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Auto merge of #3824 - phansch:adding_lints, r=phansch

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Add lint writing documentation

[Rendered](https://github.com/phansch/rust-clippy/blob/adding_lints/doc/adding_lints.md)

This adds a new documentation page that explains how to write Clippy
lints. It guides the reader through creating a `foo` function lint.

I plan to iterate a bit more on the prose of some sections, but I think the
general structure is fine now, so I'm looking forward to feedback =)

One thing I'm not sure about: I felt like this is too big for CONTRIBUTING.md
so I put it into a new `doc/` directory. I can imagine having more
documentation in the future, so we might even want to create a book using
mdbook instead? Or should everything go into CONTRIBUTING.md?

Further things left to do:

- [x] Link from CONTRIBUTING.md
- [x] Remove things covered in this guide from CONTRIBUTING.md
- [x] Section about `clippy::author` attribute
- [x] Run `remark-lint` on CI over the `doc` directory and fix things
This commit is contained in:
bors 2019-03-09 15:05:38 +00:00
commit 920112d723
4 changed files with 455 additions and 117 deletions
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117
CONTRIBUTING.md
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@ -14,11 +14,6 @@ All contributors are expected to follow the [Rust Code of Conduct](http://www.ru
* [Getting started](#getting-started)
* [Finding something to fix/improve](#finding-something-to-fiximprove)
* [Writing code](#writing-code)
* [Author lint](#author-lint)
* [Documentation](#documentation)
* [Running test suite](#running-test-suite)
* [Running rustfmt](#running-rustfmt)
* [Testing manually](#testing-manually)
* [How Clippy works](#how-clippy-works)
* [Fixing nightly build failures](#fixing-build-failures-caused-by-rust)
* [Issue and PR Triage](#issue-and-pr-triage)
@ -73,121 +68,15 @@ an AST expression). `match_def_path()` in Clippy's `utils` module can also be us
## Writing code
Clippy depends on the current git master version of rustc, which can change rapidly. Make sure you're
working near rust-clippy's master, and use the `setup-toolchain.sh` script to configure the appropriate
toolchain for this directory.
[Llogiq's blog post on lints](https://llogiq.github.io/2015/06/04/workflows.html) is a nice primer
to lint-writing, though it does get into advanced stuff. Most lints consist of an implementation of
`LintPass` with one or more of its default methods overridden. See the existing lints for examples
of this.
Have a look at the [docs for writing lints](doc/adding_lints.md) for more details. [Llogiq's blog post on lints](https://llogiq.github.io/2015/06/04/workflows.html) is also a nice primer
to lint-writing, though it does get into advanced stuff and may be a bit
outdated.
If you want to add a new lint or change existing ones apart from bugfixing, it's
also a good idea to give the [stability guarantees][rfc_stability] and
[lint categories][rfc_lint_cats] sections of the [Clippy 1.0 RFC][clippy_rfc] a
quick read.
### Author lint
There is also the internal `author` lint to generate Clippy code that detects the offending pattern. It does not work for all of the Rust syntax, but can give a good starting point.
