cast_from, cast_to);
+            fn_to_numeric_cast_with_truncation::check(cx, expr, cast_expr, cast_from, cast_to);
+            if cast_from.is_numeric() && cast_to.is_numeric() && !in_external_macro(cx.sess(), expr.span) {
+                cast_possible_truncation::check(cx, expr, cast_from, cast_to);
+                cast_possible_wrap::check(cx, expr, cast_from, cast_to);
+                cast_precision_loss::check(cx, expr, cast_from, cast_to);
+                cast_lossless::check(cx, expr, cast_expr, cast_from, cast_to);
+                cast_sign_loss::check(cx, expr, cast_expr, cast_from, cast_to);
+            }
+        }
+
+        cast_ref_to_mut::check(cx, expr);
+        cast_ptr_alignment::check(cx, expr);
+        char_lit_as_u8::check(cx, expr);
+        ptr_as_ptr::check(cx, expr, &self.msrv);
+    }
+
+    extract_msrv_attr!(LateContext);
+}

src/tools/clippy/clippy_lints/src/casts/ptr_as_ptr.rs

@@ -0,0 +1,52 @@
+use std::borrow::Cow;
+
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind, Mutability, TyKind};
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, TypeAndMut};
+use rustc_semver::RustcVersion;
+
+use if_chain::if_chain;
+
+use crate::utils::sugg::Sugg;
+use crate::utils::{meets_msrv, span_lint_and_sugg};
+
+use super::PTR_AS_PTR;
+
+const PTR_AS_PTR_MSRV: RustcVersion = RustcVersion::new(1, 38, 0);
+
+pub(super) fn check(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, msrv: &Option<RustcVersion>) {
+    if !meets_msrv(msrv.as_ref(), &PTR_AS_PTR_MSRV) {
+        return;
+    }
+
+    if_chain! {
+        if let ExprKind::Cast(cast_expr, cast_to_hir_ty) = expr.kind;
+        let (cast_from, cast_to) = (cx.typeck_results().expr_ty(cast_expr), cx.typeck_results().expr_ty(expr));
+        if let ty::RawPtr(TypeAndMut { mutbl: from_mutbl, .. }) = cast_from.kind();
+        if let ty::RawPtr(TypeAndMut { ty: to_pointee_ty, mutbl: to_mutbl }) = cast_to.kind();
+        if matches!((from_mutbl, to_mutbl),
+            (Mutability::Not, Mutability::Not) | (Mutability::Mut, Mutability::Mut));
+        // The `U` in `pointer::cast` have to be `Sized`
+        // as explained here: https://github.com/rust-lang/rust/issues/60602.
+        if to_pointee_ty.is_sized(cx.tcx.at(expr.span), cx.param_env);
+        then {
+            let mut applicability = Applicability::MachineApplicable;
+            let cast_expr_sugg = Sugg::hir_with_applicability(cx, cast_expr, "_", &mut applicability);
+            let turbofish = match &cast_to_hir_ty.kind {
+                    TyKind::Infer => Cow::Borrowed(""),
+                    TyKind::Ptr(mut_ty) if matches!(mut_ty.ty.kind, TyKind::Infer) => Cow::Borrowed(""),
+                    _ => Cow::Owned(format!("::<{}>", to_pointee_ty)),
+                };
+            span_lint_and_sugg(
+                cx,
+                PTR_AS_PTR,
+                expr.span,
+                "`as` casting between raw pointers without changing its mutability",
+                "try `pointer::cast`, a safer alternative",
+                format!("{}.cast{}()", cast_expr_sugg.maybe_par(), turbofish),
+                applicability,
+            );
+        }
+    }
+}

src/tools/clippy/clippy_lints/src/casts/unnecessary_cast.rs

@@ -0,0 +1,106 @@
+use rustc_ast::{LitFloatType, LitIntType, LitKind};
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind, Lit, UnOp};
+use rustc_lint::{LateContext, LintContext};
+use rustc_middle::lint::in_external_macro;
+use rustc_middle::ty::{self, FloatTy, InferTy, Ty};
+
+use if_chain::if_chain;
+
+use crate::utils::{numeric_literal::NumericLiteral, snippet_opt, span_lint, span_lint_and_sugg};
+
+use super::UNNECESSARY_CAST;
+
+pub(super) fn check(
+    cx: &LateContext<'_>,
+    expr: &Expr<'_>,
+    cast_expr: &Expr<'_>,
+    cast_from: Ty<'_>,
+    cast_to: Ty<'_>,
+) -> bool {
+    if let Some(lit) = get_numeric_literal(cast_expr) {
+        let literal_str = snippet_opt(cx, cast_expr.span).unwrap_or_default();
+
+        if_chain! {
+            if let LitKind::Int(n, _) = lit.node;
+            if let Some(src) = snippet_opt(cx, lit.span);
+            if cast_to.is_floating_point();
+            if let Some(num_lit) = NumericLiteral::from_lit_kind(&src, &lit.node);
+            let from_nbits = 128 - n.leading_zeros();
+            let to_nbits = fp_ty_mantissa_nbits(cast_to);
+            if from_nbits != 0 && to_nbits != 0 && from_nbits <= to_nbits && num_lit.is_decimal();
+            then {
+                let literal_str = if is_unary_neg(cast_expr) { format!("-{}", num_lit.integer) } else { num_lit.integer.into() };
+                lint_unnecessary_cast(cx, expr, &literal_str, cast_from, cast_to);
+                return true
+            }
+        }
+
+        match lit.node {
+            LitKind::Int(_, LitIntType::Unsuffixed) if cast_to.is_integral() => {
+                lint_unnecessary_cast(cx, expr, &literal_str, cast_from, cast_to);
+            },
+            LitKind::Float(_, LitFloatType::Unsuffixed) if cast_to.is_floating_point() => {
+                lint_unnecessary_cast(cx, expr, &literal_str, cast_from, cast_to);
+            },
+            LitKind::Int(_, LitIntType::Unsuffixed) | LitKind::Float(_, LitFloatType::Unsuffixed) => {},
+            _ => {
+                if cast_from.kind() == cast_to.kind() && !in_external_macro(cx.sess(), expr.span) {
+                    span_lint(
+                        cx,
+                        UNNECESSARY_CAST,
+                        expr.span,
+                        &format!(
+                            "casting to the same type is unnecessary (`{}` -> `{}`)",
+                            cast_from, cast_to
+                        ),
+                    );
+                    return true;
+                }
+            },
+        }
+    }
+
+    false
+}
+
+fn lint_unnecessary_cast(cx: &LateContext<'_>, expr: &Expr<'_>, literal_str: &str, cast_from: Ty<'_>, cast_to: Ty<'_>) {
+    let literal_kind_name = if cast_from.is_integral() { "integer" } else { "float" };
+    span_lint_and_sugg(
+        cx,
+        UNNECESSARY_CAST,
+        expr.span,
+        &format!("casting {} literal to `{}` is unnecessary", literal_kind_name, cast_to),
+        "try",
+        format!("{}_{}", literal_str.trim_end_matches('.'), cast_to),
+        Applicability::MachineApplicable,
+    );
+}
+
+fn get_numeric_literal<'e>(expr: &'e Expr<'e>) -> Option<&'e Lit> {
+    match expr.kind {
+        ExprKind::Lit(ref lit) => Some(lit),
+        ExprKind::Unary(UnOp::Neg, e) => {
+            if let ExprKind::Lit(ref lit) = e.kind {
+                Some(lit)
+            } else {
+                None
+            }
+        },
+        _ => None,
+    }
+}
+
+/// Returns the mantissa bits wide of a fp type.
+/// Will return 0 if the type is not a fp
+fn fp_ty_mantissa_nbits(typ: Ty<'_>) -> u32 {
+    match typ.kind() {
+        ty::Float(FloatTy::F32) => 23,
+        ty::Float(FloatTy::F64) | ty::Infer(InferTy::FloatVar(_)) => 52,
+        _ => 0,
+    }
+}
+
+fn is_unary_neg(expr: &Expr<'_>) -> bool {
+    matches!(expr.kind, ExprKind::Unary(UnOp::Neg, _))
+}

src/tools/clippy/clippy_lints/src/casts/utils.rs

@@ -0,0 +1,25 @@
+use rustc_middle::ty::{self, IntTy, Ty, TyCtxt, UintTy};
+
+/// Returns the size in bits of an integral type.
+/// Will return 0 if the type is not an int or uint variant
+pub(super) fn int_ty_to_nbits(typ: Ty<'_>, tcx: TyCtxt<'_>) -> u64 {
+    match typ.kind() {
+        ty::Int(i) => match i {
+            IntTy::Isize => tcx.data_layout.pointer_size.bits(),
+            IntTy::I8 => 8,
+            IntTy::I16 => 16,
+            IntTy::I32 => 32,
+            IntTy::I64 => 64,
+            IntTy::I128 => 128,
+        },
+        ty::Uint(i) => match i {
+            UintTy::Usize => tcx.data_layout.pointer_size.bits(),
+            UintTy::U8 => 8,
+            UintTy::U16 => 16,
+            UintTy::U32 => 32,
+            UintTy::U64 => 64,
+            UintTy::U128 => 128,
+        },
+        _ => 0,
+    }
+}

src/tools/clippy/clippy_lints/src/comparison_chain.rs

@@ -117,7 +117,7 @@ impl<'tcx> LateLintPass<'tcx> for ComparisonChain {
             expr.span,
             "`if` chain can be rewritten with `match`",
             None,
-            "Consider rewriting the `if` chain to use `cmp` and `match`.",
+            "consider rewriting the `if` chain to use `cmp` and `match`",
         )
     }
 }

src/tools/clippy/clippy_lints/src/default_numeric_fallback.rs

@@ -130,10 +130,9 @@ impl<'a, 'tcx> Visitor<'tcx> for NumericFallbackVisitor<'a, 'tcx> {
                 }
             },
 
-            ExprKind::Struct(qpath, fields, base) => {
+            ExprKind::Struct(_, fields, base) => {
                 if_chain! {
-                    if let Some(def_id) = self.cx.qpath_res(qpath, expr.hir_id).opt_def_id();
-                    let ty = self.cx.tcx.type_of(def_id);
+                    let ty = self.cx.typeck_results().expr_ty(expr);
                     if let Some(adt_def) = ty.ty_adt_def();
                     if adt_def.is_struct();
                     if let Some(variant) = adt_def.variants.iter().next();

src/tools/clippy/clippy_lints/src/derive.rs

@@ -1,6 +1,6 @@
 use crate::utils::paths;
 use crate::utils::{
-    get_trait_def_id, is_allowed, is_automatically_derived, is_copy, match_def_path, match_path, span_lint_and_help,
+    get_trait_def_id, is_allowed, is_automatically_derived, is_copy, match_def_path, span_lint_and_help,
     span_lint_and_note, span_lint_and_then,
 };
 use if_chain::if_chain;
@@ -294,7 +294,12 @@ fn check_ord_partial_ord<'tcx>(
 
 /// Implementation of the `EXPL_IMPL_CLONE_ON_COPY` lint.
 fn check_copy_clone<'tcx>(cx: &LateContext<'tcx>, item: &Item<'_>, trait_ref: &TraitRef<'_>, ty: Ty<'tcx>) {
-    if match_path(&trait_ref.path, &paths::CLONE_TRAIT) {
+    if cx
+        .tcx
+        .lang_items()
+        .clone_trait()
+        .map_or(false, |id| Some(id) == trait_ref.trait_def_id())
+    {
         if !is_copy(cx, ty) {
             return;
         }

src/tools/clippy/clippy_lints/src/doc.rs

@@ -329,9 +329,9 @@ fn lint_for_missing_headers<'tcx>(
             if_chain! {
                 if let Some(body_id) = body_id;
                 if let Some(future) = cx.tcx.lang_items().future_trait();
-                let def_id = cx.tcx.hir().body_owner_def_id(body_id);
-                let mir = cx.tcx.optimized_mir(def_id.to_def_id());
-                let ret_ty = mir.return_ty();
+                let typeck = cx.tcx.typeck_body(body_id);
+                let body = cx.tcx.hir().body(body_id);
+                let ret_ty = typeck.expr_ty(&body.value);
                 if implements_trait(cx, ret_ty, future, &[]);
                 if let ty::Opaque(_, subs) = ret_ty.kind();
                 if let Some(gen) = subs.types().next();

src/tools/clippy/clippy_lints/src/drop_forget_ref.rs

@@ -98,13 +98,13 @@ declare_clippy_lint! {
 }
 
 const DROP_REF_SUMMARY: &str = "calls to `std::mem::drop` with a reference instead of an owned value. \
-                                Dropping a reference does nothing.";
+                                Dropping a reference does nothing";
 const FORGET_REF_SUMMARY: &str = "calls to `std::mem::forget` with a reference instead of an owned value. \
-                                  Forgetting a reference does nothing.";
+                                  Forgetting a reference does nothing";
 const DROP_COPY_SUMMARY: &str = "calls to `std::mem::drop` with a value that implements `Copy`. \
-                                 Dropping a copy leaves the original intact.";
+                                 Dropping a copy leaves the original intact";
 const FORGET_COPY_SUMMARY: &str = "calls to `std::mem::forget` with a value that implements `Copy`. \
-                                   Forgetting a copy leaves the original intact.";
+                                   Forgetting a copy leaves the original intact";
 
 declare_lint_pass!(DropForgetRef => [DROP_REF, FORGET_REF, DROP_COPY, FORGET_COPY]);
 

src/tools/clippy/clippy_lints/src/eta_reduction.rs

@@ -10,6 +10,8 @@ use crate::utils::{
     implements_trait, is_adjusted, iter_input_pats, snippet_opt, span_lint_and_sugg, span_lint_and_then,
     type_is_unsafe_function,
 };
+use clippy_utils::higher;
+use clippy_utils::higher::VecArgs;
 
 declare_clippy_lint! {
     /// **What it does:** Checks for closures which just call another function where
@@ -74,7 +76,10 @@ impl<'tcx> LateLintPass<'tcx> for EtaReduction {
         match expr.kind {
             ExprKind::Call(_, args) | ExprKind::MethodCall(_, _, args, _) => {
                 for arg in args {
-                    check_closure(cx, arg)
+                    // skip `foo(macro!())`
+                    if arg.span.ctxt() == expr.span.ctxt() {
+                        check_closure(cx, arg)
+                    }
                 }
             },
             _ => (),
@@ -87,6 +92,23 @@ fn check_closure(cx: &LateContext<'_>, expr: &Expr<'_>) {
         let body = cx.tcx.hir().body(eid);
         let ex = &body.value;
 
+        if ex.span.ctxt() != expr.span.ctxt() {
+            if let Some(VecArgs::Vec(&[])) = higher::vec_macro(cx, ex) {
+                // replace `|| vec![]` with `Vec::new`
+                span_lint_and_sugg(
+                    cx,
+                    REDUNDANT_CLOSURE,
+                    expr.span,
+                    "redundant closure",
+                    "replace the closure with `Vec::new`",
+                    "std::vec::Vec::new".into(),
+                    Applicability::MachineApplicable,
+                );
+            }
+            // skip `foo(|| macro!())`
+            return;
+        }
+
         if_chain!(
             if let ExprKind::Call(ref caller, ref args) = ex.kind;
 
@@ -107,11 +129,11 @@ fn check_closure(cx: &LateContext<'_>, expr: &Expr<'_>) {
             if compare_inputs(&mut iter_input_pats(decl, body), &mut args.iter());
 
             then {
-                span_lint_and_then(cx, REDUNDANT_CLOSURE, expr.span, "redundant closure found", |diag| {
+                span_lint_and_then(cx, REDUNDANT_CLOSURE, expr.span, "redundant closure", |diag| {
                     if let Some(snippet) = snippet_opt(cx, caller.span) {
                         diag.span_suggestion(
                             expr.span,
-                            "remove closure as shown",
+                            "replace the closure with the function itself",
                             snippet,
                             Applicability::MachineApplicable,
                         );
@@ -141,8 +163,8 @@ fn check_closure(cx: &LateContext<'_>, expr: &Expr<'_>) {
                     cx,
                     REDUNDANT_CLOSURE_FOR_METHOD_CALLS,
                     expr.span,
-                    "redundant closure found",
-                    "remove closure as shown",
+                    "redundant closure",
+                    "replace the closure with the method itself",
                     format!("{}::{}", name, path.ident.name),
                     Applicability::MachineApplicable,
                 );

src/tools/clippy/clippy_lints/src/fallible_impl_from.rs

@@ -133,7 +133,7 @@ fn lint_impl_body<'tcx>(cx: &LateContext<'tcx>, impl_span: Span, impl_items: &[h
                         move |diag| {
                             diag.help(
                                 "`From` is intended for infallible conversions only. \
-                                Use `TryFrom` if there's a possibility for the conversion to fail.");
+                                Use `TryFrom` if there's a possibility for the conversion to fail");
                             diag.span_note(fpu.result, "potential failure(s)");
                         });
                 }

src/tools/clippy/clippy_lints/src/float_literal.rs

@@ -145,11 +145,7 @@ fn count_digits(s: &str) -> usize {
         .take_while(|c| *c != 'e' && *c != 'E')
         .fold(0, |count, c| {
             // leading zeros
-            if c == '0' && count == 0 {
-                count
-            } else {
-                count + 1
-            }
+            if c == '0' && count == 0 { count } else { count + 1 }
         })
 }
 

src/tools/clippy/clippy_lints/src/format.rs

@@ -28,11 +28,11 @@ declare_clippy_lint! {
     /// ```rust
     ///
     /// // Bad
-    /// # let foo = "foo";
+    /// let foo = "foo";
     /// format!("{}", foo);
     ///
     /// // Good
-    /// format!("foo");
+    /// foo.to_owned();
     /// ```
     pub USELESS_FORMAT,
     complexity,

src/tools/clippy/clippy_lints/src/functions.rs

@@ -317,9 +317,7 @@ impl<'tcx> LateLintPass<'tcx> for Functions {
             let attr = must_use_attr(attrs);
             if let Some(attr) = attr {
                 check_needless_must_use(cx, &sig.decl, item.hir_id(), item.span, fn_header_span, attr);
-            } else if is_public
-                && !is_proc_macro(cx.sess(), attrs)
-                && trait_ref_of_method(cx, item.hir_id()).is_none()
+            } else if is_public && !is_proc_macro(cx.sess(), attrs) && trait_ref_of_method(cx, item.hir_id()).is_none()
             {
                 check_must_use_candidate(
                     cx,

src/tools/clippy/clippy_lints/src/implicit_return.rs

@@ -1,4 +1,4 @@
-use crate::utils::{fn_has_unsatisfiable_preds, match_panic_def_id, snippet_opt, span_lint_and_then};
+use crate::utils::{match_panic_def_id, snippet_opt, span_lint_and_then};
 use if_chain::if_chain;
 use rustc_errors::Applicability;
 use rustc_hir::intravisit::FnKind;
@@ -133,19 +133,13 @@ impl<'tcx> LateLintPass<'tcx> for ImplicitReturn {
         span: Span,
         _: HirId,
     ) {
-        let def_id = cx.tcx.hir().body_owner_def_id(body.id());
-
-        // Building MIR for `fn`s with unsatisfiable preds results in ICE.
-        if fn_has_unsatisfiable_preds(cx, def_id.to_def_id()) {
+        if span.from_expansion() {
             return;
         }
-
-        let mir = cx.tcx.optimized_mir(def_id.to_def_id());
-
-        // checking return type through MIR, HIR is not able to determine inferred closure return types
-        // make sure it's not a macro
-        if !mir.return_ty().is_unit() && !span.from_expansion() {
-            expr_match(cx, &body.value);
+        let body = cx.tcx.hir().body(body.id());
+        if cx.typeck_results().expr_ty(&body.value).is_unit() {
+            return;
         }
+        expr_match(cx, &body.value);
     }
 }

src/tools/clippy/clippy_lints/src/inconsistent_struct_constructor.rs

@@ -66,8 +66,7 @@ impl LateLintPass<'_> for InconsistentStructConstructor {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>) {
         if_chain! {
             if let ExprKind::Struct(qpath, fields, base) = expr.kind;
-            if let Some(def_id)  = cx.qpath_res(qpath, expr.hir_id).opt_def_id();
-            let ty = cx.tcx.type_of(def_id);
+            let ty = cx.typeck_results().expr_ty(expr);
             if let Some(adt_def) = ty.ty_adt_def();
             if adt_def.is_struct();
             if let Some(variant) = adt_def.variants.iter().next();

src/tools/clippy/clippy_lints/src/indexing_slicing.rs

@@ -132,13 +132,13 @@ impl<'tcx> LateLintPass<'tcx> for IndexingSlicing {
                 }
 
                 let help_msg = match (range.start, range.end) {
-                    (None, Some(_)) => "Consider using `.get(..n)`or `.get_mut(..n)` instead",
-                    (Some(_), None) => "Consider using `.get(n..)` or .get_mut(n..)` instead",
-                    (Some(_), Some(_)) => "Consider using `.get(n..m)` or `.get_mut(n..m)` instead",
+                    (None, Some(_)) => "consider using `.get(..n)`or `.get_mut(..n)` instead",
+                    (Some(_), None) => "consider using `.get(n..)` or .get_mut(n..)` instead",
+                    (Some(_), Some(_)) => "consider using `.get(n..m)` or `.get_mut(n..m)` instead",
                     (None, None) => return, // [..] is ok.
                 };
 
-                span_lint_and_help(cx, INDEXING_SLICING, expr.span, "slicing may panic.", None, help_msg);
+                span_lint_and_help(cx, INDEXING_SLICING, expr.span, "slicing may panic", None, help_msg);
             } else {
                 // Catchall non-range index, i.e., [n] or [n << m]
                 if let ty::Array(..) = ty.kind() {
@@ -153,9 +153,9 @@ impl<'tcx> LateLintPass<'tcx> for IndexingSlicing {
                     cx,
                     INDEXING_SLICING,
                     expr.span,
-                    "indexing may panic.",
+                    "indexing may panic",
                     None,
-                    "Consider using `.get(n)` or `.get_mut(n)` instead",
+                    "consider using `.get(n)` or `.get_mut(n)` instead",
                 );
             }
         }

src/tools/clippy/clippy_lints/src/infinite_iter.rs

@@ -89,11 +89,7 @@ impl Finiteness {
 impl From<bool> for Finiteness {
     #[must_use]
     fn from(b: bool) -> Self {
-        if b {
-            Infinite
-        } else {
-            Finite
-        }
+        if b { Infinite } else { Finite }
     }
 }
 

src/tools/clippy/clippy_lints/src/inherent_to_string.rs

@@ -139,7 +139,7 @@ fn show_lint(cx: &LateContext<'_>, item: &ImplItem<'_>) {
                 self_type.to_string()
             ),
             None,
-            &format!("remove the inherent method from type `{}`", self_type.to_string())
+            &format!("remove the inherent method from type `{}`", self_type.to_string()),
         );
     } else {
         span_lint_and_help(

src/tools/clippy/clippy_lints/src/integer_division.rs

@@ -39,7 +39,7 @@ impl<'tcx> LateLintPass<'tcx> for IntegerDivision {
                 expr.span,
                 "integer division",
                 None,
-                "division of integers may cause loss of precision. consider using floats.",
+                "division of integers may cause loss of precision. consider using floats",
             );
         }
     }

src/tools/clippy/clippy_lints/src/len_zero.rs

@@ -1,11 +1,17 @@
-use crate::utils::{get_item_name, snippet_with_applicability, span_lint, span_lint_and_sugg};
+use crate::utils::{
+    get_item_name, get_parent_as_impl, is_allowed, snippet_with_applicability, span_lint, span_lint_and_sugg,
+    span_lint_and_then,
+};
+use if_chain::if_chain;
 use rustc_ast::ast::LitKind;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::Applicability;
-use rustc_hir::def_id::DefId;
-use rustc_hir::{AssocItemKind, BinOpKind, Expr, ExprKind, Impl, ImplItemRef, Item, ItemKind, TraitItemRef};
+use rustc_hir::{
+    def_id::DefId, AssocItemKind, BinOpKind, Expr, ExprKind, FnRetTy, ImplItem, ImplItemKind, ImplicitSelfKind, Item,
+    ItemKind, Mutability, Node, TraitItemRef, TyKind,
+};
 use rustc_lint::{LateContext, LateLintPass};
-use rustc_middle::ty;
+use rustc_middle::ty::{self, AssocKind, FnSig};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 use rustc_span::source_map::{Span, Spanned, Symbol};
 
@@ -113,14 +119,38 @@ impl<'tcx> LateLintPass<'tcx> for LenZero {
             return;
         }
 
-        match item.kind {
-            ItemKind::Trait(_, _, _, _, ref trait_items) => check_trait_items(cx, item, trait_items),
-            ItemKind::Impl(Impl {
-                of_trait: None,
-                items: ref impl_items,
-                ..
-            }) => check_impl_items(cx, item, impl_items),
-            _ => (),
+        if let ItemKind::Trait(_, _, _, _, ref trait_items) = item.kind {
+            check_trait_items(cx, item, trait_items);
+        }
+    }
+
+    fn check_impl_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx ImplItem<'_>) {
+        if_chain! {
+            if item.ident.as_str() == "len";
+            if let ImplItemKind::Fn(sig, _) = &item.kind;
+            if sig.decl.implicit_self.has_implicit_self();
+            if cx.access_levels.is_exported(item.hir_id());
+            if matches!(sig.decl.output, FnRetTy::Return(_));
+            if let Some(imp) = get_parent_as_impl(cx.tcx, item.hir_id());
+            if imp.of_trait.is_none();
+            if let TyKind::Path(ty_path) = &imp.self_ty.kind;
+            if let Some(ty_id) = cx.qpath_res(ty_path, imp.self_ty.hir_id).opt_def_id();
+            if let Some(local_id) = ty_id.as_local();
+            let ty_hir_id = cx.tcx.hir().local_def_id_to_hir_id(local_id);
+            if !is_allowed(cx, LEN_WITHOUT_IS_EMPTY, ty_hir_id);
+            then {
+                let (name, kind) = match cx.tcx.hir().find(ty_hir_id) {
+                    Some(Node::ForeignItem(x)) => (x.ident.name, "extern type"),
+                    Some(Node::Item(x)) => match x.kind {
+                        ItemKind::Struct(..) => (x.ident.name, "struct"),
+                        ItemKind::Enum(..) => (x.ident.name, "enum"),
+                        ItemKind::Union(..) => (x.ident.name, "union"),
+                        _ => (x.ident.name, "type"),
+                    }
+                    _ => return,
+                };
+                check_for_is_empty(cx, sig.span, sig.decl.implicit_self, ty_id, name, kind)
+            }
         }
     }
 
@@ -202,40 +232,94 @@ fn check_trait_items(cx: &LateContext<'_>, visited_trait: &Item<'_>, trait_items
     }
 }
 
-fn check_impl_items(cx: &LateContext<'_>, item: &Item<'_>, impl_items: &[ImplItemRef<'_>]) {
-    fn is_named_self(cx: &LateContext<'_>, item: &ImplItemRef<'_>, name: &str) -> bool {
-        item.ident.name.as_str() == name
-            && if let AssocItemKind::Fn { has_self } = item.kind {
-                has_self && cx.tcx.fn_sig(item.id.def_id).inputs().skip_binder().len() == 1
-            } else {
-                false
-            }
+/// Checks if the given signature matches the expectations for `is_empty`
+fn check_is_empty_sig(cx: &LateContext<'_>, sig: FnSig<'_>, self_kind: ImplicitSelfKind) -> bool {
+    match &**sig.inputs_and_output {
+        [arg, res] if *res == cx.tcx.types.bool => {
+            matches!(
+                (arg.kind(), self_kind),
+                (ty::Ref(_, _, Mutability::Not), ImplicitSelfKind::ImmRef)
+                    | (ty::Ref(_, _, Mutability::Mut), ImplicitSelfKind::MutRef)
+            ) || (!arg.is_ref() && matches!(self_kind, ImplicitSelfKind::Imm | ImplicitSelfKind::Mut))
+        },
+        _ => false,
     }
+}
 
-    let is_empty = if let Some(is_empty) = impl_items.iter().find(|i| is_named_self(cx, i, "is_empty")) {
-        if cx.access_levels.is_exported(is_empty.id.hir_id()) {
-            return;
-        }
-        "a private"
-    } else {
-        "no corresponding"
+/// Checks if the given type has an `is_empty` method with the appropriate signature.
+fn check_for_is_empty(
+    cx: &LateContext<'_>,
+    span: Span,
+    self_kind: ImplicitSelfKind,
+    impl_ty: DefId,
+    item_name: Symbol,
+    item_kind: &str,
+) {
+    let is_empty = Symbol::intern("is_empty");
+    let is_empty = cx
+        .tcx
+        .inherent_impls(impl_ty)
+        .iter()
+        .flat_map(|&id| cx.tcx.associated_items(id).filter_by_name_unhygienic(is_empty))
+        .find(|item| item.kind == AssocKind::Fn);
+
+    let (msg, is_empty_span, self_kind) = match is_empty {
+        None => (
+            format!(
+                "{} `{}` has a public `len` method, but no `is_empty` method",
+                item_kind,
+                item_name.as_str(),
+            ),
+            None,
+            None,
+        ),
+        Some(is_empty)
+            if !cx
+                .access_levels
+                .is_exported(cx.tcx.hir().local_def_id_to_hir_id(is_empty.def_id.expect_local())) =>
+        {
+            (
+                format!(
+                    "{} `{}` has a public `len` method, but a private `is_empty` method",
+                    item_kind,
+                    item_name.as_str(),
+                ),
+                Some(cx.tcx.def_span(is_empty.def_id)),
+                None,
+            )
+        },
+        Some(is_empty)
+            if !(is_empty.fn_has_self_parameter
+                && check_is_empty_sig(cx, cx.tcx.fn_sig(is_empty.def_id).skip_binder(), self_kind)) =>
+        {
+            (
+                format!(
+                    "{} `{}` has a public `len` method, but the `is_empty` method has an unexpected signature",
+                    item_kind,
+                    item_name.as_str(),
+                ),
+                Some(cx.tcx.def_span(is_empty.def_id)),
+                Some(self_kind),
+            )
+        },
+        Some(_) => return,
     };
 
