Auto merge of #37378 - petrochenkov:nopat, r=eddyb

Prohibit patterns in trait methods without bodies

They are not properly type checked
```rust
trait Tr {
    fn f(&a: u8); // <- This compiles
}
```
, mostly rejected by the parser already and generally don't make much sense.
This PR is kind of a missing part of https://github.com/rust-lang/rust/pull/35015.

Given the [statistics from crater](https://github.com/rust-lang/rust/pull/37378#issuecomment-256154994), the effect of this PR is mostly equivalent to improving `unused_mut` lint.

cc https://github.com/rust-lang/rust/issues/35078#issuecomment-255707355 https://github.com/rust-lang/rust/pull/35015 https://github.com/rust-lang/rfcs/pull/1685 https://github.com/rust-lang/rust/issues/35203
r? @eddyb

src/doc/reference.md

@@ -4023,9 +4023,9 @@ Methods that take either `self` or `Box<Self>` can optionally place them in a
 mutable variable by prefixing them with `mut` (similar to regular arguments):
 
 ```
-trait Changer {
-    fn change(mut self) -> Self;
-    fn modify(mut self: Box<Self>) -> Box<Self>;
+trait Changer: Sized {
+    fn change(mut self) {}
+    fn modify(mut self: Box<Self>) {}
 }
 ```
 

src/librustc/lint/builtin.rs

@@ -192,6 +192,12 @@ declare_lint! {
     "safe access to extern statics was erroneously allowed"
 }
 
+declare_lint! {
+    pub PATTERNS_IN_FNS_WITHOUT_BODY,
+    Warn,
+    "patterns in functions without body were erroneously allowed"
+}
+
 /// Does nothing as a lint pass, but registers some `Lint`s
 /// which are used by other parts of the compiler.
 #[derive(Copy, Clone)]
@@ -228,7 +234,8 @@ impl LintPass for HardwiredLints {
             SUPER_OR_SELF_IN_GLOBAL_PATH,
             HR_LIFETIME_IN_ASSOC_TYPE,
             LIFETIME_UNDERSCORE,
-            SAFE_EXTERN_STATICS
+            SAFE_EXTERN_STATICS,
+            PATTERNS_IN_FNS_WITHOUT_BODY
         )
     }
 }

src/librustc_lint/lib.rs

@@ -225,6 +225,10 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
             id: LintId::of(SAFE_EXTERN_STATICS),
             reference: "issue #36247 <https://github.com/rust-lang/rust/issues/35112>",
         },
+        FutureIncompatibleInfo {
+            id: LintId::of(PATTERNS_IN_FNS_WITHOUT_BODY),
+            reference: "issue #35203 <https://github.com/rust-lang/rust/issues/35203>",
+        },
         ]);
 
     // Register renamed and removed lints

src/librustc_passes/ast_validation.rs

@@ -190,8 +190,16 @@ impl<'a> Visitor for AstValidator<'a> {
             }
             ItemKind::Trait(.., ref trait_items) => {
                 for trait_item in trait_items {
-                    if let TraitItemKind::Method(ref sig, _) = trait_item.node {
+                    if let TraitItemKind::Method(ref sig, ref block) = trait_item.node {
                         self.check_trait_fn_not_const(sig.constness);
+                        if block.is_none() {
+                            self.check_decl_no_pat(&sig.decl, |span, _| {
+                                self.session.add_lint(lint::builtin::PATTERNS_IN_FNS_WITHOUT_BODY,
+                                                      trait_item.id, span,
+                                                      "patterns aren't allowed in methods \
+                                                       without bodies".to_string());
+                            });
+                        }
                     }
                 }
             }

src/test/compile-fail/no-patterns-in-args-2.rs

@@ -0,0 +1,23 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![deny(patterns_in_fns_without_body)]
+
+trait Tr {
+    fn f1(mut arg: u8); //~ ERROR patterns aren't allowed in methods without bodies
+                        //~^ WARN was previously accepted
+    fn f2(&arg: u8); //~ ERROR patterns aren't allowed in methods without bodies
+                     //~^ WARN was previously accepted
+    fn g1(arg: u8); // OK
+    fn g2(_: u8); // OK
+    fn g3(u8); // OK
+}
+
+fn main() {}

src/test/incremental/hashes/trait_defs.rs

@@ -264,8 +264,8 @@ trait TraitChangeModeSelfRefToMut {
 
 
 #[cfg(cfail1)]
-trait TraitChangeModeSelfOwnToMut {
-    fn method(self);
+trait TraitChangeModeSelfOwnToMut: Sized {
+    fn method(self) {}
 }
 
 #[cfg(not(cfail1))]
@@ -273,8 +273,8 @@ trait TraitChangeModeSelfOwnToMut {
 #[rustc_clean(label="Hir", cfg="cfail3")]
 #[rustc_metadata_dirty(cfg="cfail2")]
 #[rustc_metadata_clean(cfg="cfail3")]
-trait TraitChangeModeSelfOwnToMut {
-    fn method(mut self);
+trait TraitChangeModeSelfOwnToMut: Sized {
+    fn method(mut self) {}
 }
 
 

src/test/run-pass/by-value-self-in-mut-slot.rs

@@ -14,7 +14,7 @@ struct X {
 }
 
 trait Changer {
-    fn change(mut self) -> Self;
+    fn change(self) -> Self;
 }
 
 impl Changer for X {

src/test/run-pass/uniq-self-in-mut-slot.rs

@@ -17,7 +17,7 @@ struct X {
 }
 
 trait Changer {
-    fn change(mut self: Box<Self>) -> Box<Self>;
+    fn change(self: Box<Self>) -> Box<Self>;
 }
 
 impl Changer for X {

src/tools/cargotest/main.rs

@@ -24,7 +24,7 @@ struct Test {
 const TEST_REPOS: &'static [Test] = &[Test {
                                           name: "cargo",
                                           repo: "https://github.com/rust-lang/cargo",
-                                          sha: "d3bad1ab29efae414e9b4c24534b2d02b3a59782",
+                                          sha: "806e3c368a15f618244a3b4e918bf77f9c403fd0",
                                           lock: None,
                                       },
                                       Test {