First, create a new UI test file in the `tests/ui/` directory with the pattern you want to match:
```rust
// ./tests/ui/my_lint.rs
fn main() {
#[clippy::author]
let arr: [i32; 1] = [7]; // Replace line with the code you want to match
}
```
Now you run `TESTNAME=ui/my_lint cargo uitest` to produce
a `.stdout` file with the generated code:
```rust
// ./tests/ui/my_lint.stdout
if_chain! {
if let ExprKind::Array(ref elements) = stmt.node;
if elements.len() == 1;
if let ExprKind::Lit(ref lit) = elements[0].node;
if let LitKind::Int(7, _) = lit.node;
then {
// report your lint here
}
}
```
If the command was executed successfully, you can copy the code over to where you are implementing your lint.
### Documentation
Please document your lint with a doc comment akin to the following:
```rust
/// **What it does:** Checks for ... (describe what the lint matches).
///
/// **Why is this bad?** Supply the reason for linting the code.
///
/// **Known problems:** None. (Or describe where it could go wrong.)
///
/// **Example:**
///
/// ```rust
/// // Bad
/// Insert a short example of code that triggers the lint
///
/// // Good
/// Insert a short example of improved code that doesn't trigger the lint
/// ```
```
Once your lint is merged it will show up in the [lint list](https://rust-lang.github.io/rust-clippy/master/index.html)
### Running test suite
Use `cargo test` to run the whole testsuite.
If you don't want to wait for all tests to finish, you can also execute a single test file by using `TESTNAME` to specify the test to run:
```bash
TESTNAME=ui/empty_line_after_outer_attr cargo uitest
```
Clippy uses UI tests. UI tests check that the output of the compiler is exactly as expected.
Of course there's little sense in writing the output yourself or copying it around.
Therefore you should use `tests/ui/update-all-references.sh` (after running
`cargo test`) and check whether the output looks as you expect with `git diff`. Commit all
`*.stderr` files, too.
If the lint you are working on is making use of structured suggestions, the
test file should include a `// run-rustfix` comment at the top. This will
additionally run [rustfix](https://github.com/rust-lang-nursery/rustfix) for
that test. Rustfix will apply the suggestions from the lint to the code of the
test file and compare that to the contents of a `.fixed` file.
Use `tests/ui/update-all-references.sh` to automatically generate the
`.fixed` file after running `cargo test`.
### Running rustfmt
[Rustfmt](https://github.com/rust-lang/rustfmt) is a tool for formatting Rust code according
to style guidelines. The code has to be formatted by `rustfmt` before a PR will be merged.
It can be installed via `rustup`:
```bash
rustup component add rustfmt
```
Use `cargo fmt --all` to format the whole codebase.
### Testing manually
Manually testing against an example file is useful if you have added some
`println!`s and test suite output becomes unreadable. To try Clippy with your
local modifications, run `env CLIPPY_TESTS=true cargo run --bin clippy-driver -- -L ./target/debug input.rs`
from the working copy root.
## How Clippy works
Clippy is a [rustc compiler plugin][compiler_plugin]. The main entry point is at [`src/lib.rs`][main_entry]. In there, the lint registration is delegated to the [`clippy_lints`][lint_crate] crate.