-    if let Some(i) = impl_items.iter().find(|i| is_named_self(cx, i, "len")) {
-        if cx.access_levels.is_exported(i.id.hir_id()) {
-            let ty = cx.tcx.type_of(item.def_id);
-
-            span_lint(
-                cx,
-                LEN_WITHOUT_IS_EMPTY,
-                item.span,
-                &format!(
-                    "item `{}` has a public `len` method but {} `is_empty` method",
-                    ty, is_empty
-                ),
-            );
+    span_lint_and_then(cx, LEN_WITHOUT_IS_EMPTY, span, &msg, |db| {
+        if let Some(span) = is_empty_span {
+            db.span_note(span, "`is_empty` defined here");
         }
-    }
+        if let Some(self_kind) = self_kind {
+            db.note(&format!(
+                "expected signature: `({}self) -> bool`",
+                match self_kind {
+                    ImplicitSelfKind::ImmRef => "&",
+                    ImplicitSelfKind::MutRef => "&mut ",
+                    _ => "",
+                }
+            ));
+        }
+    });
 }
 
 fn check_cmp(cx: &LateContext<'_>, span: Span, method: &Expr<'_>, lit: &Expr<'_>, op: &str, compare_to: u32) {

src/tools/clippy/clippy_lints/src/lib.rs

@@ -182,6 +182,7 @@ mod booleans;
 mod bytecount;
 mod cargo_common_metadata;
 mod case_sensitive_file_extension_comparisons;
+mod casts;
 mod checked_conversions;
 mod cognitive_complexity;
 mod collapsible_if;
@@ -586,6 +587,18 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &bytecount::NAIVE_BYTECOUNT,
         &cargo_common_metadata::CARGO_COMMON_METADATA,
         &case_sensitive_file_extension_comparisons::CASE_SENSITIVE_FILE_EXTENSION_COMPARISONS,
+        &casts::CAST_LOSSLESS,
+        &casts::CAST_POSSIBLE_TRUNCATION,
+        &casts::CAST_POSSIBLE_WRAP,
+        &casts::CAST_PRECISION_LOSS,
+        &casts::CAST_PTR_ALIGNMENT,
+        &casts::CAST_REF_TO_MUT,
+        &casts::CAST_SIGN_LOSS,
+        &casts::CHAR_LIT_AS_U8,
+        &casts::FN_TO_NUMERIC_CAST,
+        &casts::FN_TO_NUMERIC_CAST_WITH_TRUNCATION,
+        &casts::PTR_AS_PTR,
+        &casts::UNNECESSARY_CAST,
         &checked_conversions::CHECKED_CONVERSIONS,
         &cognitive_complexity::COGNITIVE_COMPLEXITY,
         &collapsible_if::COLLAPSIBLE_ELSE_IF,
@@ -769,11 +782,13 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &methods::FLAT_MAP_IDENTITY,
         &methods::FROM_ITER_INSTEAD_OF_COLLECT,
         &methods::GET_UNWRAP,
+        &methods::IMPLICIT_CLONE,
         &methods::INEFFICIENT_TO_STRING,
         &methods::INSPECT_FOR_EACH,
         &methods::INTO_ITER_ON_REF,
         &methods::ITERATOR_STEP_BY_ZERO,
         &methods::ITER_CLONED_COLLECT,
+        &methods::ITER_COUNT,
         &methods::ITER_NEXT_SLICE,
         &methods::ITER_NTH,
         &methods::ITER_NTH_ZERO,
@@ -941,28 +956,16 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &types::ABSURD_EXTREME_COMPARISONS,
         &types::BORROWED_BOX,
         &types::BOX_VEC,
-        &types::CAST_LOSSLESS,
-        &types::CAST_POSSIBLE_TRUNCATION,
-        &types::CAST_POSSIBLE_WRAP,
-        &types::CAST_PRECISION_LOSS,
-        &types::CAST_PTR_ALIGNMENT,
-        &types::CAST_REF_TO_MUT,
-        &types::CAST_SIGN_LOSS,
-        &types::CHAR_LIT_AS_U8,
-        &types::FN_TO_NUMERIC_CAST,
-        &types::FN_TO_NUMERIC_CAST_WITH_TRUNCATION,
         &types::IMPLICIT_HASHER,
         &types::INVALID_UPCAST_COMPARISONS,
         &types::LET_UNIT_VALUE,
         &types::LINKEDLIST,
         &types::OPTION_OPTION,
-        &types::PTR_AS_PTR,
         &types::RC_BUFFER,
         &types::REDUNDANT_ALLOCATION,
         &types::TYPE_COMPLEXITY,
         &types::UNIT_ARG,
         &types::UNIT_CMP,
-        &types::UNNECESSARY_CAST,
         &types::VEC_BOX,
         &undropped_manually_drops::UNDROPPED_MANUALLY_DROPS,
         &unicode::INVISIBLE_CHARACTERS,
@@ -1073,6 +1076,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(move || box use_self::UseSelf::new(msrv));
     store.register_late_pass(move || box missing_const_for_fn::MissingConstForFn::new(msrv));
     store.register_late_pass(move || box needless_question_mark::NeedlessQuestionMark::new(msrv));
+    store.register_late_pass(move || box casts::Casts::new(msrv));
 
     store.register_late_pass(|| box size_of_in_element_count::SizeOfInElementCount);
     store.register_late_pass(|| box map_clone::MapClone);
@@ -1084,7 +1088,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| box main_recursion::MainRecursion::default());
     store.register_late_pass(|| box lifetimes::Lifetimes);
     store.register_late_pass(|| box entry::HashMapPass);
-    store.register_late_pass(|| box types::Casts);
     let type_complexity_threshold = conf.type_complexity_threshold;
     store.register_late_pass(move || box types::TypeComplexity::new(type_complexity_threshold));
     store.register_late_pass(|| box minmax::MinMaxPass);
@@ -1105,7 +1108,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| box panic_unimplemented::PanicUnimplemented);
     store.register_late_pass(|| box strings::StringLitAsBytes);
     store.register_late_pass(|| box derive::Derive);
-    store.register_late_pass(|| box types::CharLitAsU8);
     store.register_late_pass(|| box get_last_with_len::GetLastWithLen);
     store.register_late_pass(|| box drop_forget_ref::DropForgetRef);
     store.register_late_pass(|| box empty_enum::EmptyEnum);
@@ -1173,7 +1175,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| box slow_vector_initialization::SlowVectorInit);
     store.register_late_pass(|| box unnecessary_sort_by::UnnecessarySortBy);
     store.register_late_pass(|| box unnecessary_wraps::UnnecessaryWraps);
-    store.register_late_pass(|| box types::RefToMut);
     store.register_late_pass(|| box assertions_on_constants::AssertionsOnConstants);
     store.register_late_pass(|| box transmuting_null::TransmutingNull);
     store.register_late_pass(|| box path_buf_push_overwrite::PathBufPushOverwrite);
@@ -1275,7 +1276,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| box strings::StringToString);
     store.register_late_pass(|| box zero_sized_map_values::ZeroSizedMapValues);
     store.register_late_pass(|| box vec_init_then_push::VecInitThenPush::default());
-    store.register_late_pass(move || box types::PtrAsPtr::new(msrv));
     store.register_late_pass(|| box case_sensitive_file_extension_comparisons::CaseSensitiveFileExtensionComparisons);
     store.register_late_pass(|| box redundant_slicing::RedundantSlicing);
     store.register_late_pass(|| box from_str_radix_10::FromStrRadix10);
@@ -1341,7 +1341,15 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&await_holding_invalid::AWAIT_HOLDING_LOCK),
         LintId::of(&await_holding_invalid::AWAIT_HOLDING_REFCELL_REF),
         LintId::of(&bit_mask::VERBOSE_BIT_MASK),
+        LintId::of(&bytecount::NAIVE_BYTECOUNT),
         LintId::of(&case_sensitive_file_extension_comparisons::CASE_SENSITIVE_FILE_EXTENSION_COMPARISONS),
+        LintId::of(&casts::CAST_LOSSLESS),
+        LintId::of(&casts::CAST_POSSIBLE_TRUNCATION),
+        LintId::of(&casts::CAST_POSSIBLE_WRAP),
+        LintId::of(&casts::CAST_PRECISION_LOSS),
+        LintId::of(&casts::CAST_PTR_ALIGNMENT),
+        LintId::of(&casts::CAST_SIGN_LOSS),
+        LintId::of(&casts::PTR_AS_PTR),
         LintId::of(&checked_conversions::CHECKED_CONVERSIONS),
         LintId::of(&copies::SAME_FUNCTIONS_IN_IF_CONDITION),
         LintId::of(&copy_iterator::COPY_ITERATOR),
@@ -1380,6 +1388,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&matches::SINGLE_MATCH_ELSE),
         LintId::of(&methods::FILTER_MAP),
         LintId::of(&methods::FILTER_MAP_NEXT),
+        LintId::of(&methods::IMPLICIT_CLONE),
         LintId::of(&methods::INEFFICIENT_TO_STRING),
         LintId::of(&methods::MAP_FLATTEN),
         LintId::of(&methods::MAP_UNWRAP_OR),
@@ -1400,18 +1409,11 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&strings::STRING_ADD_ASSIGN),
         LintId::of(&trait_bounds::TRAIT_DUPLICATION_IN_BOUNDS),
         LintId::of(&trait_bounds::TYPE_REPETITION_IN_BOUNDS),
-        LintId::of(&types::CAST_LOSSLESS),
-        LintId::of(&types::CAST_POSSIBLE_TRUNCATION),
-        LintId::of(&types::CAST_POSSIBLE_WRAP),
-        LintId::of(&types::CAST_PRECISION_LOSS),
-        LintId::of(&types::CAST_PTR_ALIGNMENT),
-        LintId::of(&types::CAST_SIGN_LOSS),
         LintId::of(&types::IMPLICIT_HASHER),
         LintId::of(&types::INVALID_UPCAST_COMPARISONS),
         LintId::of(&types::LET_UNIT_VALUE),
         LintId::of(&types::LINKEDLIST),
         LintId::of(&types::OPTION_OPTION),
-        LintId::of(&types::PTR_AS_PTR),
         LintId::of(&unicode::NON_ASCII_LITERAL),
         LintId::of(&unicode::UNICODE_NOT_NFC),
         LintId::of(&unnecessary_wraps::UNNECESSARY_WRAPS),
@@ -1455,7 +1457,11 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&blocks_in_if_conditions::BLOCKS_IN_IF_CONDITIONS),
         LintId::of(&booleans::LOGIC_BUG),
         LintId::of(&booleans::NONMINIMAL_BOOL),
-        LintId::of(&bytecount::NAIVE_BYTECOUNT),
+        LintId::of(&casts::CAST_REF_TO_MUT),
+        LintId::of(&casts::CHAR_LIT_AS_U8),
+        LintId::of(&casts::FN_TO_NUMERIC_CAST),
+        LintId::of(&casts::FN_TO_NUMERIC_CAST_WITH_TRUNCATION),
+        LintId::of(&casts::UNNECESSARY_CAST),
         LintId::of(&collapsible_if::COLLAPSIBLE_ELSE_IF),
         LintId::of(&collapsible_if::COLLAPSIBLE_IF),
         LintId::of(&collapsible_match::COLLAPSIBLE_MATCH),
@@ -1576,6 +1582,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&methods::INTO_ITER_ON_REF),
         LintId::of(&methods::ITERATOR_STEP_BY_ZERO),
         LintId::of(&methods::ITER_CLONED_COLLECT),
+        LintId::of(&methods::ITER_COUNT),
         LintId::of(&methods::ITER_NEXT_SLICE),
         LintId::of(&methods::ITER_NTH),
         LintId::of(&methods::ITER_NTH_ZERO),
@@ -1695,15 +1702,10 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&types::ABSURD_EXTREME_COMPARISONS),
         LintId::of(&types::BORROWED_BOX),
         LintId::of(&types::BOX_VEC),
-        LintId::of(&types::CAST_REF_TO_MUT),
-        LintId::of(&types::CHAR_LIT_AS_U8),
-        LintId::of(&types::FN_TO_NUMERIC_CAST),
-        LintId::of(&types::FN_TO_NUMERIC_CAST_WITH_TRUNCATION),
         LintId::of(&types::REDUNDANT_ALLOCATION),
         LintId::of(&types::TYPE_COMPLEXITY),
         LintId::of(&types::UNIT_ARG),
         LintId::of(&types::UNIT_CMP),
-        LintId::of(&types::UNNECESSARY_CAST),
         LintId::of(&types::VEC_BOX),
         LintId::of(&undropped_manually_drops::UNDROPPED_MANUALLY_DROPS),
         LintId::of(&unicode::INVISIBLE_CHARACTERS),
@@ -1736,6 +1738,8 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&attrs::BLANKET_CLIPPY_RESTRICTION_LINTS),
         LintId::of(&blacklisted_name::BLACKLISTED_NAME),
         LintId::of(&blocks_in_if_conditions::BLOCKS_IN_IF_CONDITIONS),
+        LintId::of(&casts::FN_TO_NUMERIC_CAST),
+        LintId::of(&casts::FN_TO_NUMERIC_CAST_WITH_TRUNCATION),
         LintId::of(&collapsible_if::COLLAPSIBLE_ELSE_IF),
         LintId::of(&collapsible_if::COLLAPSIBLE_IF),
         LintId::of(&collapsible_match::COLLAPSIBLE_MATCH),
@@ -1832,8 +1836,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&tabs_in_doc_comments::TABS_IN_DOC_COMMENTS),
         LintId::of(&to_digit_is_some::TO_DIGIT_IS_SOME),
         LintId::of(&try_err::TRY_ERR),
-        LintId::of(&types::FN_TO_NUMERIC_CAST),
-        LintId::of(&types::FN_TO_NUMERIC_CAST_WITH_TRUNCATION),
         LintId::of(&unsafe_removed_from_name::UNSAFE_REMOVED_FROM_NAME),
         LintId::of(&unused_unit::UNUSED_UNIT),
         LintId::of(&upper_case_acronyms::UPPER_CASE_ACRONYMS),
@@ -1849,6 +1851,8 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&assign_ops::MISREFACTORED_ASSIGN_OP),
         LintId::of(&attrs::DEPRECATED_CFG_ATTR),
         LintId::of(&booleans::NONMINIMAL_BOOL),
+        LintId::of(&casts::CHAR_LIT_AS_U8),
+        LintId::of(&casts::UNNECESSARY_CAST),
         LintId::of(&double_comparison::DOUBLE_COMPARISONS),
         LintId::of(&double_parens::DOUBLE_PARENS),
         LintId::of(&duration_subsec::DURATION_SUBSEC),
@@ -1881,6 +1885,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&methods::FILTER_NEXT),
         LintId::of(&methods::FLAT_MAP_IDENTITY),
         LintId::of(&methods::INSPECT_FOR_EACH),
+        LintId::of(&methods::ITER_COUNT),
         LintId::of(&methods::MANUAL_FILTER_MAP),
         LintId::of(&methods::MANUAL_FIND_MAP),
         LintId::of(&methods::OPTION_AS_REF_DEREF),
@@ -1924,10 +1929,8 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&transmute::TRANSMUTE_PTR_TO_PTR),
         LintId::of(&transmute::TRANSMUTE_PTR_TO_REF),
         LintId::of(&types::BORROWED_BOX),
-        LintId::of(&types::CHAR_LIT_AS_U8),
         LintId::of(&types::TYPE_COMPLEXITY),
         LintId::of(&types::UNIT_ARG),
-        LintId::of(&types::UNNECESSARY_CAST),
         LintId::of(&types::VEC_BOX),
         LintId::of(&unnecessary_sort_by::UNNECESSARY_SORT_BY),
         LintId::of(&unwrap::UNNECESSARY_UNWRAP),
@@ -1945,6 +1948,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&bit_mask::BAD_BIT_MASK),
         LintId::of(&bit_mask::INEFFECTIVE_BIT_MASK),
         LintId::of(&booleans::LOGIC_BUG),
+        LintId::of(&casts::CAST_REF_TO_MUT),
         LintId::of(&copies::IFS_SAME_COND),
         LintId::of(&copies::IF_SAME_THEN_ELSE),
         LintId::of(&derive::DERIVE_HASH_XOR_EQ),
@@ -1997,7 +2001,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&transmute::WRONG_TRANSMUTE),
         LintId::of(&transmuting_null::TRANSMUTING_NULL),
         LintId::of(&types::ABSURD_EXTREME_COMPARISONS),
-        LintId::of(&types::CAST_REF_TO_MUT),
         LintId::of(&types::UNIT_CMP),
         LintId::of(&undropped_manually_drops::UNDROPPED_MANUALLY_DROPS),
         LintId::of(&unicode::INVISIBLE_CHARACTERS),
@@ -2010,7 +2013,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     ]);
 
     store.register_group(true, "clippy::perf", Some("clippy_perf"), vec![
-        LintId::of(&bytecount::NAIVE_BYTECOUNT),
         LintId::of(&entry::MAP_ENTRY),
         LintId::of(&escape::BOXED_LOCAL),
         LintId::of(&large_const_arrays::LARGE_CONST_ARRAYS),

src/tools/clippy/clippy_lints/src/lifetimes.rs

@@ -1,5 +1,4 @@
-use crate::utils::paths;
-use crate::utils::{get_trait_def_id, in_macro, span_lint, trait_ref_of_method};
+use crate::utils::{in_macro, span_lint, trait_ref_of_method};
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_hir::intravisit::{
     walk_fn_decl, walk_generic_param, walk_generics, walk_item, walk_param_bound, walk_poly_trait_ref, walk_ty,
@@ -8,8 +7,8 @@ use rustc_hir::intravisit::{
 use rustc_hir::FnRetTy::Return;
 use rustc_hir::{
     BareFnTy, BodyId, FnDecl, GenericArg, GenericBound, GenericParam, GenericParamKind, Generics, ImplItem,
-    ImplItemKind, Item, ItemKind, Lifetime, LifetimeName, ParamName, PolyTraitRef, TraitBoundModifier, TraitFn,
-    TraitItem, TraitItemKind, Ty, TyKind, WhereClause, WherePredicate,
+    ImplItemKind, Item, ItemKind, LangItem, Lifetime, LifetimeName, ParamName, PolyTraitRef, TraitBoundModifier,
+    TraitFn, TraitItem, TraitItemKind, Ty, TyKind, WhereClause, WherePredicate,
 };
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::hir::map::Map;
@@ -300,7 +299,7 @@ fn unique_lifetimes(lts: &[RefLt]) -> usize {
     lts.iter().collect::<FxHashSet<_>>().len()
 }
 
-const CLOSURE_TRAIT_BOUNDS: [&[&str]; 3] = [&paths::FN, &paths::FN_MUT, &paths::FN_ONCE];
+const CLOSURE_TRAIT_BOUNDS: [LangItem; 3] = [LangItem::Fn, LangItem::FnMut, LangItem::FnOnce];
 
 /// A visitor usable for `rustc_front::visit::walk_ty()`.
 struct RefVisitor<'a, 'tcx> {
@@ -359,10 +358,13 @@ impl<'a, 'tcx> Visitor<'tcx> for RefVisitor<'a, 'tcx> {
 
     fn visit_poly_trait_ref(&mut self, poly_tref: &'tcx PolyTraitRef<'tcx>, tbm: TraitBoundModifier) {
         let trait_ref = &poly_tref.trait_ref;
-        if CLOSURE_TRAIT_BOUNDS
-            .iter()
-            .any(|trait_path| trait_ref.trait_def_id() == get_trait_def_id(self.cx, trait_path))
-        {
+        if CLOSURE_TRAIT_BOUNDS.iter().any(|&item| {
+            self.cx
+                .tcx
+                .lang_items()
+                .require(item)
+                .map_or(false, |id| Some(id) == trait_ref.trait_def_id())
+        }) {
             let mut sub_visitor = RefVisitor::new(self.cx);
             sub_visitor.visit_trait_ref(trait_ref);
             self.nested_elision_site_lts.append(&mut sub_visitor.all_lts());

src/tools/clippy/clippy_lints/src/loops/empty_loop.rs

@@ -0,0 +1,17 @@
+use super::EMPTY_LOOP;
+use crate::utils::{is_in_panic_handler, is_no_std_crate, span_lint_and_help};
+
+use rustc_hir::{Block, Expr};
+use rustc_lint::LateContext;
+
+pub(super) fn check(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, loop_block: &'tcx Block<'_>) {
+    if loop_block.stmts.is_empty() && loop_block.expr.is_none() && !is_in_panic_handler(cx, expr) {
+        let msg = "empty `loop {}` wastes CPU cycles";
+        let help = if is_no_std_crate(cx) {
+            "you should either use `panic!()` or add a call pausing or sleeping the thread to the loop body"
+        } else {
+            "you should either use `panic!()` or add `std::thread::sleep(..);` to the loop body"
+        };
+        span_lint_and_help(cx, EMPTY_LOOP, expr.span, msg, None, help);
+    }
+}

src/tools/clippy/clippy_lints/src/loops/explicit_counter_loop.rs

@@ -0,0 +1,58 @@
+use super::{
+    get_span_of_entire_for_loop, make_iterator_snippet, IncrementVisitor, InitializeVisitor, EXPLICIT_COUNTER_LOOP,
+};
+use crate::utils::{get_enclosing_block, is_integer_const, snippet_with_applicability, span_lint_and_sugg};
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir::intravisit::{walk_block, walk_expr};
+use rustc_hir::{Expr, Pat};
+use rustc_lint::LateContext;
+
+// To trigger the EXPLICIT_COUNTER_LOOP lint, a variable must be
+// incremented exactly once in the loop body, and initialized to zero
+// at the start of the loop.
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    pat: &'tcx Pat<'_>,
+    arg: &'tcx Expr<'_>,
+    body: &'tcx Expr<'_>,
+    expr: &'tcx Expr<'_>,
+) {
+    // Look for variables that are incremented once per loop iteration.
+    let mut increment_visitor = IncrementVisitor::new(cx);
+    walk_expr(&mut increment_visitor, body);
+
+    // For each candidate, check the parent block to see if
+    // it's initialized to zero at the start of the loop.
+    if let Some(block) = get_enclosing_block(&cx, expr.hir_id) {
+        for id in increment_visitor.into_results() {
+            let mut initialize_visitor = InitializeVisitor::new(cx, expr, id);
+            walk_block(&mut initialize_visitor, block);
+
+            if_chain! {
+                if let Some((name, initializer)) = initialize_visitor.get_result();
+                if is_integer_const(cx, initializer, 0);
+                then {
+                    let mut applicability = Applicability::MachineApplicable;
+
+                    let for_span = get_span_of_entire_for_loop(expr);
+
+                    span_lint_and_sugg(
+                        cx,
+                        EXPLICIT_COUNTER_LOOP,
+                        for_span.with_hi(arg.span.hi()),
+                        &format!("the variable `{}` is used as a loop counter", name),
+                        "consider using",
+                        format!(
+                            "for ({}, {}) in {}.enumerate()",
+                            name,
+                            snippet_with_applicability(cx, pat.span, "item", &mut applicability),
+                            make_iterator_snippet(cx, arg, &mut applicability),
+                        ),
+                        applicability,
+                    );
+                }
+            }
+        }
+    }
+}

src/tools/clippy/clippy_lints/src/loops/explicit_into_iter_loop.rs

@@ -0,0 +1,27 @@
+use super::EXPLICIT_INTO_ITER_LOOP;
+use crate::utils::{snippet_with_applicability, span_lint_and_sugg};
+use rustc_errors::Applicability;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::TyS;
+
+pub(super) fn check(cx: &LateContext<'_>, args: &'hir [Expr<'hir>], arg: &Expr<'_>) {
+    let receiver_ty = cx.typeck_results().expr_ty(&args[0]);
+    let receiver_ty_adjusted = cx.typeck_results().expr_ty_adjusted(&args[0]);
+    if !TyS::same_type(receiver_ty, receiver_ty_adjusted) {
+        return;
+    }
+
+    let mut applicability = Applicability::MachineApplicable;
+    let object = snippet_with_applicability(cx, args[0].span, "_", &mut applicability);
+    span_lint_and_sugg(
+        cx,
+        EXPLICIT_INTO_ITER_LOOP,
+        arg.span,
+        "it is more concise to loop over containers instead of using explicit \
+            iteration methods",
+        "to write this more concisely, try",
+        object.to_string(),
+        applicability,
+    );
+}

src/tools/clippy/clippy_lints/src/loops/explicit_iter_loop.rs

@@ -0,0 +1,74 @@
+use super::EXPLICIT_ITER_LOOP;
+use crate::utils::{match_trait_method, snippet_with_applicability, span_lint_and_sugg};
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, Mutability};
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, Ty, TyS};
+use rustc_span::sym;
+
+use crate::utils::{is_type_diagnostic_item, match_type, paths};
+
+pub(super) fn check(cx: &LateContext<'_>, args: &[Expr<'_>], arg: &Expr<'_>, method_name: &str) {
+    let should_lint = match method_name {
+        "iter" | "iter_mut" => is_ref_iterable_type(cx, &args[0]),
+        "into_iter" if match_trait_method(cx, arg, &paths::INTO_ITERATOR) => {
+            let receiver_ty = cx.typeck_results().expr_ty(&args[0]);
+            let receiver_ty_adjusted = cx.typeck_results().expr_ty_adjusted(&args[0]);
+            let ref_receiver_ty = cx.tcx.mk_ref(
+                cx.tcx.lifetimes.re_erased,
+                ty::TypeAndMut {
+                    ty: receiver_ty,
+                    mutbl: Mutability::Not,
+                },
+            );
+            TyS::same_type(receiver_ty_adjusted, ref_receiver_ty)
+        },
+        _ => false,
+    };
+
+    if !should_lint {
+        return;
+    }
+
+    let mut applicability = Applicability::MachineApplicable;
+    let object = snippet_with_applicability(cx, args[0].span, "_", &mut applicability);
+    let muta = if method_name == "iter_mut" { "mut " } else { "" };
+    span_lint_and_sugg(
+        cx,
+        EXPLICIT_ITER_LOOP,
+        arg.span,
+        "it is more concise to loop over references to containers instead of using explicit \
+         iteration methods",
+        "to write this more concisely, try",
+        format!("&{}{}", muta, object),
+        applicability,
+    )
+}
+
+/// Returns `true` if the type of expr is one that provides `IntoIterator` impls
+/// for `&T` and `&mut T`, such as `Vec`.
+#[rustfmt::skip]
+fn is_ref_iterable_type(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
+    // no walk_ptrs_ty: calling iter() on a reference can make sense because it
+    // will allow further borrows afterwards
+    let ty = cx.typeck_results().expr_ty(e);
+    is_iterable_array(ty, cx) ||
+    is_type_diagnostic_item(cx, ty, sym::vec_type) ||
+    match_type(cx, ty, &paths::LINKED_LIST) ||
+    is_type_diagnostic_item(cx, ty, sym::hashmap_type) ||
+    is_type_diagnostic_item(cx, ty, sym::hashset_type) ||
+    is_type_diagnostic_item(cx, ty, sym::vecdeque_type) ||
+    match_type(cx, ty, &paths::BINARY_HEAP) ||
+    match_type(cx, ty, &paths::BTREEMAP) ||
+    match_type(cx, ty, &paths::BTREESET)
+}
+
+fn is_iterable_array<'tcx>(ty: Ty<'tcx>, cx: &LateContext<'tcx>) -> bool {
+    // IntoIterator is currently only implemented for array sizes <= 32 in rustc
+    match ty.kind() {
+        ty::Array(_, n) => n
+            .try_eval_usize(cx.tcx, cx.param_env)
+            .map_or(false, |val| (0..=32).contains(&val)),
+        _ => false,
+    }
+}

src/tools/clippy/clippy_lints/src/loops/for_kv_map.rs

@@ -0,0 +1,71 @@
+use super::FOR_KV_MAP;
+use crate::utils::visitors::LocalUsedVisitor;
+use crate::utils::{is_type_diagnostic_item, match_type, multispan_sugg, paths, snippet, span_lint_and_then, sugg};
+use rustc_hir::{BorrowKind, Expr, ExprKind, Mutability, Pat, PatKind};
+use rustc_lint::LateContext;
+use rustc_middle::ty;
+use rustc_span::sym;
+
+/// Checks for the `FOR_KV_MAP` lint.
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    pat: &'tcx Pat<'_>,
+    arg: &'tcx Expr<'_>,
+    body: &'tcx Expr<'_>,
+    expr: &'tcx Expr<'_>,
+) {
+    let pat_span = pat.span;
+
+    if let PatKind::Tuple(ref pat, _) = pat.kind {
+        if pat.len() == 2 {
+            let arg_span = arg.span;
+            let (new_pat_span, kind, ty, mutbl) = match *cx.typeck_results().expr_ty(arg).kind() {
+                ty::Ref(_, ty, mutbl) => match (&pat[0].kind, &pat[1].kind) {
+                    (key, _) if pat_is_wild(cx, key, body) => (pat[1].span, "value", ty, mutbl),
+                    (_, value) if pat_is_wild(cx, value, body) => (pat[0].span, "key", ty, Mutability::Not),
+                    _ => return,
+                },
+                _ => return,
+            };
+            let mutbl = match mutbl {
+                Mutability::Not => "",
+                Mutability::Mut => "_mut",
+            };
+            let arg = match arg.kind {
+                ExprKind::AddrOf(BorrowKind::Ref, _, ref expr) => &**expr,
+                _ => arg,
+            };
+
+            if is_type_diagnostic_item(cx, ty, sym::hashmap_type) || match_type(cx, ty, &paths::BTREEMAP) {
+                span_lint_and_then(
+                    cx,
+                    FOR_KV_MAP,
+                    expr.span,
+                    &format!("you seem to want to iterate on a map's {}s", kind),
+                    |diag| {
+                        let map = sugg::Sugg::hir(cx, arg, "map");
+                        multispan_sugg(
+                            diag,
+                            "use the corresponding method",
+                            vec![
+                                (pat_span, snippet(cx, new_pat_span, kind).into_owned()),
+                                (arg_span, format!("{}.{}s{}()", map.maybe_par(), kind, mutbl)),
+                            ],
+                        );
+                    },
+                );
+            }
+        }
+    }
+}
+
+/// Returns `true` if the pattern is a `PatWild` or an ident prefixed with `_`.
+fn pat_is_wild<'tcx>(cx: &LateContext<'tcx>, pat: &'tcx PatKind<'_>, body: &'tcx Expr<'_>) -> bool {
+    match *pat {
+        PatKind::Wild => true,
+        PatKind::Binding(_, id, ident, None) if ident.as_str().starts_with('_') => {
+            !LocalUsedVisitor::new(cx, id).check_expr(body)
+        },
+        _ => false,
+    }
+}

src/tools/clippy/clippy_lints/src/loops/for_loops_over_fallibles.rs

@@ -0,0 +1,45 @@
+use super::FOR_LOOPS_OVER_FALLIBLES;
+use crate::utils::{is_type_diagnostic_item, snippet, span_lint_and_help};
+use rustc_hir::{Expr, Pat};
+use rustc_lint::LateContext;
+use rustc_span::symbol::sym;
+
+/// Checks for `for` loops over `Option`s and `Result`s.
+pub(super) fn check(cx: &LateContext<'_>, pat: &Pat<'_>, arg: &Expr<'_>) {
+    let ty = cx.typeck_results().expr_ty(arg);
+    if is_type_diagnostic_item(cx, ty, sym::option_type) {
+        span_lint_and_help(
+            cx,
+            FOR_LOOPS_OVER_FALLIBLES,
+            arg.span,
+            &format!(
+                "for loop over `{0}`, which is an `Option`. This is more readably written as an \
+                `if let` statement",
+                snippet(cx, arg.span, "_")
+            ),
+            None,
+            &format!(
+                "consider replacing `for {0} in {1}` with `if let Some({0}) = {1}`",
+                snippet(cx, pat.span, "_"),
+                snippet(cx, arg.span, "_")
+            ),
+        );
+    } else if is_type_diagnostic_item(cx, ty, sym::result_type) {
+        span_lint_and_help(
+            cx,
+            FOR_LOOPS_OVER_FALLIBLES,
+            arg.span,
+            &format!(
+                "for loop over `{0}`, which is a `Result`. This is more readably written as an \
+                `if let` statement",
+                snippet(cx, arg.span, "_")
+            ),
+            None,
+            &format!(
+                "consider replacing `for {0} in {1}` with `if let Ok({0}) = {1}`",
+                snippet(cx, pat.span, "_"),
+                snippet(cx, arg.span, "_")
+            ),
+        );
+    }
+}

src/tools/clippy/clippy_lints/src/loops/iter_next_loop.rs

@@ -0,0 +1,19 @@
+use super::ITER_NEXT_LOOP;
+use crate::utils::{match_trait_method, paths, span_lint};
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+
+pub(super) fn check(cx: &LateContext<'_>, arg: &Expr<'_>, expr: &Expr<'_>) -> bool {
+    if match_trait_method(cx, arg, &paths::ITERATOR) {
+        span_lint(
+            cx,
+            ITER_NEXT_LOOP,
+            expr.span,
+            "you are iterating over `Iterator::next()` which is an Option; this will compile but is \
+            probably not what you want",
+        );
+        true
+    } else {
+        false
+    }
+}

src/tools/clippy/clippy_lints/src/loops/manual_flatten.rs

@@ -0,0 +1,79 @@
+use super::utils::make_iterator_snippet;
+use super::MANUAL_FLATTEN;
+use crate::utils::{is_ok_ctor, is_some_ctor, path_to_local_id, span_lint_and_then};
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind, MatchSource, Pat, PatKind, QPath, StmtKind};
+use rustc_lint::LateContext;
+use rustc_span::source_map::Span;
+
+/// Check for unnecessary `if let` usage in a for loop where only the `Some` or `Ok` variant of the
+/// iterator element is used.
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    pat: &'tcx Pat<'_>,
+    arg: &'tcx Expr<'_>,
+    body: &'tcx Expr<'_>,
+    span: Span,
+) {
+    if let ExprKind::Block(ref block, _) = body.kind {
+        // Ensure the `if let` statement is the only expression or statement in the for-loop
+        let inner_expr = if block.stmts.len() == 1 && block.expr.is_none() {
+            let match_stmt = &block.stmts[0];
+            if let StmtKind::Semi(inner_expr) = match_stmt.kind {
+                Some(inner_expr)
+            } else {
+                None
+            }
+        } else if block.stmts.is_empty() {
+            block.expr
+        } else {
+            None
+        };
+
+        if_chain! {
+            if let Some(inner_expr) = inner_expr;
+            if let ExprKind::Match(
+                ref match_expr, ref match_arms, MatchSource::IfLetDesugar{ contains_else_clause: false }
+            ) = inner_expr.kind;
+            // Ensure match_expr in `if let` statement is the same as the pat from the for-loop
+            if let PatKind::Binding(_, pat_hir_id, _, _) = pat.kind;
+            if path_to_local_id(match_expr, pat_hir_id);
+            // Ensure the `if let` statement is for the `Some` variant of `Option` or the `Ok` variant of `Result`
+            if let PatKind::TupleStruct(QPath::Resolved(None, path), _, _) = match_arms[0].pat.kind;
+            let some_ctor = is_some_ctor(cx, path.res);
+            let ok_ctor = is_ok_ctor(cx, path.res);
+            if some_ctor || ok_ctor;
+            let if_let_type = if some_ctor { "Some" } else { "Ok" };
+
+            then {
+                // Prepare the error message
+                let msg = format!("unnecessary `if let` since only the `{}` variant of the iterator element is used", if_let_type);
+
+                // Prepare the help message
+                let mut applicability = Applicability::MaybeIncorrect;
+                let arg_snippet = make_iterator_snippet(cx, arg, &mut applicability);
+
+                span_lint_and_then(
+                    cx,
+                    MANUAL_FLATTEN,
+                    span,
+                    &msg,
+                    |diag| {
+                        let sugg = format!("{}.flatten()", arg_snippet);
+                        diag.span_suggestion(
+                            arg.span,
+                            "try",
+                            sugg,
+                            Applicability::MaybeIncorrect,
+                        );
+                        diag.span_help(
+                            inner_expr.span,
+                            "...and remove the `if let` statement in the for loop",
+                        );
+                    }
+                );
+            }
+        }
+    }
+}

src/tools/clippy/clippy_lints/src/loops/manual_memcpy.rs

@@ -0,0 +1,451 @@
+use super::{get_span_of_entire_for_loop, IncrementVisitor, InitializeVisitor, MANUAL_MEMCPY};
+use crate::utils::sugg::Sugg;
+use crate::utils::{
+    get_enclosing_block, higher, is_type_diagnostic_item, path_to_local, snippet, span_lint_and_sugg, sugg,
+};
+use if_chain::if_chain;
+use rustc_ast::ast;
+use rustc_errors::Applicability;
+use rustc_hir::intravisit::walk_block;
+use rustc_hir::{BinOpKind, Block, Expr, ExprKind, HirId, Pat, PatKind, StmtKind};
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, Ty};
+use rustc_span::symbol::sym;
+use std::iter::Iterator;
+
+/// Checks for for loops that sequentially copy items from one slice-like
+/// object to another.
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    pat: &'tcx Pat<'_>,
+    arg: &'tcx Expr<'_>,
+    body: &'tcx Expr<'_>,
+    expr: &'tcx Expr<'_>,
+) -> bool {
+    if let Some(higher::Range {
+        start: Some(start),
+        end: Some(end),
+        limits,
+    }) = higher::range(arg)
+    {
+        // the var must be a single name
+        if let PatKind::Binding(_, canonical_id, _, _) = pat.kind {
+            let mut starts = vec![Start {
+                id: canonical_id,
+                kind: StartKind::Range,
+            }];
+
+            // This is one of few ways to return different iterators
+            // derived from: https://stackoverflow.com/questions/29760668/conditionally-iterate-over-one-of-several-possible-iterators/52064434#52064434
+            let mut iter_a = None;
+            let mut iter_b = None;
+
+            if let ExprKind::Block(block, _) = body.kind {
+                if let Some(loop_counters) = get_loop_counters(cx, block, expr) {
+                    starts.extend(loop_counters);
+                }
+                iter_a = Some(get_assignments(block, &starts));
+            } else {
+                iter_b = Some(get_assignment(body));
+            }
+
+            let assignments = iter_a.into_iter().flatten().chain(iter_b.into_iter());
+
+            let big_sugg = assignments
+                // The only statements in the for loops can be indexed assignments from
+                // indexed retrievals (except increments of loop counters).
+                .map(|o| {
+                    o.and_then(|(lhs, rhs)| {
+                        let rhs = fetch_cloned_expr(rhs);
+                        if_chain! {
+                            if let ExprKind::Index(base_left, idx_left) = lhs.kind;
+                            if let ExprKind::Index(base_right, idx_right) = rhs.kind;
+                            if is_slice_like(cx, cx.typeck_results().expr_ty(base_left))
+                                && is_slice_like(cx, cx.typeck_results().expr_ty(base_right));
+                            if let Some((start_left, offset_left)) = get_details_from_idx(cx, &idx_left, &starts);
+                            if let Some((start_right, offset_right)) = get_details_from_idx(cx, &idx_right, &starts);
+
+                            // Source and destination must be different
+                            if path_to_local(base_left) != path_to_local(base_right);
+                            then {
+                                Some((IndexExpr { base: base_left, idx: start_left, idx_offset: offset_left },
+                                    IndexExpr { base: base_right, idx: start_right, idx_offset: offset_right }))
+                            } else {
+                                None
+                            }
+                        }
+                    })
+                })
+                .map(|o| o.map(|(dst, src)| build_manual_memcpy_suggestion(cx, start, end, limits, &dst, &src)))
+                .collect::<Option<Vec<_>>>()
+                .filter(|v| !v.is_empty())
+                .map(|v| v.join("\n    "));
+
+            if let Some(big_sugg) = big_sugg {
+                span_lint_and_sugg(
+                    cx,
+                    MANUAL_MEMCPY,
+                    get_span_of_entire_for_loop(expr),
+                    "it looks like you're manually copying between slices",
+                    "try replacing the loop by",
+                    big_sugg,
+                    Applicability::Unspecified,
+                );
+                return true;
+            }
+        }
+    }
+    false
+}
+
+fn build_manual_memcpy_suggestion<'tcx>(
+    cx: &LateContext<'tcx>,
+    start: &Expr<'_>,
+    end: &Expr<'_>,
+    limits: ast::RangeLimits,
+    dst: &IndexExpr<'_>,
+    src: &IndexExpr<'_>,
+) -> String {
+    fn print_offset(offset: MinifyingSugg<'static>) -> MinifyingSugg<'static> {
+        if offset.as_str() == "0" {
+            sugg::EMPTY.into()
+        } else {
+            offset
+        }
+    }
+
+    let print_limit = |end: &Expr<'_>, end_str: &str, base: &Expr<'_>, sugg: MinifyingSugg<'static>| {
+        if_chain! {
+            if let ExprKind::MethodCall(method, _, len_args, _) = end.kind;
+            if method.ident.name == sym!(len);
+            if len_args.len() == 1;
+            if let Some(arg) = len_args.get(0);
+            if path_to_local(arg) == path_to_local(base);
+            then {
+                if sugg.as_str() == end_str {
+                    sugg::EMPTY.into()
+                } else {
+                    sugg
+                }
+            } else {
+                match limits {
+                    ast::RangeLimits::Closed => {
+                        sugg + &sugg::ONE.into()
+                    },
+                    ast::RangeLimits::HalfOpen => sugg,
+                }
+            }
+        }
+    };
+
+    let start_str = Sugg::hir(cx, start, "").into();
+    let end_str: MinifyingSugg<'_> = Sugg::hir(cx, end, "").into();
+
+    let print_offset_and_limit = |idx_expr: &IndexExpr<'_>| match idx_expr.idx {
+        StartKind::Range => (
+            print_offset(apply_offset(&start_str, &idx_expr.idx_offset)).into_sugg(),
+            print_limit(
+                end,
+                end_str.as_str(),
+                idx_expr.base,
+                apply_offset(&end_str, &idx_expr.idx_offset),
+            )
+            .into_sugg(),
+        ),
+        StartKind::Counter { initializer } => {
+            let counter_start = Sugg::hir(cx, initializer, "").into();
+            (
+                print_offset(apply_offset(&counter_start, &idx_expr.idx_offset)).into_sugg(),
+                print_limit(
+                    end,
+                    end_str.as_str(),
+                    idx_expr.base,
+                    apply_offset(&end_str, &idx_expr.idx_offset) + &counter_start - &start_str,
+                )
+                .into_sugg(),
+            )
+        },
+    };
+
+    let (dst_offset, dst_limit) = print_offset_and_limit(&dst);
+    let (src_offset, src_limit) = print_offset_and_limit(&src);
+
+    let dst_base_str = snippet(cx, dst.base.span, "???");
+    let src_base_str = snippet(cx, src.base.span, "???");
+
+    let dst = if dst_offset == sugg::EMPTY && dst_limit == sugg::EMPTY {
+        dst_base_str
+    } else {
+        format!(
+            "{}[{}..{}]",
+            dst_base_str,
+            dst_offset.maybe_par(),
+            dst_limit.maybe_par()
+        )
+        .into()
+    };
+
+    format!(
+        "{}.clone_from_slice(&{}[{}..{}]);",
+        dst,
+        src_base_str,
+        src_offset.maybe_par(),
+        src_limit.maybe_par()
+    )
+}
+
+/// a wrapper of `Sugg`. Besides what `Sugg` do, this removes unnecessary `0`;
+/// and also, it avoids subtracting a variable from the same one by replacing it with `0`.
+/// it exists for the convenience of the overloaded operators while normal functions can do the
+/// same.
+#[derive(Clone)]
+struct MinifyingSugg<'a>(Sugg<'a>);
+
+impl<'a> MinifyingSugg<'a> {
+    fn as_str(&self) -> &str {
+        let (Sugg::NonParen(s) | Sugg::MaybeParen(s) | Sugg::BinOp(_, s)) = &self.0;
+        s.as_ref()
+    }
+
+    fn into_sugg(self) -> Sugg<'a> {
+        self.0
+    }
+}
+
+impl<'a> From<Sugg<'a>> for MinifyingSugg<'a> {
+    fn from(sugg: Sugg<'a>) -> Self {
+        Self(sugg)
+    }
+}
+
+impl std::ops::Add for &MinifyingSugg<'static> {
+    type Output = MinifyingSugg<'static>;
+    fn add(self, rhs: &MinifyingSugg<'static>) -> MinifyingSugg<'static> {
+        match (self.as_str(), rhs.as_str()) {
+            ("0", _) => rhs.clone(),
+            (_, "0") => self.clone(),
+            (_, _) => (&self.0 + &rhs.0).into(),
+        }
+    }
+}
+
+impl std::ops::Sub for &MinifyingSugg<'static> {
+    type Output = MinifyingSugg<'static>;
+    fn sub(self, rhs: &MinifyingSugg<'static>) -> MinifyingSugg<'static> {
+        match (self.as_str(), rhs.as_str()) {
+            (_, "0") => self.clone(),
+            ("0", _) => (-rhs.0.clone()).into(),
+            (x, y) if x == y => sugg::ZERO.into(),
+            (_, _) => (&self.0 - &rhs.0).into(),
+        }
+    }
+}
+
+impl std::ops::Add<&MinifyingSugg<'static>> for MinifyingSugg<'static> {
+    type Output = MinifyingSugg<'static>;
+    fn add(self, rhs: &MinifyingSugg<'static>) -> MinifyingSugg<'static> {
+        match (self.as_str(), rhs.as_str()) {
+            ("0", _) => rhs.clone(),
+            (_, "0") => self,
+            (_, _) => (self.0 + &rhs.0).into(),
+        }
+    }
+}
+
+impl std::ops::Sub<&MinifyingSugg<'static>> for MinifyingSugg<'static> {
+    type Output = MinifyingSugg<'static>;
+    fn sub(self, rhs: &MinifyingSugg<'static>) -> MinifyingSugg<'static> {
+        match (self.as_str(), rhs.as_str()) {
+            (_, "0") => self,
+            ("0", _) => (-rhs.0.clone()).into(),
+            (x, y) if x == y => sugg::ZERO.into(),
+            (_, _) => (self.0 - &rhs.0).into(),
+        }
+    }
+}
+
+/// a wrapper around `MinifyingSugg`, which carries a operator like currying
+/// so that the suggested code become more efficient (e.g. `foo + -bar` `foo - bar`).
+struct Offset {
+    value: MinifyingSugg<'static>,
+    sign: OffsetSign,
+}
+
+#[derive(Clone, Copy)]
+enum OffsetSign {
+    Positive,
+    Negative,
+}
+
+impl Offset {
+    fn negative(value: Sugg<'static>) -> Self {
+        Self {
+            value: value.into(),
+            sign: OffsetSign::Negative,
+        }
+    }
+
+    fn positive(value: Sugg<'static>) -> Self {
+        Self {
+            value: value.into(),
+            sign: OffsetSign::Positive,
+        }
+    }
+
+    fn empty() -> Self {
+        Self::positive(sugg::ZERO)
+    }
+}
+
+fn apply_offset(lhs: &MinifyingSugg<'static>, rhs: &Offset) -> MinifyingSugg<'static> {
+    match rhs.sign {
+        OffsetSign::Positive => lhs + &rhs.value,
+        OffsetSign::Negative => lhs - &rhs.value,
+    }
+}
+
+#[derive(Debug, Clone, Copy)]
+enum StartKind<'hir> {
+    Range,
+    Counter { initializer: &'hir Expr<'hir> },
+}
+
+struct IndexExpr<'hir> {
+    base: &'hir Expr<'hir>,
+    idx: StartKind<'hir>,
+    idx_offset: Offset,
+}
+
+struct Start<'hir> {
+    id: HirId,
+    kind: StartKind<'hir>,
+}
+
+fn is_slice_like<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'_>) -> bool {
+    let is_slice = match ty.kind() {
+        ty::Ref(_, subty, _) => is_slice_like(cx, subty),
+        ty::Slice(..) | ty::Array(..) => true,
+        _ => false,
+    };
+
+    is_slice || is_type_diagnostic_item(cx, ty, sym::vec_type) || is_type_diagnostic_item(cx, ty, sym::vecdeque_type)
+}
+
+fn fetch_cloned_expr<'tcx>(expr: &'tcx Expr<'tcx>) -> &'tcx Expr<'tcx> {
+    if_chain! {
+        if let ExprKind::MethodCall(method, _, args, _) = expr.kind;
+        if method.ident.name == sym::clone;
+        if args.len() == 1;
+        if let Some(arg) = args.get(0);
+        then { arg } else { expr }
+    }
+}
+
+fn get_details_from_idx<'tcx>(
+    cx: &LateContext<'tcx>,
+    idx: &Expr<'_>,
+    starts: &[Start<'tcx>],
+) -> Option<(StartKind<'tcx>, Offset)> {
+    fn get_start<'tcx>(e: &Expr<'_>, starts: &[Start<'tcx>]) -> Option<StartKind<'tcx>> {
+        let id = path_to_local(e)?;
+        starts.iter().find(|start| start.id == id).map(|start| start.kind)
+    }
+
+    fn get_offset<'tcx>(cx: &LateContext<'tcx>, e: &Expr<'_>, starts: &[Start<'tcx>]) -> Option<Sugg<'static>> {
+        match &e.kind {
+            ExprKind::Lit(l) => match l.node {
+                ast::LitKind::Int(x, _ty) => Some(Sugg::NonParen(x.to_string().into())),
+                _ => None,
+            },
+            ExprKind::Path(..) if get_start(e, starts).is_none() => Some(Sugg::hir(cx, e, "???")),
+            _ => None,
+        }
+    }
+
+    match idx.kind {
+        ExprKind::Binary(op, lhs, rhs) => match op.node {
+            BinOpKind::Add => {
+                let offset_opt = get_start(lhs, starts)
+                    .and_then(|s| get_offset(cx, rhs, starts).map(|o| (s, o)))
+                    .or_else(|| get_start(rhs, starts).and_then(|s| get_offset(cx, lhs, starts).map(|o| (s, o))));
+
+                offset_opt.map(|(s, o)| (s, Offset::positive(o)))
+            },
+            BinOpKind::Sub => {
+                get_start(lhs, starts).and_then(|s| get_offset(cx, rhs, starts).map(|o| (s, Offset::negative(o))))
+            },
+            _ => None,
+        },
+        ExprKind::Path(..) => get_start(idx, starts).map(|s| (s, Offset::empty())),
+        _ => None,
+    }
+}
+
+fn get_assignment<'tcx>(e: &'tcx Expr<'tcx>) -> Option<(&'tcx Expr<'tcx>, &'tcx Expr<'tcx>)> {
+    if let ExprKind::Assign(lhs, rhs, _) = e.kind {
+        Some((lhs, rhs))
+    } else {
+        None
+    }
+}
+
+/// Get assignments from the given block.
+/// The returned iterator yields `None` if no assignment expressions are there,
+/// filtering out the increments of the given whitelisted loop counters;
+/// because its job is to make sure there's nothing other than assignments and the increments.
+fn get_assignments<'a, 'tcx>(
+    Block { stmts, expr, .. }: &'tcx Block<'tcx>,
+    loop_counters: &'a [Start<'tcx>],
+) -> impl Iterator<Item = Option<(&'tcx Expr<'tcx>, &'tcx Expr<'tcx>)>> + 'a {
+    // As the `filter` and `map` below do different things, I think putting together
+    // just increases complexity. (cc #3188 and #4193)
+    stmts
+        .iter()
+        .filter_map(move |stmt| match stmt.kind {
+            StmtKind::Local(..) | StmtKind::Item(..) => None,
+            StmtKind::Expr(e) | StmtKind::Semi(e) => Some(e),
+        })
+        .chain((*expr).into_iter())
+        .filter(move |e| {
+            if let ExprKind::AssignOp(_, place, _) = e.kind {
+                path_to_local(place).map_or(false, |id| {
+                    !loop_counters
+                        .iter()
+                        // skip the first item which should be `StartKind::Range`
+                        // this makes it possible to use the slice with `StartKind::Range` in the same iterator loop.
+                        .skip(1)
+                        .any(|counter| counter.id == id)
+                })
+            } else {
+                true
+            }
+        })
+        .map(get_assignment)
+}
+
+fn get_loop_counters<'a, 'tcx>(
+    cx: &'a LateContext<'tcx>,
+    body: &'tcx Block<'tcx>,
+    expr: &'tcx Expr<'_>,
+) -> Option<impl Iterator<Item = Start<'tcx>> + 'a> {
+    // Look for variables that are incremented once per loop iteration.
+    let mut increment_visitor = IncrementVisitor::new(cx);
+    walk_block(&mut increment_visitor, body);
+
+    // For each candidate, check the parent block to see if
+    // it's initialized to zero at the start of the loop.
+    get_enclosing_block(&cx, expr.hir_id).and_then(|block| {
+        increment_visitor
+            .into_results()
+            .filter_map(move |var_id| {
+                let mut initialize_visitor = InitializeVisitor::new(cx, expr, var_id);
+                walk_block(&mut initialize_visitor, block);
+
+                initialize_visitor.get_result().map(|(_, initializer)| Start {
+                    id: var_id,
+                    kind: StartKind::Counter { initializer },
+                })
+            })
+            .into()
+    })
+}