4
README.md
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@ -107,13 +107,13 @@ script:
- cargo clippy
# if you want the build job to fail when encountering warnings, use
- cargo clippy -- -D warnings
# in order to also check tests and none-default crate features, use
# in order to also check tests and non-default crate features, use
- cargo clippy --all-targets --all-features -- -D warnings
- cargo test
# etc.
```
It might happen that Clippy is not available for a certain nightly release.
If you are on nightly, It might happen that Clippy is not available for a certain nightly release.
In this case you can try to conditionally install Clippy from the git repo.
```yaml

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ci/base-tests.sh
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@ -4,7 +4,7 @@ echo "Running clippy base tests"
PATH=$PATH:./node_modules/.bin
if [ "$TRAVIS_OS_NAME" == "linux" ]; then
remark -f *.md > /dev/null
remark -f *.md -f doc/*.md > /dev/null
fi
# build clippy in debug mode and run tests
cargo build --features debugging

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doc/adding_lints.md Normal file
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@ -0,0 +1,449 @@
## Adding a new lint
You are probably here because you want to add a new lint to Clippy. If this is
the first time you're contributing to Clippy, this document guides you through
creating an example lint from scratch.
To get started, we will create a lint that detects functions called `foo`,
because that's clearly a non-descriptive name.
* [Setup](#Setup)
* [Testing](#Testing)
* [Rustfix tests](#Rustfix-tests)
* [Lint declaration](#Lint-declaration)
* [Lint passes](#Lint-passes)
* [Emitting a lint](#Emitting-a-lint)
* [Adding the lint logic](#Adding-the-lint-logic)
* [Author lint](#Author-lint)
* [Documentation](#Documentation)
* [Running rustfmt](#Running-rustfmt)
* [Debugging](#Debugging)
* [PR Checklist](#PR-Checklist)
* [Cheatsheet](#Cheatsheet)
### Setup
When working on Clippy, you will need the current git master version of rustc,
which can change rapidly. Make sure you're working near rust-clippy's master,
and use the `setup-toolchain.sh` script to configure the appropriate toolchain
for the Clippy directory.
### Testing
Let's write some tests first that we can execute while we iterate on our lint.
Clippy uses UI tests for testing. UI tests check that the output of Clippy is
exactly as expected. Each test is just a plain Rust file that contains the code
we want to check. The output of Clippy is compared against a `.stderr` file.
Note that you don't have to create this file yourself, we'll get to
generating the `.stderr` files further down.
We start by creating the test file at `tests/ui/foo_functions.rs`. It doesn't
really matter what the file is called, but it's a good convention to name it
after the lint it is testing, so `foo_functions.rs` it is.
Inside the file we put some examples to get started:
```rust
#![warn(clippy::foo_functions)]
// Impl methods
struct A;
impl A {
pub fn fo(&self) {}
pub fn foo(&self) {}
pub fn food(&self) {}
}
// Default trait methods
trait B {
pub fn fo(&self) {}
pub fn foo(&self) {}
pub fn food(&self) {}
}
// Plain functions
fn fo() {}
fn foo() {}
fn food() {}
fn main() {
// We also don't want to lint method calls
foo();
let a = A;
a.foo();
}
```
Now we can run the test with `TESTNAME=ui/foo_functions cargo uitest`.
Currently this test will fail. If you go through the output you will see that we
are told that `clippy::foo_functions` is an unknown lint, which is expected.
While we are working on implementing our lint, we can keep running the UI
test. That allows us to check if the output is turning into what we want.
Once we are satisfied with the output, we need to run
`tests/ui/update-all-references.sh` to update the `.stderr` file for our lint.
Running `TESTNAME=ui/foo_functions cargo uitest` should pass then. When we
commit our lint, we need to commit the generated `.stderr` files, too.
### Rustfix tests
If the lint you are working on is making use of structured suggestions, the
test file should include a `// run-rustfix` comment at the top. This will
additionally run [rustfix](https://github.com/rust-lang-nursery/rustfix) for
that test. Rustfix will apply the suggestions from the lint to the code of the
test file and compare that to the contents of a `.fixed` file.
Use `tests/ui/update-all-references.sh` to automatically generate the
`.fixed` file after running the tests.
With tests in place, let's have a look at implementing our lint now.
### Testing manually
Manually testing against an example file can be useful if you have added some
`println!`s and the test suite output becomes unreadable. To try Clippy with
your local modifications, run `env CLIPPY_TESTS=true cargo run --bin
clippy-driver -- -L ./target/debug input.rs` from the working copy root.
### Lint declaration
We start by creating a new file in the `clippy_lints` crate. That's the crate