src/tools/clippy/clippy_lints/src/loops/mod.rs

@@ -0,0 +1,627 @@
+mod empty_loop;
+mod explicit_counter_loop;
+mod explicit_into_iter_loop;
+mod explicit_iter_loop;
+mod for_kv_map;
+mod for_loops_over_fallibles;
+mod iter_next_loop;
+mod manual_flatten;
+mod manual_memcpy;
+mod mut_range_bound;
+mod needless_collect;
+mod needless_range_loop;
+mod never_loop;
+mod same_item_push;
+mod single_element_loop;
+mod utils;
+mod while_immutable_condition;
+mod while_let_loop;
+mod while_let_on_iterator;
+
+use crate::utils::higher;
+use rustc_hir::{Expr, ExprKind, LoopSource, Pat};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::source_map::Span;
+use utils::{get_span_of_entire_for_loop, make_iterator_snippet, IncrementVisitor, InitializeVisitor};
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for for-loops that manually copy items between
+    /// slices that could be optimized by having a memcpy.
+    ///
+    /// **Why is this bad?** It is not as fast as a memcpy.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// # let src = vec![1];
+    /// # let mut dst = vec![0; 65];
+    /// for i in 0..src.len() {
+    ///     dst[i + 64] = src[i];
+    /// }
+    /// ```
+    /// Could be written as:
+    /// ```rust
+    /// # let src = vec![1];
+    /// # let mut dst = vec![0; 65];
+    /// dst[64..(src.len() + 64)].clone_from_slice(&src[..]);
+    /// ```
+    pub MANUAL_MEMCPY,
+    perf,
+    "manually copying items between slices"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for looping over the range of `0..len` of some
+    /// collection just to get the values by index.
+    ///
+    /// **Why is this bad?** Just iterating the collection itself makes the intent
+    /// more clear and is probably faster.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// let vec = vec!['a', 'b', 'c'];
+    /// for i in 0..vec.len() {
+    ///     println!("{}", vec[i]);
+    /// }
+    /// ```
+    /// Could be written as:
+    /// ```rust
+    /// let vec = vec!['a', 'b', 'c'];
+    /// for i in vec {
+    ///     println!("{}", i);
+    /// }
+    /// ```
+    pub NEEDLESS_RANGE_LOOP,
+    style,
+    "for-looping over a range of indices where an iterator over items would do"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for loops on `x.iter()` where `&x` will do, and
+    /// suggests the latter.
+    ///
+    /// **Why is this bad?** Readability.
+    ///
+    /// **Known problems:** False negatives. We currently only warn on some known
+    /// types.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// // with `y` a `Vec` or slice:
+    /// # let y = vec![1];
+    /// for x in y.iter() {
+    ///     // ..
+    /// }
+    /// ```
+    /// can be rewritten to
+    /// ```rust
+    /// # let y = vec![1];
+    /// for x in &y {
+    ///     // ..
+    /// }
+    /// ```
+    pub EXPLICIT_ITER_LOOP,
+    pedantic,
+    "for-looping over `_.iter()` or `_.iter_mut()` when `&_` or `&mut _` would do"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for loops on `y.into_iter()` where `y` will do, and
+    /// suggests the latter.
+    ///
+    /// **Why is this bad?** Readability.
+    ///
+    /// **Known problems:** None
+    ///
+    /// **Example:**
+    /// ```rust
+    /// # let y = vec![1];
+    /// // with `y` a `Vec` or slice:
+    /// for x in y.into_iter() {
+    ///     // ..
+    /// }
+    /// ```
+    /// can be rewritten to
+    /// ```rust
+    /// # let y = vec![1];
+    /// for x in y {
+    ///     // ..
+    /// }
+    /// ```
+    pub EXPLICIT_INTO_ITER_LOOP,
+    pedantic,
+    "for-looping over `_.into_iter()` when `_` would do"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for loops on `x.next()`.
+    ///
+    /// **Why is this bad?** `next()` returns either `Some(value)` if there was a
+    /// value, or `None` otherwise. The insidious thing is that `Option<_>`
+    /// implements `IntoIterator`, so that possibly one value will be iterated,
+    /// leading to some hard to find bugs. No one will want to write such code
+    /// [except to win an Underhanded Rust
+    /// Contest](https://www.reddit.com/r/rust/comments/3hb0wm/underhanded_rust_contest/cu5yuhr).
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```ignore
+    /// for x in y.next() {
+    ///     ..
+    /// }
+    /// ```
+    pub ITER_NEXT_LOOP,
+    correctness,
+    "for-looping over `_.next()` which is probably not intended"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for `for` loops over `Option` or `Result` values.
+    ///
+    /// **Why is this bad?** Readability. This is more clearly expressed as an `if
+    /// let`.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// # let opt = Some(1);
+    ///
+    /// // Bad
+    /// for x in opt {
+    ///     // ..
+    /// }
+    ///
+    /// // Good
+    /// if let Some(x) = opt {
+    ///     // ..
+    /// }
+    /// ```
+    ///
+    /// // or
+    ///
+    /// ```rust
+    /// # let res: Result<i32, std::io::Error> = Ok(1);
+    ///
+    /// // Bad
+    /// for x in &res {
+    ///     // ..
+    /// }
+    ///
+    /// // Good
+    /// if let Ok(x) = res {
+    ///     // ..
+    /// }
+    /// ```
+    pub FOR_LOOPS_OVER_FALLIBLES,
+    correctness,
+    "for-looping over an `Option` or a `Result`, which is more clearly expressed as an `if let`"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Detects `loop + match` combinations that are easier
+    /// written as a `while let` loop.
+    ///
+    /// **Why is this bad?** The `while let` loop is usually shorter and more
+    /// readable.
+    ///
+    /// **Known problems:** Sometimes the wrong binding is displayed ([#383](https://github.com/rust-lang/rust-clippy/issues/383)).
+    ///
+    /// **Example:**
+    /// ```rust,no_run
+    /// # let y = Some(1);
+    /// loop {
+    ///     let x = match y {
+    ///         Some(x) => x,
+    ///         None => break,
+    ///     };
+    ///     // .. do something with x
+    /// }
+    /// // is easier written as
+    /// while let Some(x) = y {
+    ///     // .. do something with x
+    /// };
+    /// ```
+    pub WHILE_LET_LOOP,
+    complexity,
+    "`loop { if let { ... } else break }`, which can be written as a `while let` loop"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for functions collecting an iterator when collect
+    /// is not needed.
+    ///
+    /// **Why is this bad?** `collect` causes the allocation of a new data structure,
+    /// when this allocation may not be needed.
+    ///
+    /// **Known problems:**
+    /// None
+    ///
+    /// **Example:**
+    /// ```rust
+    /// # let iterator = vec![1].into_iter();
+    /// let len = iterator.clone().collect::<Vec<_>>().len();
+    /// // should be
+    /// let len = iterator.count();
+    /// ```
+    pub NEEDLESS_COLLECT,
+    perf,
+    "collecting an iterator when collect is not needed"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks `for` loops over slices with an explicit counter
+    /// and suggests the use of `.enumerate()`.
+    ///
+    /// **Why is it bad?** Using `.enumerate()` makes the intent more clear,
+    /// declutters the code and may be faster in some instances.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// # let v = vec![1];
+    /// # fn bar(bar: usize, baz: usize) {}
+    /// let mut i = 0;
+    /// for item in &v {
+    ///     bar(i, *item);
+    ///     i += 1;
+    /// }
+    /// ```
+    /// Could be written as
+    /// ```rust
+    /// # let v = vec![1];
+    /// # fn bar(bar: usize, baz: usize) {}
+    /// for (i, item) in v.iter().enumerate() { bar(i, *item); }
+    /// ```
+    pub EXPLICIT_COUNTER_LOOP,
+    complexity,
+    "for-looping with an explicit counter when `_.enumerate()` would do"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for empty `loop` expressions.
+    ///
+    /// **Why is this bad?** These busy loops burn CPU cycles without doing
+    /// anything. It is _almost always_ a better idea to `panic!` than to have
+    /// a busy loop.
+    ///
+    /// If panicking isn't possible, think of the environment and either:
+    ///   - block on something
+    ///   - sleep the thread for some microseconds
+    ///   - yield or pause the thread
+    ///
+    /// For `std` targets, this can be done with
+    /// [`std::thread::sleep`](https://doc.rust-lang.org/std/thread/fn.sleep.html)
+    /// or [`std::thread::yield_now`](https://doc.rust-lang.org/std/thread/fn.yield_now.html).
+    ///
+    /// For `no_std` targets, doing this is more complicated, especially because
+    /// `#[panic_handler]`s can't panic. To stop/pause the thread, you will
+    /// probably need to invoke some target-specific intrinsic. Examples include:
+    ///   - [`x86_64::instructions::hlt`](https://docs.rs/x86_64/0.12.2/x86_64/instructions/fn.hlt.html)
+    ///   - [`cortex_m::asm::wfi`](https://docs.rs/cortex-m/0.6.3/cortex_m/asm/fn.wfi.html)
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```no_run
+    /// loop {}
+    /// ```
+    pub EMPTY_LOOP,
+    style,
+    "empty `loop {}`, which should block or sleep"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for `while let` expressions on iterators.
+    ///
+    /// **Why is this bad?** Readability. A simple `for` loop is shorter and conveys
+    /// the intent better.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```ignore
+    /// while let Some(val) = iter() {
+    ///     ..
+    /// }
+    /// ```
+    pub WHILE_LET_ON_ITERATOR,
+    style,
+    "using a `while let` loop instead of a for loop on an iterator"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for iterating a map (`HashMap` or `BTreeMap`) and
+    /// ignoring either the keys or values.
+    ///
+    /// **Why is this bad?** Readability. There are `keys` and `values` methods that
+    /// can be used to express that don't need the values or keys.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```ignore
+    /// for (k, _) in &map {
+    ///     ..
+    /// }
+    /// ```
+    ///
+    /// could be replaced by
+    ///
+    /// ```ignore
+    /// for k in map.keys() {
+    ///     ..
+    /// }
+    /// ```
+    pub FOR_KV_MAP,
+    style,
+    "looping on a map using `iter` when `keys` or `values` would do"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for loops that will always `break`, `return` or
+    /// `continue` an outer loop.
+    ///
+    /// **Why is this bad?** This loop never loops, all it does is obfuscating the
+    /// code.
+    ///
+    /// **Known problems:** None
+    ///
+    /// **Example:**
+    /// ```rust
+    /// loop {
+    ///     ..;
+    ///     break;
+    /// }
+    /// ```
+    pub NEVER_LOOP,
+    correctness,
+    "any loop that will always `break` or `return`"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for loops which have a range bound that is a mutable variable
+    ///
+    /// **Why is this bad?** One might think that modifying the mutable variable changes the loop bounds
+    ///
+    /// **Known problems:** None
+    ///
+    /// **Example:**
+    /// ```rust
+    /// let mut foo = 42;
+    /// for i in 0..foo {
+    ///     foo -= 1;
+    ///     println!("{}", i); // prints numbers from 0 to 42, not 0 to 21
+    /// }
+    /// ```
+    pub MUT_RANGE_BOUND,
+    complexity,
+    "for loop over a range where one of the bounds is a mutable variable"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks whether variables used within while loop condition
+    /// can be (and are) mutated in the body.
+    ///
+    /// **Why is this bad?** If the condition is unchanged, entering the body of the loop
+    /// will lead to an infinite loop.
+    ///
+    /// **Known problems:** If the `while`-loop is in a closure, the check for mutation of the
+    /// condition variables in the body can cause false negatives. For example when only `Upvar` `a` is
+    /// in the condition and only `Upvar` `b` gets mutated in the body, the lint will not trigger.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// let i = 0;
+    /// while i > 10 {
+    ///     println!("let me loop forever!");
+    /// }
+    /// ```
+    pub WHILE_IMMUTABLE_CONDITION,
+    correctness,
+    "variables used within while expression are not mutated in the body"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks whether a for loop is being used to push a constant
+    /// value into a Vec.
+    ///
+    /// **Why is this bad?** This kind of operation can be expressed more succinctly with
+    /// `vec![item;SIZE]` or `vec.resize(NEW_SIZE, item)` and using these alternatives may also
+    /// have better performance.
+    /// **Known problems:** None
+    ///
+    /// **Example:**
+    /// ```rust
+    /// let item1 = 2;
+    /// let item2 = 3;
+    /// let mut vec: Vec<u8> = Vec::new();
+    /// for _ in 0..20 {
+    ///    vec.push(item1);
+    /// }
+    /// for _ in 0..30 {
+    ///     vec.push(item2);
+    /// }
+    /// ```
+    /// could be written as
+    /// ```rust
+    /// let item1 = 2;
+    /// let item2 = 3;
+    /// let mut vec: Vec<u8> = vec![item1; 20];
+    /// vec.resize(20 + 30, item2);
+    /// ```
+    pub SAME_ITEM_PUSH,
+    style,
+    "the same item is pushed inside of a for loop"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks whether a for loop has a single element.
+    ///
+    /// **Why is this bad?** There is no reason to have a loop of a
+    /// single element.
+    /// **Known problems:** None
+    ///
+    /// **Example:**
+    /// ```rust
+    /// let item1 = 2;
+    /// for item in &[item1] {
+    ///     println!("{}", item);
+    /// }
+    /// ```
+    /// could be written as
+    /// ```rust
+    /// let item1 = 2;
+    /// let item = &item1;
+    /// println!("{}", item);
+    /// ```
+    pub SINGLE_ELEMENT_LOOP,
+    complexity,
+    "there is no reason to have a single element loop"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Check for unnecessary `if let` usage in a for loop
+    /// where only the `Some` or `Ok` variant of the iterator element is used.
+    ///
+    /// **Why is this bad?** It is verbose and can be simplified
+    /// by first calling the `flatten` method on the `Iterator`.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    ///
+    /// ```rust
+    /// let x = vec![Some(1), Some(2), Some(3)];
+    /// for n in x {
+    ///     if let Some(n) = n {
+    ///         println!("{}", n);
+    ///     }
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// let x = vec![Some(1), Some(2), Some(3)];
+    /// for n in x.into_iter().flatten() {
+    ///     println!("{}", n);
+    /// }
+    /// ```
+    pub MANUAL_FLATTEN,
+    complexity,
+    "for loops over `Option`s or `Result`s with a single expression can be simplified"
+}
+
+declare_lint_pass!(Loops => [
+    MANUAL_MEMCPY,
+    MANUAL_FLATTEN,
+    NEEDLESS_RANGE_LOOP,
+    EXPLICIT_ITER_LOOP,
+    EXPLICIT_INTO_ITER_LOOP,
+    ITER_NEXT_LOOP,
+    FOR_LOOPS_OVER_FALLIBLES,
+    WHILE_LET_LOOP,
+    NEEDLESS_COLLECT,
+    EXPLICIT_COUNTER_LOOP,
+    EMPTY_LOOP,
+    WHILE_LET_ON_ITERATOR,
+    FOR_KV_MAP,
+    NEVER_LOOP,
+    MUT_RANGE_BOUND,
+    WHILE_IMMUTABLE_CONDITION,
+    SAME_ITEM_PUSH,
+    SINGLE_ELEMENT_LOOP,
+]);
+
+impl<'tcx> LateLintPass<'tcx> for Loops {
+    #[allow(clippy::too_many_lines)]
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+        if let Some((pat, arg, body, span)) = higher::for_loop(expr) {
+            // we don't want to check expanded macros
+            // this check is not at the top of the function
+            // since higher::for_loop expressions are marked as expansions
+            if body.span.from_expansion() {
+                return;
+            }
+            check_for_loop(cx, pat, arg, body, expr, span);
+        }
+
+        // we don't want to check expanded macros
+        if expr.span.from_expansion() {
+            return;
+        }
+
+        // check for never_loop
+        never_loop::check(cx, expr);
+
+        // check for `loop { if let {} else break }` that could be `while let`
+        // (also matches an explicit "match" instead of "if let")
+        // (even if the "match" or "if let" is used for declaration)
+        if let ExprKind::Loop(ref block, _, LoopSource::Loop, _) = expr.kind {
+            // also check for empty `loop {}` statements, skipping those in #[panic_handler]
+            empty_loop::check(cx, expr, block);
+            while_let_loop::check(cx, expr, block);
+        }
+
+        while_let_on_iterator::check(cx, expr);
+
+        if let Some((cond, body)) = higher::while_loop(&expr) {
+            while_immutable_condition::check(cx, cond, body);
+        }
+
+        needless_collect::check(expr, cx);
+    }
+}
+
+fn check_for_loop<'tcx>(
+    cx: &LateContext<'tcx>,
+    pat: &'tcx Pat<'_>,
+    arg: &'tcx Expr<'_>,
+    body: &'tcx Expr<'_>,
+    expr: &'tcx Expr<'_>,
+    span: Span,
+) {
+    let is_manual_memcpy_triggered = manual_memcpy::check(cx, pat, arg, body, expr);
+    if !is_manual_memcpy_triggered {
+        needless_range_loop::check(cx, pat, arg, body, expr);
+        explicit_counter_loop::check(cx, pat, arg, body, expr);
+    }
+    check_for_loop_arg(cx, pat, arg, expr);
+    for_kv_map::check(cx, pat, arg, body, expr);
+    mut_range_bound::check(cx, arg, body);
+    single_element_loop::check(cx, pat, arg, body, expr);
+    same_item_push::check(cx, pat, arg, body, expr);
+    manual_flatten::check(cx, pat, arg, body, span);
+}
+
+fn check_for_loop_arg(cx: &LateContext<'_>, pat: &Pat<'_>, arg: &Expr<'_>, expr: &Expr<'_>) {
+    let mut next_loop_linted = false; // whether or not ITER_NEXT_LOOP lint was used
+
+    if let ExprKind::MethodCall(ref method, _, ref args, _) = arg.kind {
+        // just the receiver, no arguments
+        if args.len() == 1 {
+            let method_name = &*method.ident.as_str();
+            // check for looping over x.iter() or x.iter_mut(), could use &x or &mut x
+            match method_name {
+                "iter" | "iter_mut" => explicit_iter_loop::check(cx, args, arg, method_name),
+                "into_iter" => {
+                    explicit_iter_loop::check(cx, args, arg, method_name);
+                    explicit_into_iter_loop::check(cx, args, arg);
+                },
+                "next" => {
+                    next_loop_linted = iter_next_loop::check(cx, arg, expr);
+                },
+                _ => {},
+            }
+        }
+    }
+
+    if !next_loop_linted {
+        for_loops_over_fallibles::check(cx, pat, arg);
+    }
+}

src/tools/clippy/clippy_lints/src/loops/mut_range_bound.rs

@@ -0,0 +1,115 @@
+use super::MUT_RANGE_BOUND;
+use crate::utils::{higher, path_to_local, span_lint};
+use if_chain::if_chain;
+use rustc_hir::{BindingAnnotation, Expr, HirId, Node, PatKind};
+use rustc_infer::infer::TyCtxtInferExt;
+use rustc_lint::LateContext;
+use rustc_middle::ty;
+use rustc_span::source_map::Span;
+use rustc_typeck::expr_use_visitor::{ConsumeMode, Delegate, ExprUseVisitor, PlaceBase, PlaceWithHirId};
+
+pub(super) fn check(cx: &LateContext<'_>, arg: &Expr<'_>, body: &Expr<'_>) {
+    if let Some(higher::Range {
+        start: Some(start),
+        end: Some(end),
+        ..
+    }) = higher::range(arg)
+    {
+        let mut_ids = vec![check_for_mutability(cx, start), check_for_mutability(cx, end)];
+        if mut_ids[0].is_some() || mut_ids[1].is_some() {
+            let (span_low, span_high) = check_for_mutation(cx, body, &mut_ids);
+            mut_warn_with_span(cx, span_low);
+            mut_warn_with_span(cx, span_high);
+        }
+    }
+}
+
+fn mut_warn_with_span(cx: &LateContext<'_>, span: Option<Span>) {
+    if let Some(sp) = span {
+        span_lint(
+            cx,
+            MUT_RANGE_BOUND,
+            sp,
+            "attempt to mutate range bound within loop; note that the range of the loop is unchanged",
+        );
+    }
+}
+
+fn check_for_mutability(cx: &LateContext<'_>, bound: &Expr<'_>) -> Option<HirId> {
+    if_chain! {
+        if let Some(hir_id) = path_to_local(bound);
+        if let Node::Binding(pat) = cx.tcx.hir().get(hir_id);
+        if let PatKind::Binding(BindingAnnotation::Mutable, ..) = pat.kind;
+        then {
+            return Some(hir_id);
+        }
+    }
+    None
+}
+
+fn check_for_mutation<'tcx>(
+    cx: &LateContext<'tcx>,
+    body: &Expr<'_>,
+    bound_ids: &[Option<HirId>],
+) -> (Option<Span>, Option<Span>) {
+    let mut delegate = MutatePairDelegate {
+        cx,
+        hir_id_low: bound_ids[0],
+        hir_id_high: bound_ids[1],
+        span_low: None,
+        span_high: None,
+    };
+    cx.tcx.infer_ctxt().enter(|infcx| {
+        ExprUseVisitor::new(
+            &mut delegate,
+            &infcx,
+            body.hir_id.owner,
+            cx.param_env,
+            cx.typeck_results(),
+        )
+        .walk_expr(body);
+    });
+    delegate.mutation_span()
+}
+
+struct MutatePairDelegate<'a, 'tcx> {
+    cx: &'a LateContext<'tcx>,
+    hir_id_low: Option<HirId>,
+    hir_id_high: Option<HirId>,
+    span_low: Option<Span>,
+    span_high: Option<Span>,
+}
+
+impl<'tcx> Delegate<'tcx> for MutatePairDelegate<'_, 'tcx> {
+    fn consume(&mut self, _: &PlaceWithHirId<'tcx>, _: HirId, _: ConsumeMode) {}
+
+    fn borrow(&mut self, cmt: &PlaceWithHirId<'tcx>, diag_expr_id: HirId, bk: ty::BorrowKind) {
+        if let ty::BorrowKind::MutBorrow = bk {
+            if let PlaceBase::Local(id) = cmt.place.base {
+                if Some(id) == self.hir_id_low {
+                    self.span_low = Some(self.cx.tcx.hir().span(diag_expr_id))
+                }
+                if Some(id) == self.hir_id_high {
+                    self.span_high = Some(self.cx.tcx.hir().span(diag_expr_id))
+                }
+            }
+        }
+    }
+
+    fn mutate(&mut self, cmt: &PlaceWithHirId<'tcx>, diag_expr_id: HirId) {
+        if let PlaceBase::Local(id) = cmt.place.base {
+            if Some(id) == self.hir_id_low {
+                self.span_low = Some(self.cx.tcx.hir().span(diag_expr_id))
+            }
+            if Some(id) == self.hir_id_high {
+                self.span_high = Some(self.cx.tcx.hir().span(diag_expr_id))
+            }
+        }
+    }
+}
+
+impl MutatePairDelegate<'_, '_> {
+    fn mutation_span(&self) -> (Option<Span>, Option<Span>) {
+        (self.span_low, self.span_high)
+    }
+}

src/tools/clippy/clippy_lints/src/loops/needless_collect.rs

@@ -0,0 +1,283 @@
+use super::NEEDLESS_COLLECT;
+use crate::utils::sugg::Sugg;
+use crate::utils::{
+    is_type_diagnostic_item, match_trait_method, match_type, path_to_local_id, paths, snippet, span_lint_and_sugg,
+    span_lint_and_then,
+};
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir::intravisit::{walk_block, walk_expr, NestedVisitorMap, Visitor};
+use rustc_hir::{Block, Expr, ExprKind, GenericArg, HirId, Local, Pat, PatKind, QPath, StmtKind};
+use rustc_lint::LateContext;
+use rustc_middle::hir::map::Map;
+use rustc_span::source_map::Span;
+use rustc_span::symbol::{sym, Ident};
+
+const NEEDLESS_COLLECT_MSG: &str = "avoid using `collect()` when not needed";
+
+pub(super) fn check<'tcx>(expr: &'tcx Expr<'_>, cx: &LateContext<'tcx>) {
+    check_needless_collect_direct_usage(expr, cx);
+    check_needless_collect_indirect_usage(expr, cx);
+}
+fn check_needless_collect_direct_usage<'tcx>(expr: &'tcx Expr<'_>, cx: &LateContext<'tcx>) {
+    if_chain! {
+        if let ExprKind::MethodCall(ref method, _, ref args, _) = expr.kind;
+        if let ExprKind::MethodCall(ref chain_method, _, _, _) = args[0].kind;
+        if chain_method.ident.name == sym!(collect) && match_trait_method(cx, &args[0], &paths::ITERATOR);
+        if let Some(ref generic_args) = chain_method.args;
+        if let Some(GenericArg::Type(ref ty)) = generic_args.args.get(0);
+        then {
+            let ty = cx.typeck_results().node_type(ty.hir_id);
+            if is_type_diagnostic_item(cx, ty, sym::vec_type) ||
+                is_type_diagnostic_item(cx, ty, sym::vecdeque_type) ||
+                match_type(cx, ty, &paths::BTREEMAP) ||
+                is_type_diagnostic_item(cx, ty, sym::hashmap_type) {
+                if method.ident.name == sym!(len) {
+                    let span = shorten_needless_collect_span(expr);
+                    span_lint_and_sugg(
+                        cx,
+                        NEEDLESS_COLLECT,
+                        span,
+                        NEEDLESS_COLLECT_MSG,
+                        "replace with",
+                        "count()".to_string(),
+                        Applicability::MachineApplicable,
+                    );
+                }
+                if method.ident.name == sym!(is_empty) {
+                    let span = shorten_needless_collect_span(expr);
+                    span_lint_and_sugg(
+                        cx,
+                        NEEDLESS_COLLECT,
+                        span,
+                        NEEDLESS_COLLECT_MSG,
+                        "replace with",
+                        "next().is_none()".to_string(),
+                        Applicability::MachineApplicable,
+                    );
+                }
+                if method.ident.name == sym!(contains) {
+                    let contains_arg = snippet(cx, args[1].span, "??");
+                    let span = shorten_needless_collect_span(expr);
+                    span_lint_and_then(
+                        cx,
+                        NEEDLESS_COLLECT,
+                        span,
+                        NEEDLESS_COLLECT_MSG,
+                        |diag| {
+                            let (arg, pred) = contains_arg
+                                    .strip_prefix('&')
+                                    .map_or(("&x", &*contains_arg), |s| ("x", s));
+                            diag.span_suggestion(
+                                span,
+                                "replace with",
+                                format!(
+                                    "any(|{}| x == {})",
+                                    arg, pred
+                                ),
+                                Applicability::MachineApplicable,
+                            );
+                        }
+                    );
+                }
+            }
+        }
+    }
+}
+
+fn check_needless_collect_indirect_usage<'tcx>(expr: &'tcx Expr<'_>, cx: &LateContext<'tcx>) {
+    if let ExprKind::Block(ref block, _) = expr.kind {
+        for ref stmt in block.stmts {
+            if_chain! {
+                if let StmtKind::Local(
+                    Local { pat: Pat { hir_id: pat_id, kind: PatKind::Binding(_, _, ident, .. ), .. },
+                    init: Some(ref init_expr), .. }
+                ) = stmt.kind;
+                if let ExprKind::MethodCall(ref method_name, _, &[ref iter_source], ..) = init_expr.kind;
+                if method_name.ident.name == sym!(collect) && match_trait_method(cx, &init_expr, &paths::ITERATOR);
+                if let Some(ref generic_args) = method_name.args;
+                if let Some(GenericArg::Type(ref ty)) = generic_args.args.get(0);
+                if let ty = cx.typeck_results().node_type(ty.hir_id);
+                if is_type_diagnostic_item(cx, ty, sym::vec_type) ||
+                    is_type_diagnostic_item(cx, ty, sym::vecdeque_type) ||
+                    match_type(cx, ty, &paths::LINKED_LIST);
+                if let Some(iter_calls) = detect_iter_and_into_iters(block, *ident);
+                if iter_calls.len() == 1;
+                then {
+                    let mut used_count_visitor = UsedCountVisitor {
+                        cx,
+                        id: *pat_id,
+                        count: 0,
+                    };
+                    walk_block(&mut used_count_visitor, block);
+                    if used_count_visitor.count > 1 {
+                        return;
+                    }
+
+                    // Suggest replacing iter_call with iter_replacement, and removing stmt
+                    let iter_call = &iter_calls[0];
+                    span_lint_and_then(
+                        cx,
+                        super::NEEDLESS_COLLECT,
+                        stmt.span.until(iter_call.span),
+                        NEEDLESS_COLLECT_MSG,
+                        |diag| {
+                            let iter_replacement = format!("{}{}", Sugg::hir(cx, iter_source, ".."), iter_call.get_iter_method(cx));
+                            diag.multipart_suggestion(
+                                iter_call.get_suggestion_text(),
+                                vec![
+                                    (stmt.span, String::new()),
+                                    (iter_call.span, iter_replacement)
+                                ],
+                                Applicability::MachineApplicable,// MaybeIncorrect,
+                            ).emit();
+                        },
+                    );
+                }
+            }
+        }
+    }
+}
+
+struct IterFunction {
+    func: IterFunctionKind,
+    span: Span,
+}
+impl IterFunction {
+    fn get_iter_method(&self, cx: &LateContext<'_>) -> String {
+        match &self.func {
+            IterFunctionKind::IntoIter => String::new(),
+            IterFunctionKind::Len => String::from(".count()"),
+            IterFunctionKind::IsEmpty => String::from(".next().is_none()"),
+            IterFunctionKind::Contains(span) => {
+                let s = snippet(cx, *span, "..");
+                if let Some(stripped) = s.strip_prefix('&') {
+                    format!(".any(|x| x == {})", stripped)
+                } else {
+                    format!(".any(|x| x == *{})", s)
+                }
+            },
+        }
+    }
+    fn get_suggestion_text(&self) -> &'static str {
+        match &self.func {
+            IterFunctionKind::IntoIter => {
+                "use the original Iterator instead of collecting it and then producing a new one"
+            },
+            IterFunctionKind::Len => {
+                "take the original Iterator's count instead of collecting it and finding the length"
+            },
+            IterFunctionKind::IsEmpty => {
+                "check if the original Iterator has anything instead of collecting it and seeing if it's empty"
+            },
+            IterFunctionKind::Contains(_) => {
+                "check if the original Iterator contains an element instead of collecting then checking"
+            },
+        }
+    }
+}
+enum IterFunctionKind {
+    IntoIter,
+    Len,
+    IsEmpty,
+    Contains(Span),
+}
+
+struct IterFunctionVisitor {
+    uses: Vec<IterFunction>,
+    seen_other: bool,
+    target: Ident,
+}
+impl<'tcx> Visitor<'tcx> for IterFunctionVisitor {
+    fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
+        // Check function calls on our collection
+        if_chain! {
+            if let ExprKind::MethodCall(method_name, _, ref args, _) = &expr.kind;
+            if let Some(Expr { kind: ExprKind::Path(QPath::Resolved(_, ref path)), .. }) = args.get(0);
+            if let &[name] = &path.segments;
+            if name.ident == self.target;
+            then {
+                let len = sym!(len);
+                let is_empty = sym!(is_empty);
+                let contains = sym!(contains);
+                match method_name.ident.name {
+                    sym::into_iter => self.uses.push(
+                        IterFunction { func: IterFunctionKind::IntoIter, span: expr.span }
+                    ),
+                    name if name == len => self.uses.push(
+                        IterFunction { func: IterFunctionKind::Len, span: expr.span }
+                    ),
+                    name if name == is_empty => self.uses.push(
+                        IterFunction { func: IterFunctionKind::IsEmpty, span: expr.span }
+                    ),
+                    name if name == contains => self.uses.push(
+                        IterFunction { func: IterFunctionKind::Contains(args[1].span), span: expr.span }
+                    ),
+                    _ => self.seen_other = true,
+                }
+                return
+            }
+        }
+        // Check if the collection is used for anything else
+        if_chain! {
+            if let Expr { kind: ExprKind::Path(QPath::Resolved(_, ref path)), .. } = expr;
+            if let &[name] = &path.segments;
+            if name.ident == self.target;
+            then {
+                self.seen_other = true;
+            } else {
+                walk_expr(self, expr);
+            }
+        }
+    }
+
+    type Map = Map<'tcx>;
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::None
+    }
+}
+
+struct UsedCountVisitor<'a, 'tcx> {
+    cx: &'a LateContext<'tcx>,
+    id: HirId,
+    count: usize,
+}
+
+impl<'a, 'tcx> Visitor<'tcx> for UsedCountVisitor<'a, 'tcx> {
+    type Map = Map<'tcx>;
+
+    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
+        if path_to_local_id(expr, self.id) {
+            self.count += 1;
+        } else {
+            walk_expr(self, expr);
+        }
+    }
+
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::OnlyBodies(self.cx.tcx.hir())
+    }
+}
+
+/// Detect the occurrences of calls to `iter` or `into_iter` for the
+/// given identifier
+fn detect_iter_and_into_iters<'tcx>(block: &'tcx Block<'tcx>, identifier: Ident) -> Option<Vec<IterFunction>> {
+    let mut visitor = IterFunctionVisitor {
+        uses: Vec::new(),
+        target: identifier,
+        seen_other: false,
+    };
+    visitor.visit_block(block);
+    if visitor.seen_other { None } else { Some(visitor.uses) }
+}
+
+fn shorten_needless_collect_span(expr: &Expr<'_>) -> Span {
+    if_chain! {
+        if let ExprKind::MethodCall(.., args, _) = &expr.kind;
+        if let ExprKind::MethodCall(_, span, ..) = &args[0].kind;
+        then {
+            return expr.span.with_lo(span.lo());
+        }
+    }
+    unreachable!();
+}

src/tools/clippy/clippy_lints/src/loops/needless_range_loop.rs

@@ -0,0 +1,391 @@
+use super::NEEDLESS_RANGE_LOOP;
+use crate::utils::visitors::LocalUsedVisitor;
+use crate::utils::{
+    contains_name, has_iter_method, higher, is_integer_const, match_trait_method, multispan_sugg, path_to_local_id,
+    paths, snippet, span_lint_and_then, sugg, SpanlessEq,
+};
+use if_chain::if_chain;
+use rustc_ast::ast;
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_hir::def::{DefKind, Res};
+use rustc_hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
+use rustc_hir::{BinOpKind, BorrowKind, Expr, ExprKind, HirId, Mutability, Pat, PatKind, QPath};
+use rustc_lint::LateContext;
+use rustc_middle::hir::map::Map;
+use rustc_middle::middle::region;
+use rustc_middle::ty::{self, Ty};
+use rustc_span::symbol::{sym, Symbol};
+use std::iter::Iterator;
+use std::mem;
+
+/// Checks for looping over a range and then indexing a sequence with it.
+/// The iteratee must be a range literal.
+#[allow(clippy::too_many_lines)]
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    pat: &'tcx Pat<'_>,
+    arg: &'tcx Expr<'_>,
+    body: &'tcx Expr<'_>,
+    expr: &'tcx Expr<'_>,
+) {
+    if let Some(higher::Range {
+        start: Some(start),
+        ref end,
+        limits,
+    }) = higher::range(arg)
+    {
+        // the var must be a single name
+        if let PatKind::Binding(_, canonical_id, ident, _) = pat.kind {
+            let mut visitor = VarVisitor {
+                cx,
+                var: canonical_id,
+                indexed_mut: FxHashSet::default(),
+                indexed_indirectly: FxHashMap::default(),
+                indexed_directly: FxHashMap::default(),
+                referenced: FxHashSet::default(),
+                nonindex: false,
+                prefer_mutable: false,
+            };
+            walk_expr(&mut visitor, body);
+
+            // linting condition: we only indexed one variable, and indexed it directly
+            if visitor.indexed_indirectly.is_empty() && visitor.indexed_directly.len() == 1 {
+                let (indexed, (indexed_extent, indexed_ty)) = visitor
+                    .indexed_directly
+                    .into_iter()
+                    .next()
+                    .expect("already checked that we have exactly 1 element");
+
+                // ensure that the indexed variable was declared before the loop, see #601
+                if let Some(indexed_extent) = indexed_extent {
+                    let parent_id = cx.tcx.hir().get_parent_item(expr.hir_id);
+                    let parent_def_id = cx.tcx.hir().local_def_id(parent_id);
+                    let region_scope_tree = cx.tcx.region_scope_tree(parent_def_id);
+                    let pat_extent = region_scope_tree.var_scope(pat.hir_id.local_id);
+                    if region_scope_tree.is_subscope_of(indexed_extent, pat_extent) {
+                        return;
+                    }
+                }
+
+                // don't lint if the container that is indexed does not have .iter() method
+                let has_iter = has_iter_method(cx, indexed_ty);
+                if has_iter.is_none() {
+                    return;
+                }
+
+                // don't lint if the container that is indexed into is also used without
+                // indexing
+                if visitor.referenced.contains(&indexed) {
+                    return;
+                }
+
+                let starts_at_zero = is_integer_const(cx, start, 0);
+
+                let skip = if starts_at_zero {
+                    String::new()
+                } else if visitor.indexed_mut.contains(&indexed) && contains_name(indexed, start) {
+                    return;
+                } else {
+                    format!(".skip({})", snippet(cx, start.span, ".."))
+                };
+
+                let mut end_is_start_plus_val = false;
+
+                let take = if let Some(end) = *end {
+                    let mut take_expr = end;
+
+                    if let ExprKind::Binary(ref op, ref left, ref right) = end.kind {
+                        if let BinOpKind::Add = op.node {
+                            let start_equal_left = SpanlessEq::new(cx).eq_expr(start, left);
+                            let start_equal_right = SpanlessEq::new(cx).eq_expr(start, right);
+
+                            if start_equal_left {
+                                take_expr = right;
+                            } else if start_equal_right {
+                                take_expr = left;
+                            }
+
+                            end_is_start_plus_val = start_equal_left | start_equal_right;
+                        }
+                    }
+
+                    if is_len_call(end, indexed) || is_end_eq_array_len(cx, end, limits, indexed_ty) {
+                        String::new()
+                    } else if visitor.indexed_mut.contains(&indexed) && contains_name(indexed, take_expr) {
+                        return;
+                    } else {
+                        match limits {
+                            ast::RangeLimits::Closed => {
+                                let take_expr = sugg::Sugg::hir(cx, take_expr, "<count>");
+                                format!(".take({})", take_expr + sugg::ONE)
+                            },
+                            ast::RangeLimits::HalfOpen => format!(".take({})", snippet(cx, take_expr.span, "..")),
+                        }
+                    }
+                } else {
+                    String::new()
+                };
+
+                let (ref_mut, method) = if visitor.indexed_mut.contains(&indexed) {
+                    ("mut ", "iter_mut")
+                } else {
+                    ("", "iter")
+                };
+
+                let take_is_empty = take.is_empty();
+                let mut method_1 = take;
+                let mut method_2 = skip;
+
+                if end_is_start_plus_val {
+                    mem::swap(&mut method_1, &mut method_2);
+                }
+
+                if visitor.nonindex {
+                    span_lint_and_then(
+                        cx,
+                        NEEDLESS_RANGE_LOOP,
+                        expr.span,
+                        &format!("the loop variable `{}` is used to index `{}`", ident.name, indexed),
+                        |diag| {
+                            multispan_sugg(
+                                diag,
+                                "consider using an iterator",
+                                vec![
+                                    (pat.span, format!("({}, <item>)", ident.name)),
+                                    (
+                                        arg.span,
+                                        format!("{}.{}().enumerate(){}{}", indexed, method, method_1, method_2),
+                                    ),
+                                ],
+                            );
+                        },
+                    );
+                } else {
+                    let repl = if starts_at_zero && take_is_empty {
+                        format!("&{}{}", ref_mut, indexed)
+                    } else {
+                        format!("{}.{}(){}{}", indexed, method, method_1, method_2)
+                    };
+
+                    span_lint_and_then(
+                        cx,
+                        NEEDLESS_RANGE_LOOP,
+                        expr.span,
+                        &format!("the loop variable `{}` is only used to index `{}`", ident.name, indexed),
+                        |diag| {
+                            multispan_sugg(
+                                diag,
+                                "consider using an iterator",
+                                vec![(pat.span, "<item>".to_string()), (arg.span, repl)],
+                            );
+                        },
+                    );
+                }
+            }
+        }
+    }
+}
+
+fn is_len_call(expr: &Expr<'_>, var: Symbol) -> bool {
+    if_chain! {
+        if let ExprKind::MethodCall(ref method, _, ref len_args, _) = expr.kind;
+        if len_args.len() == 1;
+        if method.ident.name == sym!(len);
+        if let ExprKind::Path(QPath::Resolved(_, ref path)) = len_args[0].kind;
+        if path.segments.len() == 1;
+        if path.segments[0].ident.name == var;
+        then {
+            return true;
+        }
+    }
+
+    false
+}
+
+fn is_end_eq_array_len<'tcx>(
+    cx: &LateContext<'tcx>,
+    end: &Expr<'_>,
+    limits: ast::RangeLimits,
+    indexed_ty: Ty<'tcx>,
+) -> bool {
+    if_chain! {
+        if let ExprKind::Lit(ref lit) = end.kind;
+        if let ast::LitKind::Int(end_int, _) = lit.node;
+        if let ty::Array(_, arr_len_const) = indexed_ty.kind();
+        if let Some(arr_len) = arr_len_const.try_eval_usize(cx.tcx, cx.param_env);
+        then {
+            return match limits {
+                ast::RangeLimits::Closed => end_int + 1 >= arr_len.into(),
+                ast::RangeLimits::HalfOpen => end_int >= arr_len.into(),
+            };
+        }
+    }
+
+    false
+}
+
+struct VarVisitor<'a, 'tcx> {
+    /// context reference
+    cx: &'a LateContext<'tcx>,
+    /// var name to look for as index
+    var: HirId,
+    /// indexed variables that are used mutably
+    indexed_mut: FxHashSet<Symbol>,
+    /// indirectly indexed variables (`v[(i + 4) % N]`), the extend is `None` for global
+    indexed_indirectly: FxHashMap<Symbol, Option<region::Scope>>,
+    /// subset of `indexed` of vars that are indexed directly: `v[i]`
+    /// this will not contain cases like `v[calc_index(i)]` or `v[(i + 4) % N]`
+    indexed_directly: FxHashMap<Symbol, (Option<region::Scope>, Ty<'tcx>)>,
+    /// Any names that are used outside an index operation.
+    /// Used to detect things like `&mut vec` used together with `vec[i]`
+    referenced: FxHashSet<Symbol>,
+    /// has the loop variable been used in expressions other than the index of
+    /// an index op?
+    nonindex: bool,
+    /// Whether we are inside the `$` in `&mut $` or `$ = foo` or `$.bar`, where bar
+    /// takes `&mut self`
+    prefer_mutable: bool,
+}
+
+impl<'a, 'tcx> VarVisitor<'a, 'tcx> {
+    fn check(&mut self, idx: &'tcx Expr<'_>, seqexpr: &'tcx Expr<'_>, expr: &'tcx Expr<'_>) -> bool {
+        if_chain! {
+            // the indexed container is referenced by a name
+            if let ExprKind::Path(ref seqpath) = seqexpr.kind;
+            if let QPath::Resolved(None, ref seqvar) = *seqpath;
+            if seqvar.segments.len() == 1;
+            then {
+                let index_used_directly = path_to_local_id(idx, self.var);
+                let indexed_indirectly = {
+                    let mut used_visitor = LocalUsedVisitor::new(self.cx, self.var);
+                    walk_expr(&mut used_visitor, idx);
+                    used_visitor.used
+                };
+
+                if indexed_indirectly || index_used_directly {
+                    if self.prefer_mutable {
+                        self.indexed_mut.insert(seqvar.segments[0].ident.name);
+                    }
+                    let res = self.cx.qpath_res(seqpath, seqexpr.hir_id);
+                    match res {
+                        Res::Local(hir_id) => {
+                            let parent_id = self.cx.tcx.hir().get_parent_item(expr.hir_id);
+                            let parent_def_id = self.cx.tcx.hir().local_def_id(parent_id);
+                            let extent = self.cx.tcx.region_scope_tree(parent_def_id).var_scope(hir_id.local_id);
+                            if indexed_indirectly {
+                                self.indexed_indirectly.insert(seqvar.segments[0].ident.name, Some(extent));
+                            }
+                            if index_used_directly {
+                                self.indexed_directly.insert(
+                                    seqvar.segments[0].ident.name,
+                                    (Some(extent), self.cx.typeck_results().node_type(seqexpr.hir_id)),
+                                );
+                            }
+                            return false;  // no need to walk further *on the variable*
+                        }
+                        Res::Def(DefKind::Static | DefKind::Const, ..) => {
+                            if indexed_indirectly {
+                                self.indexed_indirectly.insert(seqvar.segments[0].ident.name, None);
+                            }
+                            if index_used_directly {
+                                self.indexed_directly.insert(
+                                    seqvar.segments[0].ident.name,
+                                    (None, self.cx.typeck_results().node_type(seqexpr.hir_id)),
+                                );
+                            }
+                            return false;  // no need to walk further *on the variable*
+                        }
+                        _ => (),
+                    }
+                }
+            }
+        }
+        true
+    }
+}
+
+impl<'a, 'tcx> Visitor<'tcx> for VarVisitor<'a, 'tcx> {
+    type Map = Map<'tcx>;
+
+    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
+        if_chain! {
+            // a range index op
+            if let ExprKind::MethodCall(ref meth, _, ref args, _) = expr.kind;
+            if (meth.ident.name == sym::index && match_trait_method(self.cx, expr, &paths::INDEX))
+                || (meth.ident.name == sym::index_mut && match_trait_method(self.cx, expr, &paths::INDEX_MUT));
+            if !self.check(&args[1], &args[0], expr);
+            then { return }
+        }
+
+        if_chain! {
+            // an index op
+            if let ExprKind::Index(ref seqexpr, ref idx) = expr.kind;
+            if !self.check(idx, seqexpr, expr);
+            then { return }
+        }
+
+        if_chain! {
+            // directly using a variable
+            if let ExprKind::Path(QPath::Resolved(None, path)) = expr.kind;
+            if let Res::Local(local_id) = path.res;
+            then {
+                if local_id == self.var {
+                    self.nonindex = true;
+                } else {
+                    // not the correct variable, but still a variable
+                    self.referenced.insert(path.segments[0].ident.name);
+                }
+            }
+        }
+
+        let old = self.prefer_mutable;
+        match expr.kind {
+            ExprKind::AssignOp(_, ref lhs, ref rhs) | ExprKind::Assign(ref lhs, ref rhs, _) => {
+                self.prefer_mutable = true;
+                self.visit_expr(lhs);
+                self.prefer_mutable = false;
+                self.visit_expr(rhs);
+            },
+            ExprKind::AddrOf(BorrowKind::Ref, mutbl, ref expr) => {
+                if mutbl == Mutability::Mut {
+                    self.prefer_mutable = true;
+                }
+                self.visit_expr(expr);
+            },
+            ExprKind::Call(ref f, args) => {
+                self.visit_expr(f);
+                for expr in args {
+                    let ty = self.cx.typeck_results().expr_ty_adjusted(expr);
+                    self.prefer_mutable = false;
+                    if let ty::Ref(_, _, mutbl) = *ty.kind() {
+                        if mutbl == Mutability::Mut {
+                            self.prefer_mutable = true;
+                        }
+                    }
+                    self.visit_expr(expr);
+                }
+            },
+            ExprKind::MethodCall(_, _, args, _) => {
+                let def_id = self.cx.typeck_results().type_dependent_def_id(expr.hir_id).unwrap();
+                for (ty, expr) in self.cx.tcx.fn_sig(def_id).inputs().skip_binder().iter().zip(args) {
+                    self.prefer_mutable = false;
+                    if let ty::Ref(_, _, mutbl) = *ty.kind() {
+                        if mutbl == Mutability::Mut {
+                            self.prefer_mutable = true;
+                        }
+                    }
+                    self.visit_expr(expr);
+                }
+            },
+            ExprKind::Closure(_, _, body_id, ..) => {
+                let body = self.cx.tcx.hir().body(body_id);
+                self.visit_expr(&body.value);
+            },
+            _ => walk_expr(self, expr),
+        }
+        self.prefer_mutable = old;
+    }
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::None
+    }
+}

src/tools/clippy/clippy_lints/src/loops/never_loop.rs

@@ -0,0 +1,172 @@
+use super::NEVER_LOOP;
+use crate::utils::span_lint;
+use rustc_hir::{Block, Expr, ExprKind, HirId, InlineAsmOperand, Stmt, StmtKind};
+use rustc_lint::LateContext;
+use std::iter::{once, Iterator};
+
+pub(super) fn check(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+    if let ExprKind::Loop(ref block, _, _, _) = expr.kind {
+        match never_loop_block(block, expr.hir_id) {
+            NeverLoopResult::AlwaysBreak => span_lint(cx, NEVER_LOOP, expr.span, "this loop never actually loops"),
+            NeverLoopResult::MayContinueMainLoop | NeverLoopResult::Otherwise => (),
+        }
+    }
+}
+
+enum NeverLoopResult {
+    // A break/return always get triggered but not necessarily for the main loop.
+    AlwaysBreak,
+    // A continue may occur for the main loop.
+    MayContinueMainLoop,
+    Otherwise,
+}
+
+#[must_use]
+fn absorb_break(arg: &NeverLoopResult) -> NeverLoopResult {
+    match *arg {
+        NeverLoopResult::AlwaysBreak | NeverLoopResult::Otherwise => NeverLoopResult::Otherwise,
+        NeverLoopResult::MayContinueMainLoop => NeverLoopResult::MayContinueMainLoop,
+    }
+}
+
+// Combine two results for parts that are called in order.
+#[must_use]
+fn combine_seq(first: NeverLoopResult, second: NeverLoopResult) -> NeverLoopResult {
+    match first {
+        NeverLoopResult::AlwaysBreak | NeverLoopResult::MayContinueMainLoop => first,
+        NeverLoopResult::Otherwise => second,
+    }
+}
+
+// Combine two results where both parts are called but not necessarily in order.
+#[must_use]
+fn combine_both(left: NeverLoopResult, right: NeverLoopResult) -> NeverLoopResult {
+    match (left, right) {
+        (NeverLoopResult::MayContinueMainLoop, _) | (_, NeverLoopResult::MayContinueMainLoop) => {
+            NeverLoopResult::MayContinueMainLoop
+        },
+        (NeverLoopResult::AlwaysBreak, _) | (_, NeverLoopResult::AlwaysBreak) => NeverLoopResult::AlwaysBreak,
+        (NeverLoopResult::Otherwise, NeverLoopResult::Otherwise) => NeverLoopResult::Otherwise,
+    }
+}
+
+// Combine two results where only one of the part may have been executed.
+#[must_use]
+fn combine_branches(b1: NeverLoopResult, b2: NeverLoopResult) -> NeverLoopResult {
+    match (b1, b2) {
+        (NeverLoopResult::AlwaysBreak, NeverLoopResult::AlwaysBreak) => NeverLoopResult::AlwaysBreak,
+        (NeverLoopResult::MayContinueMainLoop, _) | (_, NeverLoopResult::MayContinueMainLoop) => {
+            NeverLoopResult::MayContinueMainLoop
+        },
+        (NeverLoopResult::Otherwise, _) | (_, NeverLoopResult::Otherwise) => NeverLoopResult::Otherwise,
+    }
+}
+
+fn never_loop_block(block: &Block<'_>, main_loop_id: HirId) -> NeverLoopResult {
+    let stmts = block.stmts.iter().map(stmt_to_expr);
+    let expr = once(block.expr.as_deref());
+    let mut iter = stmts.chain(expr).flatten();
+    never_loop_expr_seq(&mut iter, main_loop_id)
+}
+
+fn never_loop_expr_seq<'a, T: Iterator<Item = &'a Expr<'a>>>(es: &mut T, main_loop_id: HirId) -> NeverLoopResult {
+    es.map(|e| never_loop_expr(e, main_loop_id))
+        .fold(NeverLoopResult::Otherwise, combine_seq)
+}
+
+fn stmt_to_expr<'tcx>(stmt: &Stmt<'tcx>) -> Option<&'tcx Expr<'tcx>> {
+    match stmt.kind {
+        StmtKind::Semi(ref e, ..) | StmtKind::Expr(ref e, ..) => Some(e),
+        StmtKind::Local(ref local) => local.init.as_deref(),
+        _ => None,
+    }
+}
+
+fn never_loop_expr(expr: &Expr<'_>, main_loop_id: HirId) -> NeverLoopResult {
+    match expr.kind {
+        ExprKind::Box(ref e)
+        | ExprKind::Unary(_, ref e)
+        | ExprKind::Cast(ref e, _)
+        | ExprKind::Type(ref e, _)
+        | ExprKind::Field(ref e, _)
+        | ExprKind::AddrOf(_, _, ref e)
+        | ExprKind::Struct(_, _, Some(ref e))
+        | ExprKind::Repeat(ref e, _)
+        | ExprKind::DropTemps(ref e) => never_loop_expr(e, main_loop_id),
+        ExprKind::Array(ref es) | ExprKind::MethodCall(_, _, ref es, _) | ExprKind::Tup(ref es) => {
+            never_loop_expr_all(&mut es.iter(), main_loop_id)
+        },
+        ExprKind::Call(ref e, ref es) => never_loop_expr_all(&mut once(&**e).chain(es.iter()), main_loop_id),
+        ExprKind::Binary(_, ref e1, ref e2)
+        | ExprKind::Assign(ref e1, ref e2, _)
+        | ExprKind::AssignOp(_, ref e1, ref e2)
+        | ExprKind::Index(ref e1, ref e2) => never_loop_expr_all(&mut [&**e1, &**e2].iter().cloned(), main_loop_id),
+        ExprKind::Loop(ref b, _, _, _) => {
+            // Break can come from the inner loop so remove them.
+            absorb_break(&never_loop_block(b, main_loop_id))
+        },
+        ExprKind::If(ref e, ref e2, ref e3) => {
+            let e1 = never_loop_expr(e, main_loop_id);
+            let e2 = never_loop_expr(e2, main_loop_id);
+            let e3 = e3
+                .as_ref()
+                .map_or(NeverLoopResult::Otherwise, |e| never_loop_expr(e, main_loop_id));
+            combine_seq(e1, combine_branches(e2, e3))
+        },
+        ExprKind::Match(ref e, ref arms, _) => {
+            let e = never_loop_expr(e, main_loop_id);
+            if arms.is_empty() {
+                e
+            } else {
+                let arms = never_loop_expr_branch(&mut arms.iter().map(|a| &*a.body), main_loop_id);
+                combine_seq(e, arms)
+            }
+        },
+        ExprKind::Block(ref b, _) => never_loop_block(b, main_loop_id),
+        ExprKind::Continue(d) => {
+            let id = d
+                .target_id
+                .expect("target ID can only be missing in the presence of compilation errors");
+            if id == main_loop_id {
+                NeverLoopResult::MayContinueMainLoop
+            } else {
+                NeverLoopResult::AlwaysBreak
+            }
+        },
+        ExprKind::Break(_, ref e) | ExprKind::Ret(ref e) => e.as_ref().map_or(NeverLoopResult::AlwaysBreak, |e| {
+            combine_seq(never_loop_expr(e, main_loop_id), NeverLoopResult::AlwaysBreak)
+        }),
+        ExprKind::InlineAsm(ref asm) => asm
+            .operands
+            .iter()
+            .map(|(o, _)| match o {
+                InlineAsmOperand::In { expr, .. }
+                | InlineAsmOperand::InOut { expr, .. }
+                | InlineAsmOperand::Const { expr }
+                | InlineAsmOperand::Sym { expr } => never_loop_expr(expr, main_loop_id),
+                InlineAsmOperand::Out { expr, .. } => never_loop_expr_all(&mut expr.iter(), main_loop_id),
+                InlineAsmOperand::SplitInOut { in_expr, out_expr, .. } => {
+                    never_loop_expr_all(&mut once(in_expr).chain(out_expr.iter()), main_loop_id)
+                },
+            })
+            .fold(NeverLoopResult::Otherwise, combine_both),
+        ExprKind::Struct(_, _, None)
+        | ExprKind::Yield(_, _)
+        | ExprKind::Closure(_, _, _, _, _)
+        | ExprKind::LlvmInlineAsm(_)
+        | ExprKind::Path(_)
+        | ExprKind::ConstBlock(_)
+        | ExprKind::Lit(_)
+        | ExprKind::Err => NeverLoopResult::Otherwise,
+    }
+}
+
+fn never_loop_expr_all<'a, T: Iterator<Item = &'a Expr<'a>>>(es: &mut T, main_loop_id: HirId) -> NeverLoopResult {
+    es.map(|e| never_loop_expr(e, main_loop_id))
+        .fold(NeverLoopResult::Otherwise, combine_both)
+}
+
+fn never_loop_expr_branch<'a, T: Iterator<Item = &'a Expr<'a>>>(e: &mut T, main_loop_id: HirId) -> NeverLoopResult {
+    e.map(|e| never_loop_expr(e, main_loop_id))
+        .fold(NeverLoopResult::AlwaysBreak, combine_branches)
+}

src/tools/clippy/clippy_lints/src/loops/same_item_push.rs

@@ -0,0 +1,169 @@
+use super::SAME_ITEM_PUSH;
+use crate::utils::{implements_trait, is_type_diagnostic_item, snippet_with_macro_callsite, span_lint_and_help};
+use if_chain::if_chain;
+use rustc_hir::def::{DefKind, Res};
+use rustc_hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
+use rustc_hir::{BindingAnnotation, Block, Expr, ExprKind, Node, Pat, PatKind, Stmt, StmtKind};
+use rustc_lint::LateContext;
+use rustc_middle::hir::map::Map;
+use rustc_span::symbol::sym;
+use std::iter::Iterator;
+
+/// Detects for loop pushing the same item into a Vec
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    pat: &'tcx Pat<'_>,
+    _: &'tcx Expr<'_>,
+    body: &'tcx Expr<'_>,
+    _: &'tcx Expr<'_>,
+) {
+    fn emit_lint(cx: &LateContext<'_>, vec: &Expr<'_>, pushed_item: &Expr<'_>) {
+        let vec_str = snippet_with_macro_callsite(cx, vec.span, "");
+        let item_str = snippet_with_macro_callsite(cx, pushed_item.span, "");
+
+        span_lint_and_help(
+            cx,
+            SAME_ITEM_PUSH,
+            vec.span,
+            "it looks like the same item is being pushed into this Vec",
+            None,
+            &format!(
+                "try using vec![{};SIZE] or {}.resize(NEW_SIZE, {})",
+                item_str, vec_str, item_str
+            ),
+        )
+    }
+
+    if !matches!(pat.kind, PatKind::Wild) {
+        return;
+    }
+
+    // Determine whether it is safe to lint the body
+    let mut same_item_push_visitor = SameItemPushVisitor {
+        should_lint: true,
+        vec_push: None,
+        cx,
+    };
+    walk_expr(&mut same_item_push_visitor, body);
+    if same_item_push_visitor.should_lint {
+        if let Some((vec, pushed_item)) = same_item_push_visitor.vec_push {
+            let vec_ty = cx.typeck_results().expr_ty(vec);
+            let ty = vec_ty.walk().nth(1).unwrap().expect_ty();
+            if cx
+                .tcx
+                .lang_items()
+                .clone_trait()
+                .map_or(false, |id| implements_trait(cx, ty, id, &[]))
+            {
+                // Make sure that the push does not involve possibly mutating values
+                match pushed_item.kind {
+                    ExprKind::Path(ref qpath) => {
+                        match cx.qpath_res(qpath, pushed_item.hir_id) {
+                            // immutable bindings that are initialized with literal or constant
+                            Res::Local(hir_id) => {
+                                if_chain! {
+                                    let node = cx.tcx.hir().get(hir_id);
+                                    if let Node::Binding(pat) = node;
+                                    if let PatKind::Binding(bind_ann, ..) = pat.kind;
+                                    if !matches!(bind_ann, BindingAnnotation::RefMut | BindingAnnotation::Mutable);
+                                    let parent_node = cx.tcx.hir().get_parent_node(hir_id);
+                                    if let Some(Node::Local(parent_let_expr)) = cx.tcx.hir().find(parent_node);
+                                    if let Some(init) = parent_let_expr.init;
+                                    then {
+                                        match init.kind {
+                                            // immutable bindings that are initialized with literal
+                                            ExprKind::Lit(..) => emit_lint(cx, vec, pushed_item),
+                                            // immutable bindings that are initialized with constant
+                                            ExprKind::Path(ref path) => {
+                                                if let Res::Def(DefKind::Const, ..) = cx.qpath_res(path, init.hir_id) {
+                                                    emit_lint(cx, vec, pushed_item);
+                                                }
+                                            }
+                                            _ => {},
+                                        }
+                                    }
+                                }
+                            },
+                            // constant
+                            Res::Def(DefKind::Const, ..) => emit_lint(cx, vec, pushed_item),
+                            _ => {},
+                        }
+                    },
+                    ExprKind::Lit(..) => emit_lint(cx, vec, pushed_item),
+                    _ => {},
+                }
+            }
+        }
+    }
+}
+
+// Scans the body of the for loop and determines whether lint should be given
+struct SameItemPushVisitor<'a, 'tcx> {
+    should_lint: bool,
+    // this field holds the last vec push operation visited, which should be the only push seen
+    vec_push: Option<(&'tcx Expr<'tcx>, &'tcx Expr<'tcx>)>,
+    cx: &'a LateContext<'tcx>,
+}
+
+impl<'a, 'tcx> Visitor<'tcx> for SameItemPushVisitor<'a, 'tcx> {
+    type Map = Map<'tcx>;
+
+    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
+        match &expr.kind {
+            // Non-determinism may occur ... don't give a lint
+            ExprKind::Loop(..) | ExprKind::Match(..) => self.should_lint = false,
+            ExprKind::Block(block, _) => self.visit_block(block),
+            _ => {},
+        }
+    }
+
+    fn visit_block(&mut self, b: &'tcx Block<'_>) {
+        for stmt in b.stmts.iter() {
+            self.visit_stmt(stmt);
+        }
+    }
+
+    fn visit_stmt(&mut self, s: &'tcx Stmt<'_>) {
+        let vec_push_option = get_vec_push(self.cx, s);
+        if vec_push_option.is_none() {
+            // Current statement is not a push so visit inside
+            match &s.kind {
+                StmtKind::Expr(expr) | StmtKind::Semi(expr) => self.visit_expr(&expr),
+                _ => {},
+            }
+        } else {
+            // Current statement is a push ...check whether another
+            // push had been previously done
+            if self.vec_push.is_none() {
+                self.vec_push = vec_push_option;
+            } else {
+                // There are multiple pushes ... don't lint
+                self.should_lint = false;
+            }
+        }
+    }
+
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::None
+    }
+}
+
+// Given some statement, determine if that statement is a push on a Vec. If it is, return
+// the Vec being pushed into and the item being pushed
+fn get_vec_push<'tcx>(cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) -> Option<(&'tcx Expr<'tcx>, &'tcx Expr<'tcx>)> {
+    if_chain! {
+            // Extract method being called
+            if let StmtKind::Semi(semi_stmt) = &stmt.kind;
+            if let ExprKind::MethodCall(path, _, args, _) = &semi_stmt.kind;
+            // Figure out the parameters for the method call
+            if let Some(self_expr) = args.get(0);
+            if let Some(pushed_item) = args.get(1);
+            // Check that the method being called is push() on a Vec
+            if is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(self_expr), sym::vec_type);
+            if path.ident.name.as_str() == "push";
+            then {
+                return Some((self_expr, pushed_item))
+            }
+    }
+    None
+}

src/tools/clippy/clippy_lints/src/loops/single_element_loop.rs

@@ -0,0 +1,42 @@
+use super::{get_span_of_entire_for_loop, SINGLE_ELEMENT_LOOP};
+use crate::utils::{indent_of, single_segment_path, snippet, span_lint_and_sugg};
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir::{BorrowKind, Expr, ExprKind, Pat, PatKind};
+use rustc_lint::LateContext;
+
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    pat: &'tcx Pat<'_>,
+    arg: &'tcx Expr<'_>,
+    body: &'tcx Expr<'_>,
+    expr: &'tcx Expr<'_>,
+) {
+    if_chain! {
+        if let ExprKind::AddrOf(BorrowKind::Ref, _, ref arg_expr) = arg.kind;
+        if let PatKind::Binding(.., target, _) = pat.kind;
+        if let ExprKind::Array([arg_expression]) = arg_expr.kind;
+        if let ExprKind::Path(ref list_item) = arg_expression.kind;
+        if let Some(list_item_name) = single_segment_path(list_item).map(|ps| ps.ident.name);
+        if let ExprKind::Block(ref block, _) = body.kind;
+        if !block.stmts.is_empty();
+
+        then {
+            let for_span = get_span_of_entire_for_loop(expr);
+            let mut block_str = snippet(cx, block.span, "..").into_owned();
+            block_str.remove(0);
+            block_str.pop();
+
+
+            span_lint_and_sugg(
+                cx,
+                SINGLE_ELEMENT_LOOP,
+                for_span,
+                "for loop over a single element",
+                "try",
+                format!("{{\n{}let {} = &{};{}}}", " ".repeat(indent_of(cx, block.stmts[0].span).unwrap_or(0)), target.name, list_item_name, block_str),
+                Applicability::MachineApplicable
+            )
+        }
+    }
+}

src/tools/clippy/clippy_lints/src/loops/utils.rs

@@ -0,0 +1,350 @@
+use crate::utils::{
+    get_parent_expr, get_trait_def_id, has_iter_method, implements_trait, is_integer_const, path_to_local,
+    path_to_local_id, paths, sugg,
+};
+use if_chain::if_chain;
+use rustc_data_structures::fx::FxHashMap;
+use rustc_errors::Applicability;
+use rustc_hir::intravisit::{walk_expr, walk_pat, walk_stmt, NestedVisitorMap, Visitor};
+use rustc_hir::{BinOpKind, BorrowKind, Expr, ExprKind, HirId, Mutability, Pat, PatKind, Stmt, StmtKind};
+use rustc_lint::LateContext;
+use rustc_middle::hir::map::Map;
+use rustc_span::source_map::Span;
+use rustc_span::symbol::Symbol;
+use std::iter::Iterator;
+
+#[derive(Debug, PartialEq)]
+enum IncrementVisitorVarState {
+    Initial,  // Not examined yet
+    IncrOnce, // Incremented exactly once, may be a loop counter
+    DontWarn,
+}
+
+/// Scan a for loop for variables that are incremented exactly once and not used after that.
+pub(super) struct IncrementVisitor<'a, 'tcx> {
+    cx: &'a LateContext<'tcx>,                          // context reference
+    states: FxHashMap<HirId, IncrementVisitorVarState>, // incremented variables
+    depth: u32,                                         // depth of conditional expressions
+    done: bool,
+}
+
+impl<'a, 'tcx> IncrementVisitor<'a, 'tcx> {
+    pub(super) fn new(cx: &'a LateContext<'tcx>) -> Self {
+        Self {
+            cx,
+            states: FxHashMap::default(),
+            depth: 0,
+            done: false,
+        }
+    }
+
+    pub(super) fn into_results(self) -> impl Iterator<Item = HirId> {
+        self.states.into_iter().filter_map(|(id, state)| {
+            if state == IncrementVisitorVarState::IncrOnce {
+                Some(id)
+            } else {
+                None
+            }
+        })
+    }
+}
+
+impl<'a, 'tcx> Visitor<'tcx> for IncrementVisitor<'a, 'tcx> {
+    type Map = Map<'tcx>;
+
+    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
+        if self.done {
+            return;
+        }
+
+        // If node is a variable
+        if let Some(def_id) = path_to_local(expr) {
+            if let Some(parent) = get_parent_expr(self.cx, expr) {
+                let state = self.states.entry(def_id).or_insert(IncrementVisitorVarState::Initial);
+                if *state == IncrementVisitorVarState::IncrOnce {
+                    *state = IncrementVisitorVarState::DontWarn;
+                    return;
+                }
+
+                match parent.kind {
+                    ExprKind::AssignOp(op, ref lhs, ref rhs) => {
+                        if lhs.hir_id == expr.hir_id {
+                            *state = if op.node == BinOpKind::Add
+                                && is_integer_const(self.cx, rhs, 1)
+                                && *state == IncrementVisitorVarState::Initial
+                                && self.depth == 0
+                            {
+                                IncrementVisitorVarState::IncrOnce
+                            } else {
+                                // Assigned some other value or assigned multiple times
+                                IncrementVisitorVarState::DontWarn
+                            };
+                        }
+                    },
+                    ExprKind::Assign(ref lhs, _, _) if lhs.hir_id == expr.hir_id => {
+                        *state = IncrementVisitorVarState::DontWarn
+                    },
+                    ExprKind::AddrOf(BorrowKind::Ref, mutability, _) if mutability == Mutability::Mut => {
+                        *state = IncrementVisitorVarState::DontWarn
+                    },
+                    _ => (),
+                }
+            }
+
+            walk_expr(self, expr);
+        } else if is_loop(expr) || is_conditional(expr) {
+            self.depth += 1;
+            walk_expr(self, expr);
+            self.depth -= 1;
+        } else if let ExprKind::Continue(_) = expr.kind {
+            self.done = true;
+        } else {
+            walk_expr(self, expr);
+        }
+    }
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::None
+    }
+}
+
+enum InitializeVisitorState<'hir> {
+    Initial,          // Not examined yet
+    Declared(Symbol), // Declared but not (yet) initialized
+    Initialized {
+        name: Symbol,
+        initializer: &'hir Expr<'hir>,
+    },
+    DontWarn,
+}
+
+/// Checks whether a variable is initialized at the start of a loop and not modified
+/// and used after the loop.
+pub(super) struct InitializeVisitor<'a, 'tcx> {
+    cx: &'a LateContext<'tcx>,  // context reference
+    end_expr: &'tcx Expr<'tcx>, // the for loop. Stop scanning here.
+    var_id: HirId,
+    state: InitializeVisitorState<'tcx>,
+    depth: u32, // depth of conditional expressions
+    past_loop: bool,
+}
+
+impl<'a, 'tcx> InitializeVisitor<'a, 'tcx> {
+    pub(super) fn new(cx: &'a LateContext<'tcx>, end_expr: &'tcx Expr<'tcx>, var_id: HirId) -> Self {
+        Self {
+            cx,
+            end_expr,
+            var_id,
+            state: InitializeVisitorState::Initial,
+            depth: 0,
+            past_loop: false,
+        }
+    }
+
+    pub(super) fn get_result(&self) -> Option<(Symbol, &'tcx Expr<'tcx>)> {
+        if let InitializeVisitorState::Initialized { name, initializer } = self.state {
+            Some((name, initializer))
+        } else {
+            None
+        }
+    }
+}
+
+impl<'a, 'tcx> Visitor<'tcx> for InitializeVisitor<'a, 'tcx> {
+    type Map = Map<'tcx>;
+
+    fn visit_stmt(&mut self, stmt: &'tcx Stmt<'_>) {
+        // Look for declarations of the variable
+        if_chain! {
+            if let StmtKind::Local(ref local) = stmt.kind;
+            if local.pat.hir_id == self.var_id;
+            if let PatKind::Binding(.., ident, _) = local.pat.kind;
+            then {
+                self.state = local.init.map_or(InitializeVisitorState::Declared(ident.name), |init| {
+                    InitializeVisitorState::Initialized {
+                        initializer: init,
+                        name: ident.name,
+                    }
+                })
+            }
+        }
+        walk_stmt(self, stmt);
+    }
+
+    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
+        if matches!(self.state, InitializeVisitorState::DontWarn) {
+            return;
+        }
+        if expr.hir_id == self.end_expr.hir_id {
+            self.past_loop = true;
+            return;
+        }
+        // No need to visit expressions before the variable is
+        // declared
+        if matches!(self.state, InitializeVisitorState::Initial) {
+            return;
+        }
+
+        // If node is the desired variable, see how it's used
+        if path_to_local_id(expr, self.var_id) {
+            if self.past_loop {
+                self.state = InitializeVisitorState::DontWarn;
+                return;
+            }
+
+            if let Some(parent) = get_parent_expr(self.cx, expr) {
+                match parent.kind {
+                    ExprKind::AssignOp(_, ref lhs, _) if lhs.hir_id == expr.hir_id => {
+                        self.state = InitializeVisitorState::DontWarn;
+                    },
+                    ExprKind::Assign(ref lhs, ref rhs, _) if lhs.hir_id == expr.hir_id => {
+                        self.state = if_chain! {
+                            if self.depth == 0;
+                            if let InitializeVisitorState::Declared(name)
+                                | InitializeVisitorState::Initialized { name, ..} = self.state;
+                            then {
+                                InitializeVisitorState::Initialized { initializer: rhs, name }
+                            } else {
+                                InitializeVisitorState::DontWarn
+                            }
+                        }
+                    },
+                    ExprKind::AddrOf(BorrowKind::Ref, mutability, _) if mutability == Mutability::Mut => {
+                        self.state = InitializeVisitorState::DontWarn
+                    },
+                    _ => (),
+                }
+            }
+
+            walk_expr(self, expr);
+        } else if !self.past_loop && is_loop(expr) {
+            self.state = InitializeVisitorState::DontWarn;
+        } else if is_conditional(expr) {
+            self.depth += 1;
+            walk_expr(self, expr);
+            self.depth -= 1;
+        } else {
+            walk_expr(self, expr);
+        }
+    }
+
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::OnlyBodies(self.cx.tcx.hir())
+    }
+}
+
+fn is_loop(expr: &Expr<'_>) -> bool {
+    matches!(expr.kind, ExprKind::Loop(..))
+}
+
+fn is_conditional(expr: &Expr<'_>) -> bool {
+    matches!(expr.kind, ExprKind::If(..) | ExprKind::Match(..))
+}
+
+#[derive(PartialEq, Eq)]
+pub(super) enum Nesting {
+    Unknown,     // no nesting detected yet
+    RuledOut,    // the iterator is initialized or assigned within scope
+    LookFurther, // no nesting detected, no further walk required
+}
+
+use self::Nesting::{LookFurther, RuledOut, Unknown};
+
+pub(super) struct LoopNestVisitor {
+    pub(super) hir_id: HirId,
+    pub(super) iterator: HirId,
+    pub(super) nesting: Nesting,
+}
+
+impl<'tcx> Visitor<'tcx> for LoopNestVisitor {
+    type Map = Map<'tcx>;
+
+    fn visit_stmt(&mut self, stmt: &'tcx Stmt<'_>) {
+        if stmt.hir_id == self.hir_id {
+            self.nesting = LookFurther;
+        } else if self.nesting == Unknown {
+            walk_stmt(self, stmt);
+        }
+    }
+
+    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
+        if self.nesting != Unknown {
+            return;
+        }
+        if expr.hir_id == self.hir_id {
+            self.nesting = LookFurther;
+            return;
+        }
+        match expr.kind {
+            ExprKind::Assign(ref path, _, _) | ExprKind::AssignOp(_, ref path, _) => {
+                if path_to_local_id(path, self.iterator) {
+                    self.nesting = RuledOut;
+                }
+            },
+            _ => walk_expr(self, expr),
+        }
+    }
+
+    fn visit_pat(&mut self, pat: &'tcx Pat<'_>) {
+        if self.nesting != Unknown {
+            return;
+        }
+        if let PatKind::Binding(_, id, ..) = pat.kind {
+            if id == self.iterator {
+                self.nesting = RuledOut;
+                return;
+            }
+        }
+        walk_pat(self, pat)
+    }
+
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::None
+    }
+}
+
+// this function assumes the given expression is a `for` loop.
+pub(super) fn get_span_of_entire_for_loop(expr: &Expr<'_>) -> Span {
+    // for some reason this is the only way to get the `Span`
+    // of the entire `for` loop
+    if let ExprKind::Match(_, arms, _) = &expr.kind {
+        arms[0].body.span
+    } else {
+        unreachable!()
+    }
+}
+
+/// If `arg` was the argument to a `for` loop, return the "cleanest" way of writing the
+/// actual `Iterator` that the loop uses.
+pub(super) fn make_iterator_snippet(cx: &LateContext<'_>, arg: &Expr<'_>, applic_ref: &mut Applicability) -> String {
+    let impls_iterator = get_trait_def_id(cx, &paths::ITERATOR).map_or(false, |id| {
+        implements_trait(cx, cx.typeck_results().expr_ty(arg), id, &[])
+    });
+    if impls_iterator {
+        format!(
+            "{}",
+            sugg::Sugg::hir_with_applicability(cx, arg, "_", applic_ref).maybe_par()
+        )
+    } else {
+        // (&x).into_iter() ==> x.iter()
+        // (&mut x).into_iter() ==> x.iter_mut()
+        match &arg.kind {
+            ExprKind::AddrOf(BorrowKind::Ref, mutability, arg_inner)
+                if has_iter_method(cx, cx.typeck_results().expr_ty(&arg_inner)).is_some() =>
+            {
+                let meth_name = match mutability {
+                    Mutability::Mut => "iter_mut",
+                    Mutability::Not => "iter",
+                };
+                format!(
+                    "{}.{}()",
+                    sugg::Sugg::hir_with_applicability(cx, &arg_inner, "_", applic_ref).maybe_par(),
+                    meth_name,
+                )
+            }
+            _ => format!(
+                "{}.into_iter()",
+                sugg::Sugg::hir_with_applicability(cx, arg, "_", applic_ref).maybe_par()
+            ),
+        }
+    }
+}

src/tools/clippy/clippy_lints/src/loops/while_immutable_condition.rs

@@ -0,0 +1,139 @@
+use super::WHILE_IMMUTABLE_CONDITION;
+use crate::consts::constant;
+use crate::utils::span_lint_and_then;
+use crate::utils::usage::mutated_variables;
+use if_chain::if_chain;
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_hir::def::{DefKind, Res};
+use rustc_hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
+use rustc_hir::{def_id, Expr, ExprKind, HirId, QPath};
+use rustc_lint::LateContext;
+use rustc_middle::hir::map::Map;
+use std::iter::Iterator;
+
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, cond: &'tcx Expr<'_>, expr: &'tcx Expr<'_>) {
+    if constant(cx, cx.typeck_results(), cond).is_some() {
+        // A pure constant condition (e.g., `while false`) is not linted.
+        return;
+    }
+
+    let mut var_visitor = VarCollectorVisitor {
+        cx,
+        ids: FxHashSet::default(),
+        def_ids: FxHashMap::default(),
+        skip: false,
+    };
+    var_visitor.visit_expr(cond);
+    if var_visitor.skip {
+        return;
+    }
+    let used_in_condition = &var_visitor.ids;
+    let no_cond_variable_mutated = if let Some(used_mutably) = mutated_variables(expr, cx) {
+        used_in_condition.is_disjoint(&used_mutably)
+    } else {
+        return;
+    };
+    let mutable_static_in_cond = var_visitor.def_ids.iter().any(|(_, v)| *v);
+
+    let mut has_break_or_return_visitor = HasBreakOrReturnVisitor {
+        has_break_or_return: false,
+    };
+    has_break_or_return_visitor.visit_expr(expr);
+    let has_break_or_return = has_break_or_return_visitor.has_break_or_return;
+
+    if no_cond_variable_mutated && !mutable_static_in_cond {
+        span_lint_and_then(
+            cx,
+            WHILE_IMMUTABLE_CONDITION,
+            cond.span,
+            "variables in the condition are not mutated in the loop body",
+            |diag| {
+                diag.note("this may lead to an infinite or to a never running loop");
+
+                if has_break_or_return {
+                    diag.note("this loop contains `return`s or `break`s");
+                    diag.help("rewrite it as `if cond { loop { } }`");
+                }
+            },
+        );
+    }
+}
+
+struct HasBreakOrReturnVisitor {
+    has_break_or_return: bool,
+}
+
+impl<'tcx> Visitor<'tcx> for HasBreakOrReturnVisitor {
+    type Map = Map<'tcx>;
+
+    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
+        if self.has_break_or_return {
+            return;
+        }
+
+        match expr.kind {
+            ExprKind::Ret(_) | ExprKind::Break(_, _) => {
+                self.has_break_or_return = true;
+                return;
+            },
+            _ => {},
+        }
+
+        walk_expr(self, expr);
+    }
+
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::None
+    }
+}
+
+/// Collects the set of variables in an expression
+/// Stops analysis if a function call is found
+/// Note: In some cases such as `self`, there are no mutable annotation,
+/// All variables definition IDs are collected
+struct VarCollectorVisitor<'a, 'tcx> {
+    cx: &'a LateContext<'tcx>,
+    ids: FxHashSet<HirId>,
+    def_ids: FxHashMap<def_id::DefId, bool>,
+    skip: bool,
+}
+
+impl<'a, 'tcx> VarCollectorVisitor<'a, 'tcx> {
+    fn insert_def_id(&mut self, ex: &'tcx Expr<'_>) {
+        if_chain! {
+            if let ExprKind::Path(ref qpath) = ex.kind;
+            if let QPath::Resolved(None, _) = *qpath;
+            let res = self.cx.qpath_res(qpath, ex.hir_id);
+            then {
+                match res {
+                    Res::Local(hir_id) => {
+                        self.ids.insert(hir_id);
+                    },
+                    Res::Def(DefKind::Static, def_id) => {
+                        let mutable = self.cx.tcx.is_mutable_static(def_id);
+                        self.def_ids.insert(def_id, mutable);
+                    },
+                    _ => {},
+                }
+            }
+        }
+    }
+}
+
+impl<'a, 'tcx> Visitor<'tcx> for VarCollectorVisitor<'a, 'tcx> {
+    type Map = Map<'tcx>;
+
+    fn visit_expr(&mut self, ex: &'tcx Expr<'_>) {
+        match ex.kind {
+            ExprKind::Path(_) => self.insert_def_id(ex),
+            // If there is any function/method call… we just stop analysis
+            ExprKind::Call(..) | ExprKind::MethodCall(..) => self.skip = true,
+
+            _ => walk_expr(self, ex),
+        }
+    }
+
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::None
+    }
+}

src/tools/clippy/clippy_lints/src/loops/while_let_loop.rs

@@ -0,0 +1,87 @@
+use super::WHILE_LET_LOOP;
+use crate::utils::{snippet_with_applicability, span_lint_and_sugg};
+use rustc_errors::Applicability;
+use rustc_hir::{Block, Expr, ExprKind, MatchSource, StmtKind};
+use rustc_lint::{LateContext, LintContext};
+use rustc_middle::lint::in_external_macro;
+
+pub(super) fn check(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, loop_block: &'tcx Block<'_>) {
+    // extract the expression from the first statement (if any) in a block
+    let inner_stmt_expr = extract_expr_from_first_stmt(loop_block);
+    // or extract the first expression (if any) from the block
+    if let Some(inner) = inner_stmt_expr.or_else(|| extract_first_expr(loop_block)) {
+        if let ExprKind::Match(ref matchexpr, ref arms, ref source) = inner.kind {
+            // ensure "if let" compatible match structure
+            match *source {
+                MatchSource::Normal | MatchSource::IfLetDesugar { .. } => {
+                    if arms.len() == 2
+                        && arms[0].guard.is_none()
+                        && arms[1].guard.is_none()
+                        && is_simple_break_expr(&arms[1].body)
+                    {
+                        if in_external_macro(cx.sess(), expr.span) {
+                            return;
+                        }
+
+                        // NOTE: we used to build a body here instead of using
+                        // ellipsis, this was removed because:
+                        // 1) it was ugly with big bodies;
+                        // 2) it was not indented properly;
+                        // 3) it wasn’t very smart (see #675).
+                        let mut applicability = Applicability::HasPlaceholders;
+                        span_lint_and_sugg(
+                            cx,
+                            WHILE_LET_LOOP,
+                            expr.span,
+                            "this loop could be written as a `while let` loop",
+                            "try",
+                            format!(
+                                "while let {} = {} {{ .. }}",
+                                snippet_with_applicability(cx, arms[0].pat.span, "..", &mut applicability),
+                                snippet_with_applicability(cx, matchexpr.span, "..", &mut applicability),
+                            ),
+                            applicability,
+                        );
+                    }
+                },
+                _ => (),
+            }
+        }
+    }
+}
+
+/// If a block begins with a statement (possibly a `let` binding) and has an
+/// expression, return it.
+fn extract_expr_from_first_stmt<'tcx>(block: &Block<'tcx>) -> Option<&'tcx Expr<'tcx>> {
+    if block.stmts.is_empty() {
+        return None;
+    }
+    if let StmtKind::Local(ref local) = block.stmts[0].kind {
+        local.init //.map(|expr| expr)
+    } else {
+        None
+    }
+}
+
+/// If a block begins with an expression (with or without semicolon), return it.
+fn extract_first_expr<'tcx>(block: &Block<'tcx>) -> Option<&'tcx Expr<'tcx>> {
+    match block.expr {
+        Some(ref expr) if block.stmts.is_empty() => Some(expr),
+        None if !block.stmts.is_empty() => match block.stmts[0].kind {
+            StmtKind::Expr(ref expr) | StmtKind::Semi(ref expr) => Some(expr),
+            StmtKind::Local(..) | StmtKind::Item(..) => None,
+        },
+        _ => None,
+    }
+}
+
+/// Returns `true` if expr contains a single break expr without destination label
+/// and
+/// passed expression. The expression may be within a block.
+fn is_simple_break_expr(expr: &Expr<'_>) -> bool {
+    match expr.kind {
+        ExprKind::Break(dest, ref passed_expr) if dest.label.is_none() && passed_expr.is_none() => true,
+        ExprKind::Block(ref b, _) => extract_first_expr(b).map_or(false, |subexpr| is_simple_break_expr(subexpr)),
+        _ => false,
+    }
+}

src/tools/clippy/clippy_lints/src/loops/while_let_on_iterator.rs

@@ -0,0 +1,171 @@
+use super::utils::{LoopNestVisitor, Nesting};
+use super::WHILE_LET_ON_ITERATOR;
+use crate::utils::usage::mutated_variables;
+use crate::utils::{
+    get_enclosing_block, get_trait_def_id, implements_trait, is_refutable, last_path_segment, match_trait_method,
+    path_to_local, path_to_local_id, paths, snippet_with_applicability, span_lint_and_sugg,
+};
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir::intravisit::{walk_block, walk_expr, NestedVisitorMap, Visitor};
+use rustc_hir::{Expr, ExprKind, HirId, MatchSource, Node, PatKind};
+use rustc_lint::LateContext;
+use rustc_middle::hir::map::Map;
+
+use rustc_span::symbol::sym;
+
+pub(super) fn check(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+    if let ExprKind::Match(ref match_expr, ref arms, MatchSource::WhileLetDesugar) = expr.kind {
+        let pat = &arms[0].pat.kind;
+        if let (
+            &PatKind::TupleStruct(ref qpath, ref pat_args, _),
+            &ExprKind::MethodCall(ref method_path, _, ref method_args, _),
+        ) = (pat, &match_expr.kind)
+        {
+            let iter_expr = &method_args[0];
+
+            // Don't lint when the iterator is recreated on every iteration
+            if_chain! {
+                if let ExprKind::MethodCall(..) | ExprKind::Call(..) = iter_expr.kind;
+                if let Some(iter_def_id) = get_trait_def_id(cx, &paths::ITERATOR);
+                if implements_trait(cx, cx.typeck_results().expr_ty(iter_expr), iter_def_id, &[]);
+                then {
+                    return;
+                }
+            }
+
+            let lhs_constructor = last_path_segment(qpath);
+            if method_path.ident.name == sym::next
+                && match_trait_method(cx, match_expr, &paths::ITERATOR)
+                && lhs_constructor.ident.name == sym::Some
+                && (pat_args.is_empty()
+                    || !is_refutable(cx, &pat_args[0])
+                        && !is_used_inside(cx, iter_expr, &arms[0].body)
+                        && !is_iterator_used_after_while_let(cx, iter_expr)
+                        && !is_nested(cx, expr, &method_args[0]))
+            {
+                let mut applicability = Applicability::MachineApplicable;
+                let iterator = snippet_with_applicability(cx, method_args[0].span, "_", &mut applicability);
+                let loop_var = if pat_args.is_empty() {
+                    "_".to_string()
+                } else {
+                    snippet_with_applicability(cx, pat_args[0].span, "_", &mut applicability).into_owned()
+                };
+                span_lint_and_sugg(
+                    cx,
+                    WHILE_LET_ON_ITERATOR,
+                    expr.span.with_hi(match_expr.span.hi()),
+                    "this loop could be written as a `for` loop",
+                    "try",
+                    format!("for {} in {}", loop_var, iterator),
+                    applicability,
+                );
+            }
+        }
+    }
+}
+
+fn is_used_inside<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, container: &'tcx Expr<'_>) -> bool {
+    let def_id = match path_to_local(expr) {
+        Some(id) => id,
+        None => return false,
+    };
+    if let Some(used_mutably) = mutated_variables(container, cx) {
+        if used_mutably.contains(&def_id) {
+            return true;
+        }
+    }
+    false
+}
+
+fn is_iterator_used_after_while_let<'tcx>(cx: &LateContext<'tcx>, iter_expr: &'tcx Expr<'_>) -> bool {
+    let def_id = match path_to_local(iter_expr) {
+        Some(id) => id,
+        None => return false,
+    };
+    let mut visitor = VarUsedAfterLoopVisitor {
+        def_id,
+        iter_expr_id: iter_expr.hir_id,
+        past_while_let: false,
+        var_used_after_while_let: false,
+    };
+    if let Some(enclosing_block) = get_enclosing_block(cx, def_id) {
+        walk_block(&mut visitor, enclosing_block);
+    }
+    visitor.var_used_after_while_let
+}
+
+fn is_nested(cx: &LateContext<'_>, match_expr: &Expr<'_>, iter_expr: &Expr<'_>) -> bool {
+    if_chain! {
+        if let Some(loop_block) = get_enclosing_block(cx, match_expr.hir_id);
+        let parent_node = cx.tcx.hir().get_parent_node(loop_block.hir_id);
+        if let Some(Node::Expr(loop_expr)) = cx.tcx.hir().find(parent_node);
+        then {
+            return is_loop_nested(cx, loop_expr, iter_expr)
+        }
+    }
+    false
+}
+
+fn is_loop_nested(cx: &LateContext<'_>, loop_expr: &Expr<'_>, iter_expr: &Expr<'_>) -> bool {
+    let mut id = loop_expr.hir_id;
+    let iter_id = if let Some(id) = path_to_local(iter_expr) {
+        id
+    } else {
+        return true;
+    };
+    loop {
+        let parent = cx.tcx.hir().get_parent_node(id);
+        if parent == id {
+            return false;
+        }
+        match cx.tcx.hir().find(parent) {
+            Some(Node::Expr(expr)) => {
+                if let ExprKind::Loop(..) = expr.kind {
+                    return true;
+                };
+            },
+            Some(Node::Block(block)) => {
+                let mut block_visitor = LoopNestVisitor {
+                    hir_id: id,
+                    iterator: iter_id,
+                    nesting: Nesting::Unknown,
+                };
+                walk_block(&mut block_visitor, block);
+                if block_visitor.nesting == Nesting::RuledOut {
+                    return false;
+                }
+            },
+            Some(Node::Stmt(_)) => (),
+            _ => {
+                return false;
+            },
+        }
+        id = parent;
+    }
+}
+
+struct VarUsedAfterLoopVisitor {
+    def_id: HirId,
+    iter_expr_id: HirId,
+    past_while_let: bool,
+    var_used_after_while_let: bool,
+}
+
+impl<'tcx> Visitor<'tcx> for VarUsedAfterLoopVisitor {
+    type Map = Map<'tcx>;
+
+    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
+        if self.past_while_let {
+            if path_to_local_id(expr, self.def_id) {
+                self.var_used_after_while_let = true;
+            }
+        } else if self.iter_expr_id == expr.hir_id {
+            self.past_while_let = true;
+        }
+        walk_expr(self, expr);
+    }
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::None
+    }
+}

src/tools/clippy/clippy_lints/src/manual_map.rs

@@ -2,17 +2,25 @@ use crate::{
     map_unit_fn::OPTION_MAP_UNIT_FN,
     matches::MATCH_AS_REF,
     utils::{
-        is_allowed, is_type_diagnostic_item, match_def_path, match_var, paths, peel_hir_expr_refs,
-        peel_mid_ty_refs_is_mutable, snippet_with_applicability, span_lint_and_sugg,
+        can_partially_move_ty, is_allowed, is_type_diagnostic_item, match_def_path, match_var, paths,
+        peel_hir_expr_refs, peel_mid_ty_refs_is_mutable, snippet_with_applicability, snippet_with_context,
+        span_lint_and_sugg,
     },
 };
 use rustc_ast::util::parser::PREC_POSTFIX;
 use rustc_errors::Applicability;
-use rustc_hir::{Arm, BindingAnnotation, Block, Expr, ExprKind, Mutability, Pat, PatKind, QPath};
+use rustc_hir::{
+    def::Res,
+    intravisit::{walk_expr, ErasedMap, NestedVisitorMap, Visitor},
+    Arm, BindingAnnotation, Block, Expr, ExprKind, Mutability, Pat, PatKind, Path, QPath,
+};
 use rustc_lint::{LateContext, LateLintPass, LintContext};
 use rustc_middle::lint::in_external_macro;
 use rustc_session::{declare_lint_pass, declare_tool_lint};
-use rustc_span::symbol::{sym, Ident};
+use rustc_span::{
+    symbol::{sym, Ident},
+    SyntaxContext,
+};
 
 declare_clippy_lint! {
     /// **What it does:** Checks for usages of `match` which could be implemented using `map`
@@ -52,43 +60,46 @@ impl LateLintPass<'_> for ManualMap {
         {
             let (scrutinee_ty, ty_ref_count, ty_mutability) =
                 peel_mid_ty_refs_is_mutable(cx.typeck_results().expr_ty(scrutinee));
-            if !is_type_diagnostic_item(cx, scrutinee_ty, sym::option_type)
-                || !is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(expr), sym::option_type)
+            if !(is_type_diagnostic_item(cx, scrutinee_ty, sym::option_type)
+                && is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(expr), sym::option_type))
             {
                 return;
             }
 
-            let (some_expr, some_pat, pat_ref_count, is_wild_none) =
-                match (try_parse_pattern(cx, arm1.pat), try_parse_pattern(cx, arm2.pat)) {
-                    (Some(OptionPat::Wild), Some(OptionPat::Some { pattern, ref_count }))
-                        if is_none_expr(cx, arm1.body) =>
-                    {
-                        (arm2.body, pattern, ref_count, true)
-                    },
-                    (Some(OptionPat::None), Some(OptionPat::Some { pattern, ref_count }))
-                        if is_none_expr(cx, arm1.body) =>
-                    {
-                        (arm2.body, pattern, ref_count, false)
-                    },
-                    (Some(OptionPat::Some { pattern, ref_count }), Some(OptionPat::Wild))
-                        if is_none_expr(cx, arm2.body) =>
-                    {
-                        (arm1.body, pattern, ref_count, true)
-                    },
-                    (Some(OptionPat::Some { pattern, ref_count }), Some(OptionPat::None))
-                        if is_none_expr(cx, arm2.body) =>
-                    {
-                        (arm1.body, pattern, ref_count, false)
-                    },
-                    _ => return,
-                };
+            let expr_ctxt = expr.span.ctxt();
+            let (some_expr, some_pat, pat_ref_count, is_wild_none) = match (
+                try_parse_pattern(cx, arm1.pat, expr_ctxt),
+                try_parse_pattern(cx, arm2.pat, expr_ctxt),
+            ) {
+                (Some(OptionPat::Wild), Some(OptionPat::Some { pattern, ref_count }))
+                    if is_none_expr(cx, arm1.body) =>
+                {
+                    (arm2.body, pattern, ref_count, true)
+                },
+                (Some(OptionPat::None), Some(OptionPat::Some { pattern, ref_count }))
+                    if is_none_expr(cx, arm1.body) =>
+                {
+                    (arm2.body, pattern, ref_count, false)
+                },
+                (Some(OptionPat::Some { pattern, ref_count }), Some(OptionPat::Wild))
+                    if is_none_expr(cx, arm2.body) =>
+                {
+                    (arm1.body, pattern, ref_count, true)
+                },
+                (Some(OptionPat::Some { pattern, ref_count }), Some(OptionPat::None))
+                    if is_none_expr(cx, arm2.body) =>
+                {
+                    (arm1.body, pattern, ref_count, false)
+                },
+                _ => return,
+            };
 
             // Top level or patterns aren't allowed in closures.
             if matches!(some_pat.kind, PatKind::Or(_)) {
                 return;
             }
 
-            let some_expr = match get_some_expr(cx, some_expr) {
+            let some_expr = match get_some_expr(cx, some_expr, expr_ctxt) {
                 Some(expr) => expr,
                 None => return,
             };
@@ -99,6 +110,10 @@ impl LateLintPass<'_> for ManualMap {
                 return;
             }
 
+            if !can_move_expr_to_closure(cx, some_expr) {
+                return;
+            }
+
             // Determine which binding mode to use.
             let explicit_ref = some_pat.contains_explicit_ref_binding();
             let binding_ref = explicit_ref.or_else(|| (ty_ref_count != pat_ref_count).then(|| ty_mutability));
@@ -111,47 +126,50 @@ impl LateLintPass<'_> for ManualMap {
 
             let mut app = Applicability::MachineApplicable;
 
-            // Remove address-of expressions from the scrutinee. `as_ref` will be called,
-            // the type is copyable, or the option is being passed by value.
+            // Remove address-of expressions from the scrutinee. Either `as_ref` will be called, or
+            // it's being passed by value.
             let scrutinee = peel_hir_expr_refs(scrutinee).0;
-            let scrutinee_str = snippet_with_applicability(cx, scrutinee.span, "_", &mut app);
-            let scrutinee_str = if expr.precedence().order() < PREC_POSTFIX {
-                // Parens are needed to chain method calls.
-                format!("({})", scrutinee_str)
-            } else {
-                scrutinee_str.into()
-            };
+            let scrutinee_str = snippet_with_context(cx, scrutinee.span, expr_ctxt, "..", &mut app);
+            let scrutinee_str =
+                if scrutinee.span.ctxt() == expr.span.ctxt() && scrutinee.precedence().order() < PREC_POSTFIX {
+                    format!("({})", scrutinee_str)
+                } else {
+                    scrutinee_str.into()
+                };
 
             let body_str = if let PatKind::Binding(annotation, _, some_binding, None) = some_pat.kind {
-                if let Some(func) = can_pass_as_func(cx, some_binding, some_expr) {
-                    snippet_with_applicability(cx, func.span, "..", &mut app).into_owned()
-                } else {
-                    if match_var(some_expr, some_binding.name)
-                        && !is_allowed(cx, MATCH_AS_REF, expr.hir_id)
-                        && binding_ref.is_some()
-                    {
-                        return;
-                    }
+                match can_pass_as_func(cx, some_binding, some_expr) {
+                    Some(func) if func.span.ctxt() == some_expr.span.ctxt() => {
+                        snippet_with_applicability(cx, func.span, "..", &mut app).into_owned()
+                    },
+                    _ => {
+                        if match_var(some_expr, some_binding.name)
+                            && !is_allowed(cx, MATCH_AS_REF, expr.hir_id)
+                            && binding_ref.is_some()
+                        {
+                            return;
+                        }
 
-                    // `ref` and `ref mut` annotations were handled earlier.
-                    let annotation = if matches!(annotation, BindingAnnotation::Mutable) {
-                        "mut "
-                    } else {
-                        ""
-                    };
-                    format!(
-                        "|{}{}| {}",
-                        annotation,
-                        some_binding,
-                        snippet_with_applicability(cx, some_expr.span, "..", &mut app)
-                    )
+                        // `ref` and `ref mut` annotations were handled earlier.
+                        let annotation = if matches!(annotation, BindingAnnotation::Mutable) {
+                            "mut "
+                        } else {
+                            ""
+                        };
+                        format!(
+                            "|{}{}| {}",
+                            annotation,
+                            some_binding,
+                            snippet_with_context(cx, some_expr.span, expr_ctxt, "..", &mut app)
+                        )
+                    },
                 }
             } else if !is_wild_none && explicit_ref.is_none() {
                 // TODO: handle explicit reference annotations.
                 format!(
                     "|{}| {}",
-                    snippet_with_applicability(cx, some_pat.span, "..", &mut app),
-                    snippet_with_applicability(cx, some_expr.span, "..", &mut app)
+                    snippet_with_context(cx, some_pat.span, expr_ctxt, "..", &mut app),
+                    snippet_with_context(cx, some_expr.span, expr_ctxt, "..", &mut app)
                 )
             } else {
                 // Refutable bindings and mixed reference annotations can't be handled by `map`.
@@ -171,6 +189,51 @@ impl LateLintPass<'_> for ManualMap {
     }
 }
 
+// Checks if the expression can be moved into a closure as is.
+fn can_move_expr_to_closure(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) -> bool {
+    struct V<'cx, 'tcx> {
+        cx: &'cx LateContext<'tcx>,
+        make_closure: bool,
+    }
+    impl Visitor<'tcx> for V<'_, 'tcx> {
+        type Map = ErasedMap<'tcx>;
+        fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+            NestedVisitorMap::None
+        }
+
+        fn visit_expr(&mut self, e: &'tcx Expr<'_>) {
+            match e.kind {
+                ExprKind::Break(..)
+                | ExprKind::Continue(_)
+                | ExprKind::Ret(_)
+                | ExprKind::Yield(..)
+                | ExprKind::InlineAsm(_)
+                | ExprKind::LlvmInlineAsm(_) => {
+                    self.make_closure = false;
+                },
+                // Accessing a field of a local value can only be done if the type isn't
+                // partially moved.
+                ExprKind::Field(base_expr, _)
+                    if matches!(
+                        base_expr.kind,
+                        ExprKind::Path(QPath::Resolved(_, Path { res: Res::Local(_), .. }))
+                    ) && can_partially_move_ty(self.cx, self.cx.typeck_results().expr_ty(base_expr)) =>
+                {
+                    // TODO: check if the local has been partially moved. Assume it has for now.
+                    self.make_closure = false;
+                    return;
+                }
+                _ => (),
+            };
+            walk_expr(self, e);
+        }
+    }
+
+    let mut v = V { cx, make_closure: true };
+    v.visit_expr(expr);
+    v.make_closure
+}
+
 // Checks whether the expression could be passed as a function, or whether a closure is needed.
 // Returns the function to be passed to `map` if it exists.
 fn can_pass_as_func(cx: &LateContext<'tcx>, binding: Ident, expr: &'tcx Expr<'_>) -> Option<&'tcx Expr<'tcx>> {
@@ -198,11 +261,11 @@ enum OptionPat<'a> {
 
 // Try to parse into a recognized `Option` pattern.
 // i.e. `_`, `None`, `Some(..)`, or a reference to any of those.
-fn try_parse_pattern(cx: &LateContext<'tcx>, pat: &'tcx Pat<'_>) -> Option<OptionPat<'tcx>> {
-    fn f(cx: &LateContext<'tcx>, pat: &'tcx Pat<'_>, ref_count: usize) -> Option<OptionPat<'tcx>> {
+fn try_parse_pattern(cx: &LateContext<'tcx>, pat: &'tcx Pat<'_>, ctxt: SyntaxContext) -> Option<OptionPat<'tcx>> {
+    fn f(cx: &LateContext<'tcx>, pat: &'tcx Pat<'_>, ref_count: usize, ctxt: SyntaxContext) -> Option<OptionPat<'tcx>> {
         match pat.kind {
             PatKind::Wild => Some(OptionPat::Wild),
-            PatKind::Ref(pat, _) => f(cx, pat, ref_count + 1),
+            PatKind::Ref(pat, _) => f(cx, pat, ref_count + 1, ctxt),
             PatKind::Path(QPath::Resolved(None, path))
                 if path
                     .res
@@ -215,18 +278,19 @@ fn try_parse_pattern(cx: &LateContext<'tcx>, pat: &'tcx Pat<'_>) -> Option<Optio
                 if path
                     .res
                     .opt_def_id()
-                    .map_or(false, |id| match_def_path(cx, id, &paths::OPTION_SOME)) =>
+                    .map_or(false, |id| match_def_path(cx, id, &paths::OPTION_SOME))
+                    && pat.span.ctxt() == ctxt =>
             {
                 Some(OptionPat::Some { pattern, ref_count })
             },
             _ => None,
         }
     }
-    f(cx, pat, 0)
+    f(cx, pat, 0, ctxt)
 }
 
 // Checks for an expression wrapped by the `Some` constructor. Returns the contained expression.
-fn get_some_expr(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) -> Option<&'tcx Expr<'tcx>> {
+fn get_some_expr(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, ctxt: SyntaxContext) -> Option<&'tcx Expr<'tcx>> {
     // TODO: Allow more complex expressions.
     match expr.kind {
         ExprKind::Call(
@@ -235,7 +299,7 @@ fn get_some_expr(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) -> Option<&'tcx E
                 ..
             },
             [arg],
-        ) => {
+        ) if ctxt == expr.span.ctxt() => {
             if match_def_path(cx, path.res.opt_def_id()?, &paths::OPTION_SOME) {
                 Some(arg)
             } else {
@@ -249,7 +313,7 @@ fn get_some_expr(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) -> Option<&'tcx E
                 ..
             },
             _,
-        ) => get_some_expr(cx, expr),
+        ) => get_some_expr(cx, expr, ctxt),
         _ => None,
     }
 }

src/tools/clippy/clippy_lints/src/map_clone.rs

@@ -79,7 +79,9 @@ impl<'tcx> LateLintPass<'tcx> for MapClone {
                             },
                             hir::ExprKind::MethodCall(ref method, _, [obj], _) => if_chain! {
                                 if ident_eq(name, obj) && method.ident.name == sym::clone;
-                                if match_trait_method(cx, closure_expr, &paths::CLONE_TRAIT);
+                                if let Some(fn_id) = cx.typeck_results().type_dependent_def_id(closure_expr.hir_id);
+                                if let Some(trait_id) = cx.tcx.trait_of_item(fn_id);
+                                if cx.tcx.lang_items().clone_trait().map_or(false, |id| id == trait_id);
                                 // no autoderefs
                                 if !cx.typeck_results().expr_adjustments(obj).iter()
                                     .any(|a| matches!(a.kind, Adjust::Deref(Some(..))));

src/tools/clippy/clippy_lints/src/matches.rs

@@ -1173,9 +1173,9 @@ fn check_wild_in_or_pats(cx: &LateContext<'_>, arms: &[Arm<'_>]) {
                     cx,
                     WILDCARD_IN_OR_PATTERNS,
                     arm.pat.span,
-                    "wildcard pattern covers any other pattern as it will match anyway.",
+                    "wildcard pattern covers any other pattern as it will match anyway",
                     None,
-                    "Consider handling `_` separately.",
+                    "consider handling `_` separately",
                 );
             }
         }

src/tools/clippy/clippy_lints/src/methods/bind_instead_of_map.rs

@@ -158,7 +158,7 @@ pub(crate) trait BindInsteadOfMap {
     }
 
     /// Lint use of `_.and_then(|x| Some(y))` for `Option`s
-    fn lint(cx: &LateContext<'_>, expr: &hir::Expr<'_>, args: &[hir::Expr<'_>]) -> bool {
+    fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, args: &[hir::Expr<'_>]) -> bool {
         if !match_type(cx, cx.typeck_results().expr_ty(&args[0]), Self::TYPE_QPATH) {
             return false;
         }

src/tools/clippy/clippy_lints/src/methods/bytes_nth.rs

@@ -7,7 +7,7 @@ use rustc_span::sym;
 
 use super::BYTES_NTH;
 
-pub(super) fn lints<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'_>, iter_args: &'tcx [Expr<'tcx>]) {
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'_>, iter_args: &'tcx [Expr<'tcx>]) {
     if_chain! {
         if let ExprKind::MethodCall(_, _, ref args, _) = expr.kind;
         let ty = cx.typeck_results().expr_ty(&iter_args[0]).peel_refs();

src/tools/clippy/clippy_lints/src/methods/clone_on_copy.rs

@@ -0,0 +1,109 @@
+use crate::utils::{is_copy, span_lint_and_then, sugg};
+use rustc_errors::Applicability;
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, Ty};
+use std::iter;
+
+use super::CLONE_DOUBLE_REF;
+use super::CLONE_ON_COPY;
+
+/// Checks for the `CLONE_ON_COPY` lint.
+pub(super) fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, arg: &hir::Expr<'_>, arg_ty: Ty<'_>) {
+    let ty = cx.typeck_results().expr_ty(expr);
+    if let ty::Ref(_, inner, _) = arg_ty.kind() {
+        if let ty::Ref(_, innermost, _) = inner.kind() {
+            span_lint_and_then(
+                cx,
+                CLONE_DOUBLE_REF,
+                expr.span,
+                &format!(
+                    "using `clone` on a double-reference; \
+                    this will copy the reference of type `{}` instead of cloning the inner type",
+                    ty
+                ),
+                |diag| {
+                    if let Some(snip) = sugg::Sugg::hir_opt(cx, arg) {
+                        let mut ty = innermost;
+                        let mut n = 0;
+                        while let ty::Ref(_, inner, _) = ty.kind() {
+                            ty = inner;
+                            n += 1;
+                        }
+                        let refs: String = iter::repeat('&').take(n + 1).collect();
+                        let derefs: String = iter::repeat('*').take(n).collect();
+                        let explicit = format!("<{}{}>::clone({})", refs, ty, snip);
+                        diag.span_suggestion(
+                            expr.span,
+                            "try dereferencing it",
+                            format!("{}({}{}).clone()", refs, derefs, snip.deref()),
+                            Applicability::MaybeIncorrect,
+                        );
+                        diag.span_suggestion(
+                            expr.span,
+                            "or try being explicit if you are sure, that you want to clone a reference",
+                            explicit,
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
+                },
+            );
+            return; // don't report clone_on_copy
+        }
+    }
+
+    if is_copy(cx, ty) {
+        let snip;
+        if let Some(snippet) = sugg::Sugg::hir_opt(cx, arg) {
+            let parent = cx.tcx.hir().get_parent_node(expr.hir_id);
+            match &cx.tcx.hir().get(parent) {
+                hir::Node::Expr(parent) => match parent.kind {
+                    // &*x is a nop, &x.clone() is not
+                    hir::ExprKind::AddrOf(..) => return,
+                    // (*x).func() is useless, x.clone().func() can work in case func borrows mutably
+                    hir::ExprKind::MethodCall(_, _, parent_args, _) if expr.hir_id == parent_args[0].hir_id => {
+                        return;
+                    },
+
+                    _ => {},
+                },
+                hir::Node::Stmt(stmt) => {
+                    if let hir::StmtKind::Local(ref loc) = stmt.kind {
+                        if let hir::PatKind::Ref(..) = loc.pat.kind {
+                            // let ref y = *x borrows x, let ref y = x.clone() does not
+                            return;
+                        }
+                    }
+                },
+                _ => {},
+            }
+
+            // x.clone() might have dereferenced x, possibly through Deref impls
+            if cx.typeck_results().expr_ty(arg) == ty {
+                snip = Some(("try removing the `clone` call", format!("{}", snippet)));
+            } else {
+                let deref_count = cx
+                    .typeck_results()
+                    .expr_adjustments(arg)
+                    .iter()
+                    .filter(|adj| matches!(adj.kind, ty::adjustment::Adjust::Deref(_)))
+                    .count();
+                let derefs: String = iter::repeat('*').take(deref_count).collect();
+                snip = Some(("try dereferencing it", format!("{}{}", derefs, snippet)));
+            }
+        } else {
+            snip = None;
+        }
+        span_lint_and_then(
+            cx,
+            CLONE_ON_COPY,
+            expr.span,
+            &format!("using `clone` on type `{}` which implements the `Copy` trait", ty),
+            |diag| {
+                if let Some((text, snip)) = snip {
+                    diag.span_suggestion(expr.span, text, snip, Applicability::MachineApplicable);
+                }
+            },
+        );
+    }
+}

src/tools/clippy/clippy_lints/src/methods/clone_on_ref_ptr.rs

@@ -0,0 +1,36 @@
+use crate::utils::{is_type_diagnostic_item, match_type, paths, snippet_with_macro_callsite, span_lint_and_sugg};
+use rustc_errors::Applicability;
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+use rustc_middle::ty;
+use rustc_span::symbol::sym;
+
+use super::CLONE_ON_REF_PTR;
+
+pub(super) fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, arg: &hir::Expr<'_>) {
+    let obj_ty = cx.typeck_results().expr_ty(arg).peel_refs();
+
+    if let ty::Adt(_, subst) = obj_ty.kind() {
+        let caller_type = if is_type_diagnostic_item(cx, obj_ty, sym::Rc) {
+            "Rc"
+        } else if is_type_diagnostic_item(cx, obj_ty, sym::Arc) {
+            "Arc"
+        } else if match_type(cx, obj_ty, &paths::WEAK_RC) || match_type(cx, obj_ty, &paths::WEAK_ARC) {
+            "Weak"
+        } else {
+            return;
+        };
+
+        let snippet = snippet_with_macro_callsite(cx, arg.span, "..");
+
+        span_lint_and_sugg(
+            cx,
+            CLONE_ON_REF_PTR,
+            expr.span,
+            "using `.clone()` on a ref-counted pointer",
+            "try this",
+            format!("{}::<{}>::clone(&{})", caller_type, subst.type_at(0), snippet),
+            Applicability::Unspecified, // Sometimes unnecessary ::<_> after Rc/Arc/Weak
+        );
+    }
+}

src/tools/clippy/clippy_lints/src/methods/expect_fun_call.rs

@@ -0,0 +1,199 @@
+use crate::utils::{is_expn_of, is_type_diagnostic_item, snippet, snippet_with_applicability, span_lint_and_sugg};
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+use rustc_middle::ty;
+use rustc_span::source_map::Span;
+use rustc_span::symbol::sym;
+use std::borrow::Cow;
+
+use super::EXPECT_FUN_CALL;
+
+/// Checks for the `EXPECT_FUN_CALL` lint.
+#[allow(clippy::too_many_lines)]
+pub(super) fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, method_span: Span, name: &str, args: &[hir::Expr<'_>]) {
+    // Strip `&`, `as_ref()` and `as_str()` off `arg` until we're left with either a `String` or
+    // `&str`
+    fn get_arg_root<'a>(cx: &LateContext<'_>, arg: &'a hir::Expr<'a>) -> &'a hir::Expr<'a> {
+        let mut arg_root = arg;
+        loop {
+            arg_root = match &arg_root.kind {
+                hir::ExprKind::AddrOf(hir::BorrowKind::Ref, _, expr) => expr,
+                hir::ExprKind::MethodCall(method_name, _, call_args, _) => {
+                    if call_args.len() == 1
+                        && (method_name.ident.name == sym::as_str || method_name.ident.name == sym!(as_ref))
+                        && {
+                            let arg_type = cx.typeck_results().expr_ty(&call_args[0]);
+                            let base_type = arg_type.peel_refs();
+                            *base_type.kind() == ty::Str || is_type_diagnostic_item(cx, base_type, sym::string_type)
+                        }
+                    {
+                        &call_args[0]
+                    } else {
+                        break;
+                    }
+                },
+                _ => break,
+            };
+        }
+        arg_root
+    }
+
+    // Only `&'static str` or `String` can be used directly in the `panic!`. Other types should be
+    // converted to string.
+    fn requires_to_string(cx: &LateContext<'_>, arg: &hir::Expr<'_>) -> bool {
+        let arg_ty = cx.typeck_results().expr_ty(arg);
+        if is_type_diagnostic_item(cx, arg_ty, sym::string_type) {
+            return false;
+        }
+        if let ty::Ref(_, ty, ..) = arg_ty.kind() {
+            if *ty.kind() == ty::Str && can_be_static_str(cx, arg) {
+                return false;
+            }
+        };
+        true
+    }
+
+    // Check if an expression could have type `&'static str`, knowing that it
+    // has type `&str` for some lifetime.
+    fn can_be_static_str(cx: &LateContext<'_>, arg: &hir::Expr<'_>) -> bool {
+        match arg.kind {
+            hir::ExprKind::Lit(_) => true,
+            hir::ExprKind::Call(fun, _) => {
+                if let hir::ExprKind::Path(ref p) = fun.kind {
+                    match cx.qpath_res(p, fun.hir_id) {
+                        hir::def::Res::Def(hir::def::DefKind::Fn | hir::def::DefKind::AssocFn, def_id) => matches!(
+                            cx.tcx.fn_sig(def_id).output().skip_binder().kind(),
+                            ty::Ref(ty::ReStatic, ..)
+                        ),
+                        _ => false,
+                    }
+                } else {
+                    false
+                }
+            },
+            hir::ExprKind::MethodCall(..) => {
+                cx.typeck_results()
+                    .type_dependent_def_id(arg.hir_id)
+                    .map_or(false, |method_id| {
+                        matches!(
+                            cx.tcx.fn_sig(method_id).output().skip_binder().kind(),
+                            ty::Ref(ty::ReStatic, ..)
+                        )
+                    })
+            },
+            hir::ExprKind::Path(ref p) => matches!(
+                cx.qpath_res(p, arg.hir_id),
+                hir::def::Res::Def(hir::def::DefKind::Const | hir::def::DefKind::Static, _)
+            ),
+            _ => false,
+        }
+    }
+
+    fn generate_format_arg_snippet(
+        cx: &LateContext<'_>,
+        a: &hir::Expr<'_>,
+        applicability: &mut Applicability,
+    ) -> Vec<String> {
+        if_chain! {
+            if let hir::ExprKind::AddrOf(hir::BorrowKind::Ref, _, ref format_arg) = a.kind;
+            if let hir::ExprKind::Match(ref format_arg_expr, _, _) = format_arg.kind;
+            if let hir::ExprKind::Tup(ref format_arg_expr_tup) = format_arg_expr.kind;
+
+            then {
+                format_arg_expr_tup
+                    .iter()
+                    .map(|a| snippet_with_applicability(cx, a.span, "..", applicability).into_owned())
+                    .collect()
+            } else {
+                unreachable!()
+            }
+        }
+    }
+
+    fn is_call(node: &hir::ExprKind<'_>) -> bool {
+        match node {
+            hir::ExprKind::AddrOf(hir::BorrowKind::Ref, _, expr) => {
+                is_call(&expr.kind)
+            },
+            hir::ExprKind::Call(..)
+            | hir::ExprKind::MethodCall(..)
+            // These variants are debatable or require further examination
+            | hir::ExprKind::If(..)
+            | hir::ExprKind::Match(..)
+            | hir::ExprKind::Block{ .. } => true,
+            _ => false,
+        }
+    }
+
+    if args.len() != 2 || name != "expect" || !is_call(&args[1].kind) {
+        return;
+    }
+
+    let receiver_type = cx.typeck_results().expr_ty_adjusted(&args[0]);
+    let closure_args = if is_type_diagnostic_item(cx, receiver_type, sym::option_type) {
+        "||"
+    } else if is_type_diagnostic_item(cx, receiver_type, sym::result_type) {
+        "|_|"
+    } else {
+        return;
+    };
+
+    let arg_root = get_arg_root(cx, &args[1]);
+
+    let span_replace_word = method_span.with_hi(expr.span.hi());
+
+    let mut applicability = Applicability::MachineApplicable;
+
+    //Special handling for `format!` as arg_root
+    if_chain! {
+        if let hir::ExprKind::Block(block, None) = &arg_root.kind;
+        if block.stmts.len() == 1;
+        if let hir::StmtKind::Local(local) = &block.stmts[0].kind;
+        if let Some(arg_root) = &local.init;
+        if let hir::ExprKind::Call(ref inner_fun, ref inner_args) = arg_root.kind;
+        if is_expn_of(inner_fun.span, "format").is_some() && inner_args.len() == 1;
+        if let hir::ExprKind::Call(_, format_args) = &inner_args[0].kind;
+        then {
+            let fmt_spec = &format_args[0];
+            let fmt_args = &format_args[1];
+
+            let mut args = vec![snippet(cx, fmt_spec.span, "..").into_owned()];
+
+            args.extend(generate_format_arg_snippet(cx, fmt_args, &mut applicability));
+
+            let sugg = args.join(", ");
+
+            span_lint_and_sugg(
+                cx,
+                EXPECT_FUN_CALL,
+                span_replace_word,
+                &format!("use of `{}` followed by a function call", name),
+                "try this",
+                format!("unwrap_or_else({} panic!({}))", closure_args, sugg),
+                applicability,
+            );
+
+            return;
+        }
+    }
+
+    let mut arg_root_snippet: Cow<'_, _> = snippet_with_applicability(cx, arg_root.span, "..", &mut applicability);
+    if requires_to_string(cx, arg_root) {
+        arg_root_snippet.to_mut().push_str(".to_string()");
+    }
+
+    span_lint_and_sugg(
+        cx,
+        EXPECT_FUN_CALL,
+        span_replace_word,
+        &format!("use of `{}` followed by a function call", name),
+        "try this",
+        format!(
+            "unwrap_or_else({} {{ panic!(\"{{}}\", {}) }})",
+            closure_args, arg_root_snippet
+        ),
+        applicability,
+    );
+}

src/tools/clippy/clippy_lints/src/methods/expect_used.rs

@@ -0,0 +1,30 @@
+use crate::utils::{is_type_diagnostic_item, span_lint_and_help};
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+use rustc_span::sym;
+
+use super::EXPECT_USED;
+
+/// lint use of `expect()` for `Option`s and `Result`s
+pub(super) fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, expect_args: &[hir::Expr<'_>]) {
+    let obj_ty = cx.typeck_results().expr_ty(&expect_args[0]).peel_refs();
+
+    let mess = if is_type_diagnostic_item(cx, obj_ty, sym::option_type) {
+        Some((EXPECT_USED, "an Option", "None"))
+    } else if is_type_diagnostic_item(cx, obj_ty, sym::result_type) {
+        Some((EXPECT_USED, "a Result", "Err"))
+    } else {
+        None
+    };
+
+    if let Some((lint, kind, none_value)) = mess {
+        span_lint_and_help(
+            cx,
+            lint,
+            expr.span,
+            &format!("used `expect()` on `{}` value", kind,),
+            None,
+            &format!("if this value is an `{}`, it will panic", none_value,),
+        );
+    }
+}

src/tools/clippy/clippy_lints/src/methods/filetype_is_file.rs

@@ -0,0 +1,39 @@
+use crate::utils::{get_parent_expr, match_type, paths, span_lint_and_help};
+use if_chain::if_chain;
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+use rustc_span::source_map::Span;
+
+use super::FILETYPE_IS_FILE;
+
+pub(super) fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, args: &[hir::Expr<'_>]) {
+    let ty = cx.typeck_results().expr_ty(&args[0]);
+
+    if !match_type(cx, ty, &paths::FILE_TYPE) {
+        return;
+    }
+
+    let span: Span;
+    let verb: &str;
+    let lint_unary: &str;
+    let help_unary: &str;
+    if_chain! {
+        if let Some(parent) = get_parent_expr(cx, expr);
+        if let hir::ExprKind::Unary(op, _) = parent.kind;
+        if op == hir::UnOp::Not;
+        then {
+            lint_unary = "!";
+            verb = "denies";
+            help_unary = "";
+            span = parent.span;
+        } else {
+            lint_unary = "";
+            verb = "covers";
+            help_unary = "!";
+            span = expr.span;
+        }
+    }
+    let lint_msg = format!("`{}FileType::is_file()` only {} regular files", lint_unary, verb);
+    let help_msg = format!("use `{}FileType::is_dir()` instead", help_unary);
+    span_lint_and_help(cx, FILETYPE_IS_FILE, span, &lint_msg, None, &help_msg);
+}

src/tools/clippy/clippy_lints/src/methods/filter_flat_map.rs

@@ -0,0 +1,21 @@
+use crate::utils::{match_trait_method, paths, span_lint_and_help};
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+
+use super::FILTER_MAP;
+
+/// lint use of `filter().flat_map()` for `Iterators`
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    expr: &'tcx hir::Expr<'_>,
+    _filter_args: &'tcx [hir::Expr<'_>],
+    _map_args: &'tcx [hir::Expr<'_>],
+) {
+    // lint if caller of `.filter().flat_map()` is an Iterator
+    if match_trait_method(cx, expr, &paths::ITERATOR) {
+        let msg = "called `filter(..).flat_map(..)` on an `Iterator`";
+        let hint = "this is more succinctly expressed by calling `.flat_map(..)` \
+                    and filtering by returning `iter::empty()`";
+        span_lint_and_help(cx, FILTER_MAP, expr.span, msg, None, hint);
+    }
+}

src/tools/clippy/clippy_lints/src/methods/filter_map.rs

@@ -0,0 +1,85 @@
+use crate::utils::{match_trait_method, path_to_local_id, paths, snippet, span_lint_and_sugg, SpanlessEq};
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir as hir;
+use rustc_hir::{Expr, ExprKind, PatKind, UnOp};
+use rustc_lint::LateContext;
+use rustc_middle::ty::TyS;
+use rustc_span::symbol::sym;
+
+use super::MANUAL_FILTER_MAP;
+use super::MANUAL_FIND_MAP;
+
+/// lint use of `filter().map()` or `find().map()` for `Iterators`
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>, is_find: bool) {
+    if_chain! {
+        if let ExprKind::MethodCall(_, _, [map_recv, map_arg], map_span) = expr.kind;
+        if let ExprKind::MethodCall(_, _, [_, filter_arg], filter_span) = map_recv.kind;
+        if match_trait_method(cx, map_recv, &paths::ITERATOR);
+
+        // filter(|x| ...is_some())...
+        if let ExprKind::Closure(_, _, filter_body_id, ..) = filter_arg.kind;
+        let filter_body = cx.tcx.hir().body(filter_body_id);
+        if let [filter_param] = filter_body.params;
+        // optional ref pattern: `filter(|&x| ..)`
+        let (filter_pat, is_filter_param_ref) = if let PatKind::Ref(ref_pat, _) = filter_param.pat.kind {
+            (ref_pat, true)
+        } else {
+            (filter_param.pat, false)
+        };
+        // closure ends with is_some() or is_ok()
+        if let PatKind::Binding(_, filter_param_id, _, None) = filter_pat.kind;
+        if let ExprKind::MethodCall(path, _, [filter_arg], _) = filter_body.value.kind;
+        if let Some(opt_ty) = cx.typeck_results().expr_ty(filter_arg).ty_adt_def();
+        if let Some(is_result) = if cx.tcx.is_diagnostic_item(sym::option_type, opt_ty.did) {
+            Some(false)
+        } else if cx.tcx.is_diagnostic_item(sym::result_type, opt_ty.did) {
+            Some(true)
+        } else {
+            None
+        };
+        if path.ident.name.as_str() == if is_result { "is_ok" } else { "is_some" };
+
+        // ...map(|x| ...unwrap())
+        if let ExprKind::Closure(_, _, map_body_id, ..) = map_arg.kind;
+        let map_body = cx.tcx.hir().body(map_body_id);
+        if let [map_param] = map_body.params;
+        if let PatKind::Binding(_, map_param_id, map_param_ident, None) = map_param.pat.kind;
+        // closure ends with expect() or unwrap()
+        if let ExprKind::MethodCall(seg, _, [map_arg, ..], _) = map_body.value.kind;
+        if matches!(seg.ident.name, sym::expect | sym::unwrap | sym::unwrap_or);
+
+        let eq_fallback = |a: &Expr<'_>, b: &Expr<'_>| {
+            // in `filter(|x| ..)`, replace `*x` with `x`
+            let a_path = if_chain! {
+                if !is_filter_param_ref;
+                if let ExprKind::Unary(UnOp::Deref, expr_path) = a.kind;
+                then { expr_path } else { a }
+            };
+            // let the filter closure arg and the map closure arg be equal
+            if_chain! {
+                if path_to_local_id(a_path, filter_param_id);
+                if path_to_local_id(b, map_param_id);
+                if TyS::same_type(cx.typeck_results().expr_ty_adjusted(a), cx.typeck_results().expr_ty_adjusted(b));
+                then {
+                    return true;
+                }
+            }
+            false
+        };
+        if SpanlessEq::new(cx).expr_fallback(eq_fallback).eq_expr(filter_arg, map_arg);
+        then {
+            let span = filter_span.to(map_span);
+            let (filter_name, lint) = if is_find {
+                ("find", MANUAL_FIND_MAP)
+            } else {
+                ("filter", MANUAL_FILTER_MAP)
+            };
+            let msg = format!("`{}(..).map(..)` can be simplified as `{0}_map(..)`", filter_name);
+            let to_opt = if is_result { ".ok()" } else { "" };
+            let sugg = format!("{}_map(|{}| {}{})", filter_name, map_param_ident,
+                snippet(cx, map_arg.span, ".."), to_opt);
+            span_lint_and_sugg(cx, lint, span, &msg, "try", sugg, Applicability::MachineApplicable);
+        }
+    }
+}

src/tools/clippy/clippy_lints/src/methods/filter_map_flat_map.rs

@@ -0,0 +1,21 @@
+use crate::utils::{match_trait_method, paths, span_lint_and_help};
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+
+use super::FILTER_MAP;
+
+/// lint use of `filter_map().flat_map()` for `Iterators`
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    expr: &'tcx hir::Expr<'_>,
+    _filter_args: &'tcx [hir::Expr<'_>],
+    _map_args: &'tcx [hir::Expr<'_>],
+) {
+    // lint if caller of `.filter_map().flat_map()` is an Iterator
+    if match_trait_method(cx, expr, &paths::ITERATOR) {
+        let msg = "called `filter_map(..).flat_map(..)` on an `Iterator`";
+        let hint = "this is more succinctly expressed by calling `.flat_map(..)` \
+                    and filtering by returning `iter::empty()`";
+        span_lint_and_help(cx, FILTER_MAP, expr.span, msg, None, hint);
+    }
+}

src/tools/clippy/clippy_lints/src/methods/filter_map_map.rs

@@ -0,0 +1,20 @@
+use crate::utils::{match_trait_method, paths, span_lint_and_help};
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+
+use super::FILTER_MAP;
+
+/// lint use of `filter_map().map()` for `Iterators`
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    expr: &'tcx hir::Expr<'_>,
+    _filter_args: &'tcx [hir::Expr<'_>],
+    _map_args: &'tcx [hir::Expr<'_>],
+) {
+    // lint if caller of `.filter_map().map()` is an Iterator
+    if match_trait_method(cx, expr, &paths::ITERATOR) {
+        let msg = "called `filter_map(..).map(..)` on an `Iterator`";
+        let hint = "this is more succinctly expressed by only calling `.filter_map(..)` instead";
+        span_lint_and_help(cx, FILTER_MAP, expr.span, msg, None, hint);
+    }
+}

src/tools/clippy/clippy_lints/src/methods/filter_map_next.rs

@@ -0,0 +1,40 @@
+use crate::utils::{match_trait_method, meets_msrv, paths, snippet, span_lint, span_lint_and_sugg};
+use rustc_errors::Applicability;
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+use rustc_semver::RustcVersion;
+
+use super::FILTER_MAP_NEXT;
+
+const FILTER_MAP_NEXT_MSRV: RustcVersion = RustcVersion::new(1, 30, 0);
+
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    expr: &'tcx hir::Expr<'_>,
+    filter_args: &'tcx [hir::Expr<'_>],
+    msrv: Option<&RustcVersion>,
+) {
+    if match_trait_method(cx, expr, &paths::ITERATOR) {
+        if !meets_msrv(msrv, &FILTER_MAP_NEXT_MSRV) {
+            return;
+        }
+
+        let msg = "called `filter_map(..).next()` on an `Iterator`. This is more succinctly expressed by calling \
+                   `.find_map(..)` instead";
+        let filter_snippet = snippet(cx, filter_args[1].span, "..");
+        if filter_snippet.lines().count() <= 1 {
+            let iter_snippet = snippet(cx, filter_args[0].span, "..");
+            span_lint_and_sugg(
+                cx,
+                FILTER_MAP_NEXT,
+                expr.span,
+                msg,
+                "try this",
+                format!("{}.find_map({})", iter_snippet, filter_snippet),
+                Applicability::MachineApplicable,
+            );
+        } else {
+            span_lint(cx, FILTER_MAP_NEXT, expr.span, msg);
+        }
+    }
+}

src/tools/clippy/clippy_lints/src/methods/filter_next.rs

@@ -0,0 +1,31 @@
+use crate::utils::{match_trait_method, paths, snippet, span_lint, span_lint_and_sugg};
+use rustc_errors::Applicability;
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+
+use super::FILTER_NEXT;
+
+/// lint use of `filter().next()` for `Iterators`
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>, filter_args: &'tcx [hir::Expr<'_>]) {
+    // lint if caller of `.filter().next()` is an Iterator
+    if match_trait_method(cx, expr, &paths::ITERATOR) {
+        let msg = "called `filter(..).next()` on an `Iterator`. This is more succinctly expressed by calling \
+                   `.find(..)` instead";
+        let filter_snippet = snippet(cx, filter_args[1].span, "..");
+        if filter_snippet.lines().count() <= 1 {
+            let iter_snippet = snippet(cx, filter_args[0].span, "..");
+            // add note if not multi-line
+            span_lint_and_sugg(
+                cx,
+                FILTER_NEXT,
+                expr.span,
+                msg,
+                "try this",
+                format!("{}.find({})", iter_snippet, filter_snippet),
+                Applicability::MachineApplicable,
+            );
+        } else {
+            span_lint(cx, FILTER_NEXT, expr.span, msg);
+        }
+    }
+}

src/tools/clippy/clippy_lints/src/methods/flat_map_identity.rs

@@ -0,0 +1,57 @@
+use crate::utils::{match_qpath, match_trait_method, paths, span_lint_and_sugg};
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+use rustc_span::source_map::Span;
+
+use super::FLAT_MAP_IDENTITY;
+
+/// lint use of `flat_map` for `Iterators` where `flatten` would be sufficient
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    expr: &'tcx hir::Expr<'_>,
+    flat_map_args: &'tcx [hir::Expr<'_>],
+    flat_map_span: Span,
+) {
+    if match_trait_method(cx, expr, &paths::ITERATOR) {
+        let arg_node = &flat_map_args[1].kind;
+
+        let apply_lint = |message: &str| {
+            span_lint_and_sugg(
+                cx,
+                FLAT_MAP_IDENTITY,
+                flat_map_span.with_hi(expr.span.hi()),
+                message,
+                "try",
+                "flatten()".to_string(),
+                Applicability::MachineApplicable,
+            );
+        };
+
+        if_chain! {
+            if let hir::ExprKind::Closure(_, _, body_id, _, _) = arg_node;
+            let body = cx.tcx.hir().body(*body_id);
+
+            if let hir::PatKind::Binding(_, _, binding_ident, _) = body.params[0].pat.kind;
+            if let hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) = body.value.kind;
+
+            if path.segments.len() == 1;
+            if path.segments[0].ident.name == binding_ident.name;
+
+            then {
+                apply_lint("called `flat_map(|x| x)` on an `Iterator`");
+            }
+        }
+
+        if_chain! {
+            if let hir::ExprKind::Path(ref qpath) = arg_node;
+
+            if match_qpath(qpath, &paths::STD_CONVERT_IDENTITY);
+
+            then {
+                apply_lint("called `flat_map(std::convert::identity)` on an `Iterator`");
+            }
+        }
+    }
+}

src/tools/clippy/clippy_lints/src/methods/from_iter_instead_of_collect.rs

@@ -0,0 +1,67 @@
+use crate::utils::{get_trait_def_id, implements_trait, paths, span_lint_and_sugg, sugg};
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir as hir;
+use rustc_lint::{LateContext, LintContext};
+use rustc_middle::ty::Ty;
+
+use super::FROM_ITER_INSTEAD_OF_COLLECT;
+
+pub(super) fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, args: &[hir::Expr<'_>]) {
+    let ty = cx.typeck_results().expr_ty(expr);
+    let arg_ty = cx.typeck_results().expr_ty(&args[0]);
+
+    if_chain! {
+        if let Some(from_iter_id) = get_trait_def_id(cx, &paths::FROM_ITERATOR);
+        if let Some(iter_id) = get_trait_def_id(cx, &paths::ITERATOR);
+
+        if implements_trait(cx, ty, from_iter_id, &[]) && implements_trait(cx, arg_ty, iter_id, &[]);
+        then {
+            // `expr` implements `FromIterator` trait
+            let iter_expr = sugg::Sugg::hir(cx, &args[0], "..").maybe_par();
+            let turbofish = extract_turbofish(cx, expr, ty);
+            let sugg = format!("{}.collect::<{}>()", iter_expr, turbofish);
+            span_lint_and_sugg(
+                cx,
+                FROM_ITER_INSTEAD_OF_COLLECT,
+                expr.span,
+                "usage of `FromIterator::from_iter`",
+                "use `.collect()` instead of `::from_iter()`",
+                sugg,
+                Applicability::MaybeIncorrect,
+            );
+        }
+    }
+}
+
+fn extract_turbofish(cx: &LateContext<'_>, expr: &hir::Expr<'_>, ty: Ty<'tcx>) -> String {
+    if_chain! {
+        let call_site = expr.span.source_callsite();
+        if let Ok(snippet) = cx.sess().source_map().span_to_snippet(call_site);
+        let snippet_split = snippet.split("::").collect::<Vec<_>>();
+        if let Some((_, elements)) = snippet_split.split_last();
+
+        then {
+            // is there a type specifier? (i.e.: like `<u32>` in `collections::BTreeSet::<u32>::`)
+            if let Some(type_specifier) = snippet_split.iter().find(|e| e.starts_with('<') && e.ends_with('>')) {
+                // remove the type specifier from the path elements
+                let without_ts = elements.iter().filter_map(|e| {
+                    if e == type_specifier { None } else { Some((*e).to_string()) }
+                }).collect::<Vec<_>>();
+                // join and add the type specifier at the end (i.e.: `collections::BTreeSet<u32>`)
+                format!("{}{}", without_ts.join("::"), type_specifier)
+            } else {
+                // type is not explicitly specified so wildcards are needed
+                // i.e.: 2 wildcards in `std::collections::BTreeMap<&i32, &char>`
+                let ty_str = ty.to_string();
+                let start = ty_str.find('<').unwrap_or(0);
+                let end = ty_str.find('>').unwrap_or_else(|| ty_str.len());
+                let nb_wildcard = ty_str[start..end].split(',').count();
+                let wildcards = format!("_{}", ", _".repeat(nb_wildcard - 1));
+                format!("{}<{}>", elements.join("::"), wildcards)
+            }
+        } else {
+            ty.to_string()
+        }
+    }
+}

src/tools/clippy/clippy_lints/src/methods/get_unwrap.rs

@@ -0,0 +1,84 @@
+use crate::methods::derefs_to_slice;
+use crate::utils::{
+    get_parent_expr, is_type_diagnostic_item, match_type, paths, snippet_with_applicability, span_lint_and_sugg,
+};
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+use rustc_span::sym;
+
+use super::GET_UNWRAP;
+
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &hir::Expr<'_>, get_args: &'tcx [hir::Expr<'_>], is_mut: bool) {
+    // Note: we don't want to lint `get_mut().unwrap` for `HashMap` or `BTreeMap`,
+    // because they do not implement `IndexMut`
+    let mut applicability = Applicability::MachineApplicable;
+    let expr_ty = cx.typeck_results().expr_ty(&get_args[0]);
+    let get_args_str = if get_args.len() > 1 {
+        snippet_with_applicability(cx, get_args[1].span, "..", &mut applicability)
+    } else {
+        return; // not linting on a .get().unwrap() chain or variant
+    };
+    let mut needs_ref;
+    let caller_type = if derefs_to_slice(cx, &get_args[0], expr_ty).is_some() {
+        needs_ref = get_args_str.parse::<usize>().is_ok();
+        "slice"
+    } else if is_type_diagnostic_item(cx, expr_ty, sym::vec_type) {
+        needs_ref = get_args_str.parse::<usize>().is_ok();
+        "Vec"
+    } else if is_type_diagnostic_item(cx, expr_ty, sym::vecdeque_type) {
+        needs_ref = get_args_str.parse::<usize>().is_ok();
+        "VecDeque"
+    } else if !is_mut && is_type_diagnostic_item(cx, expr_ty, sym::hashmap_type) {
+        needs_ref = true;
+        "HashMap"
+    } else if !is_mut && match_type(cx, expr_ty, &paths::BTREEMAP) {
+        needs_ref = true;
+        "BTreeMap"
+    } else {
+        return; // caller is not a type that we want to lint
+    };
+
+    let mut span = expr.span;
+
+    // Handle the case where the result is immediately dereferenced
+    // by not requiring ref and pulling the dereference into the
+    // suggestion.
+    if_chain! {
+        if needs_ref;
+        if let Some(parent) = get_parent_expr(cx, expr);
+        if let hir::ExprKind::Unary(hir::UnOp::Deref, _) = parent.kind;
+        then {
+            needs_ref = false;
+            span = parent.span;
+        }
+    }
+
+    let mut_str = if is_mut { "_mut" } else { "" };
+    let borrow_str = if !needs_ref {
+        ""
+    } else if is_mut {
+        "&mut "
+    } else {
+        "&"
+    };
+
+    span_lint_and_sugg(
+        cx,
+        GET_UNWRAP,
+        span,
+        &format!(
+            "called `.get{0}().unwrap()` on a {1}. Using `[]` is more clear and more concise",
+            mut_str, caller_type
+        ),
+        "try this",
+        format!(
+            "{}{}[{}]",
+            borrow_str,
+            snippet_with_applicability(cx, get_args[0].span, "..", &mut applicability),
+            get_args_str
+        ),
+        applicability,
+    );
+}

src/tools/clippy/clippy_lints/src/methods/implicit_clone.rs

@@ -0,0 +1,32 @@
+use crate::utils::span_lint_and_sugg;
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir as hir;
+use rustc_hir::ExprKind;
+use rustc_lint::LateContext;
+use rustc_middle::ty::TyS;
+use rustc_span::symbol::Symbol;
+
+use super::IMPLICIT_CLONE;
+use clippy_utils::is_diagnostic_assoc_item;
+
+pub fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, trait_diagnostic: Symbol) {
+    if_chain! {
+        if let ExprKind::MethodCall(method_path, _, [arg], _) = &expr.kind;
+        let return_type = cx.typeck_results().expr_ty(&expr);
+        let input_type = cx.typeck_results().expr_ty(arg).peel_refs();
+        if let Some(expr_def_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id);
+        if let Some(ty_name) = input_type.ty_adt_def().map(|adt_def| cx.tcx.item_name(adt_def.did));
+        if TyS::same_type(return_type, input_type);
+        if is_diagnostic_assoc_item(cx, expr_def_id, trait_diagnostic);
+        then {
+            span_lint_and_sugg(
+                cx,IMPLICIT_CLONE,method_path.ident.span,
+                &format!("implicitly cloning a `{}` by calling `{}` on its dereferenced type", ty_name, method_path.ident.name),
+                "consider using",
+                "clone".to_string(),
+                Applicability::MachineApplicable
+            );
+        }
+    }
+}

src/tools/clippy/clippy_lints/src/methods/inefficient_to_string.rs

@@ -10,7 +10,7 @@ use rustc_middle::ty::{self, Ty};
 use rustc_span::sym;
 
 /// Checks for the `INEFFICIENT_TO_STRING` lint
-pub fn lint<'tcx>(cx: &LateContext<'tcx>, expr: &hir::Expr<'_>, arg: &hir::Expr<'_>, arg_ty: Ty<'tcx>) {
+pub fn check<'tcx>(cx: &LateContext<'tcx>, expr: &hir::Expr<'_>, arg: &hir::Expr<'_>, arg_ty: Ty<'tcx>) {
     if_chain! {
         if let Some(to_string_meth_did) = cx.typeck_results().type_dependent_def_id(expr.hir_id);
         if match_def_path(cx, to_string_meth_did, &paths::TO_STRING_METHOD);

src/tools/clippy/clippy_lints/src/methods/inspect_for_each.rs

@@ -7,7 +7,7 @@ use crate::utils::{match_trait_method, paths, span_lint_and_help};
 use super::INSPECT_FOR_EACH;
 
 /// lint use of `inspect().for_each()` for `Iterators`
-pub(super) fn lint<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>, inspect_span: Span) {
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>, inspect_span: Span) {
     if match_trait_method(cx, expr, &paths::ITERATOR) {
         let msg = "called `inspect(..).for_each(..)` on an `Iterator`";
         let hint = "move the code from `inspect(..)` to `for_each(..)` and remove the `inspect(..)`";

src/tools/clippy/clippy_lints/src/methods/into_iter_on_ref.rs

@@ -0,0 +1,43 @@
+use crate::utils::{has_iter_method, match_trait_method, paths, span_lint_and_sugg};
+use rustc_errors::Applicability;
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, Ty};
+use rustc_span::source_map::Span;
+use rustc_span::symbol::Symbol;
+
+use super::INTO_ITER_ON_REF;
+
+pub(super) fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, self_ref_ty: Ty<'_>, method_span: Span) {
+    if !match_trait_method(cx, expr, &paths::INTO_ITERATOR) {
+        return;
+    }
+    if let Some((kind, method_name)) = ty_has_iter_method(cx, self_ref_ty) {
+        span_lint_and_sugg(
+            cx,
+            INTO_ITER_ON_REF,
+            method_span,
+            &format!(
+                "this `.into_iter()` call is equivalent to `.{}()` and will not consume the `{}`",
+                method_name, kind,
+            ),
+            "call directly",
+            method_name.to_string(),
+            Applicability::MachineApplicable,
+        );
+    }
+}
+
+fn ty_has_iter_method(cx: &LateContext<'_>, self_ref_ty: Ty<'_>) -> Option<(Symbol, &'static str)> {
+    has_iter_method(cx, self_ref_ty).map(|ty_name| {
+        let mutbl = match self_ref_ty.kind() {
+            ty::Ref(_, _, mutbl) => mutbl,
+            _ => unreachable!(),
+        };
+        let method_name = match mutbl {
+            hir::Mutability::Not => "iter",
+            hir::Mutability::Mut => "iter_mut",
+        };
+        (ty_name, method_name)
+    })
+}

src/tools/clippy/clippy_lints/src/methods/iter_cloned_collect.rs

@@ -0,0 +1,30 @@
+use crate::methods::derefs_to_slice;
+use crate::utils::{is_type_diagnostic_item, span_lint_and_sugg};
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+use rustc_span::sym;
+
+use super::ITER_CLONED_COLLECT;
+
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &hir::Expr<'_>, iter_args: &'tcx [hir::Expr<'_>]) {
+    if_chain! {
+        if is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(expr), sym::vec_type);
+        if let Some(slice) = derefs_to_slice(cx, &iter_args[0], cx.typeck_results().expr_ty(&iter_args[0]));
+        if let Some(to_replace) = expr.span.trim_start(slice.span.source_callsite());
+
+        then {
+            span_lint_and_sugg(
+                cx,
+                ITER_CLONED_COLLECT,
+                to_replace,
+                "called `iter().cloned().collect()` on a slice to create a `Vec`. Calling `to_vec()` is both faster and \
+                more readable",
+                "try",
+                ".to_vec()".to_string(),
+                Applicability::MachineApplicable,
+            );
+        }
+    }
+}

src/tools/clippy/clippy_lints/src/methods/iter_count.rs

@@ -0,0 +1,47 @@
+use crate::methods::derefs_to_slice;
+use crate::utils::{is_type_diagnostic_item, match_type, paths, snippet_with_applicability, span_lint_and_sugg};
+
+use rustc_errors::Applicability;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_span::sym;
+
+use super::ITER_COUNT;
+
+pub(crate) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'_>, iter_args: &'tcx [Expr<'tcx>], iter_method: &str) {
+    let ty = cx.typeck_results().expr_ty(&iter_args[0]);
+    let caller_type = if derefs_to_slice(cx, &iter_args[0], ty).is_some() {
+        "slice"
+    } else if is_type_diagnostic_item(cx, ty, sym::vec_type) {
+        "Vec"
+    } else if is_type_diagnostic_item(cx, ty, sym::vecdeque_type) {
+        "VecDeque"
+    } else if is_type_diagnostic_item(cx, ty, sym::hashset_type) {
+        "HashSet"
+    } else if is_type_diagnostic_item(cx, ty, sym::hashmap_type) {
+        "HashMap"
+    } else if match_type(cx, ty, &paths::BTREEMAP) {
+        "BTreeMap"
+    } else if match_type(cx, ty, &paths::BTREESET) {
+        "BTreeSet"
+    } else if match_type(cx, ty, &paths::LINKED_LIST) {
+        "LinkedList"
+    } else if match_type(cx, ty, &paths::BINARY_HEAP) {
+        "BinaryHeap"
+    } else {
+        return;
+    };
+    let mut applicability = Applicability::MachineApplicable;
+    span_lint_and_sugg(
+        cx,
+        ITER_COUNT,
+        expr.span,
+        &format!("called `.{}().count()` on a `{}`", iter_method, caller_type),
+        "try",
+        format!(
+            "{}.len()",
+            snippet_with_applicability(cx, iter_args[0].span, "..", &mut applicability),
+        ),
+        applicability,
+    );
+}

src/tools/clippy/clippy_lints/src/methods/iter_next_slice.rs

@@ -0,0 +1,68 @@
+use crate::methods::derefs_to_slice;
+use crate::utils::{get_parent_expr, higher, is_type_diagnostic_item, snippet_with_applicability, span_lint_and_sugg};
+use if_chain::if_chain;
+use rustc_ast::ast;
+use rustc_errors::Applicability;
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+use rustc_middle::ty;
+use rustc_span::symbol::sym;
+
+use super::ITER_NEXT_SLICE;
+
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>, iter_args: &'tcx [hir::Expr<'_>]) {
+    let caller_expr = &iter_args[0];
+
+    // Skip lint if the `iter().next()` expression is a for loop argument,
+    // since it is already covered by `&loops::ITER_NEXT_LOOP`
+    let mut parent_expr_opt = get_parent_expr(cx, expr);
+    while let Some(parent_expr) = parent_expr_opt {
+        if higher::for_loop(parent_expr).is_some() {
+            return;
+        }
+        parent_expr_opt = get_parent_expr(cx, parent_expr);
+    }
+
+    if derefs_to_slice(cx, caller_expr, cx.typeck_results().expr_ty(caller_expr)).is_some() {
+        // caller is a Slice
+        if_chain! {
+            if let hir::ExprKind::Index(ref caller_var, ref index_expr) = &caller_expr.kind;
+            if let Some(higher::Range { start: Some(start_expr), end: None, limits: ast::RangeLimits::HalfOpen })
+                = higher::range(index_expr);
+            if let hir::ExprKind::Lit(ref start_lit) = &start_expr.kind;
+            if let ast::LitKind::Int(start_idx, _) = start_lit.node;
+            then {
+                let mut applicability = Applicability::MachineApplicable;
+                span_lint_and_sugg(
+                    cx,
+                    ITER_NEXT_SLICE,
+                    expr.span,
+                    "using `.iter().next()` on a Slice without end index",
+                    "try calling",
+                    format!("{}.get({})", snippet_with_applicability(cx, caller_var.span, "..", &mut applicability), start_idx),
+                    applicability,
+                );
+            }
+        }
+    } else if is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(caller_expr), sym::vec_type)
+        || matches!(
+            &cx.typeck_results().expr_ty(caller_expr).peel_refs().kind(),
+            ty::Array(_, _)
+        )
+    {
+        // caller is a Vec or an Array
+        let mut applicability = Applicability::MachineApplicable;
+        span_lint_and_sugg(
+            cx,
+            ITER_NEXT_SLICE,
+            expr.span,
+            "using `.iter().next()` on an array",
+            "try calling",
+            format!(
+                "{}.get(0)",
+                snippet_with_applicability(cx, caller_expr.span, "..", &mut applicability)
+            ),
+            applicability,
+        );
+    }
+}

src/tools/clippy/clippy_lints/src/methods/iter_nth.rs

@@ -0,0 +1,38 @@
+use crate::methods::derefs_to_slice;
+use crate::methods::iter_nth_zero;
+use crate::utils::{is_type_diagnostic_item, span_lint_and_help};
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+use rustc_span::symbol::sym;
+
+use super::ITER_NTH;
+
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    expr: &hir::Expr<'_>,
+    nth_and_iter_args: &[&'tcx [hir::Expr<'tcx>]],
+    is_mut: bool,
+) {
+    let iter_args = nth_and_iter_args[1];
+    let mut_str = if is_mut { "_mut" } else { "" };
+    let caller_type = if derefs_to_slice(cx, &iter_args[0], cx.typeck_results().expr_ty(&iter_args[0])).is_some() {
+        "slice"
+    } else if is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(&iter_args[0]), sym::vec_type) {
+        "Vec"
+    } else if is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(&iter_args[0]), sym::vecdeque_type) {
+        "VecDeque"
+    } else {
+        let nth_args = nth_and_iter_args[0];
+        iter_nth_zero::check(cx, expr, &nth_args);
+        return; // caller is not a type that we want to lint
+    };
+
+    span_lint_and_help(
+        cx,
+        ITER_NTH,
+        expr.span,
+        &format!("called `.iter{0}().nth()` on a {1}", mut_str, caller_type),
+        None,
+        &format!("calling `.get{}()` is both faster and more readable", mut_str),
+    );
+}