where all the lint code is. We are going to call the file
`clippy_lints/src/foo_functions.rs` and import some initial things we need:
```rust
use rustc::lint::{LintArray, LintPass};
use rustc::{declare_tool_lint, lint_array};
```
The next step is to provide a lint declaration. Lints are declared using the
[`declare_clippy_lint!`][declare_clippy_lint] macro:
```rust
declare_clippy_lint! {
pub FOO_FUNCTIONS,
pedantic,
"function named `foo`, which is not a descriptive name"
}
```
* `FOO_FUNCTIONS` is the name of our lint. Be sure to follow the [lint naming
guidelines][lint_naming] here when naming your lint. In short, the name should
state the thing that is being checked for and read well when used with
`allow`/`warn`/`deny`.
* `pedantic` sets the lint level to `Allow`.
The exact mapping can be found [here][category_level_mapping]
* The last part should be a text that explains what exactly is wrong with the
code
With our lint declaration done, we will now make sure that it is assigned to a
lint pass:
```rust
// clippy_lints/src/foo_functions.rs
// .. imports and lint declaration ..
#[derive(Copy, Clone)]
pub struct FooFunctionsPass;
impl LintPass for FooFunctionsPass {
fn get_lints(&self) -> LintArray {
lint_array!(
FOO_FUNCTIONS,
)
}
fn name(&self) -> &'static str {
"FooFunctions"
}
}
```
Don't worry about the `name` method here. As long as it includes the name of the
lint pass it should be fine.
Next we need to run `util/dev update_lints` to register the lint in various
places, mainly in `clippy_lints/src/lib.rs`.
While `update_lints` automates some things, it doesn't automate everything. We
will have to register our lint pass manually in the `register_plugins` function
in `clippy_lints/src/lib.rs`:
```rust
reg.register_early_lint_pass(box foo_functions::FooFunctionsPass);
```
This should fix the `unknown clippy lint: clippy::foo_functions` error that we
saw when we executed our tests the first time. The next decision we have to make
is which lint pass our lint is going to need.
### Lint passes
Writing a lint that only checks for the name of a function means that we only
have to deal with the AST and don't have to deal with the type system at all.
This is good, because it makes writing this particular lint less complicated.
We have to make this decision with every new Clippy lint. It boils down to using
either [`EarlyLintPass`][early_lint_pass] or [`LateLintPass`][late_lint_pass].
In short, the `LateLintPass` has access to type information while the
`EarlyLintPass` doesn't. If you don't need access to type information, use the
`EarlyLintPass`. The `EarlyLintPass` is also faster. However linting speed
hasn't really been a concern with Clippy so far.
Since we don't need type information for checking the function name, we are
going to use the `EarlyLintPass`. It has to be imported as well, changing our
imports to:
```rust
use rustc::lint::{LintArray, LintPass, EarlyLintPass, EarlyContext};
use rustc::{declare_tool_lint, lint_array};
```
### Emitting a lint
With UI tests and the lint declaration in place, we can start working on the
implementation of the lint logic.
Let's start by implementing the `EarlyLintPass` for our `FooFunctionsPass`:
```rust
impl EarlyLintPass for FooFunctionsPass {
fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, _: &FnDecl, span: Span, _: NodeId) {
// TODO: Emit lint here
}
}
```
We implement the [`check_fn`][check_fn] method from the
[`EarlyLintPass`][early_lint_pass] trait. This gives us access to various
information about the function that is currently being checked. More on that in
the next section. Let's worry about the details later and emit our lint for
*every* function definition first.
Depending on how complex we want our lint message to be, we can choose from a
variety of lint emission functions. They can all be found in
[`clippy_lints/src/utils/diagnostics.rs`][diagnostics].
`span_help_and_lint` seems most appropriate in this case. It allows us to
provide an extra help message and we can't really suggest a better name
automatically. This is how it looks:
```rust
impl EarlyLintPass for Pass {
fn check_fn(&mut self, cx: &EarlyContext<'_>, _: FnKind<'_>, _: &FnDecl, span: Span, _: NodeId) {
span_help_and_lint(
cx,
FOO_FUNCTIONS,
span,
"function named `foo`",
"consider using a more meaningful name"
);
}
}
```
Running our UI test should now produce output that contains the lint message.
### Adding the lint logic
Writing the logic for your lint will most likely be different from our example,
so this section is kept rather short.
Using the [`check_fn`][check_fn] method gives us access to [`FnKind`][fn_kind]
that has two relevant variants for us `FnKind::ItemFn` and `FnKind::Method`.
Both provide access to the name of the function/method via an [`Ident`][ident].
With that we can expand our `check_fn` method to:
```rust
impl EarlyLintPass for Pass {
fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, _: &FnDecl, span: Span, _: NodeId) {
if is_foo_fn(fn_kind) {
span_help_and_lint(
cx,
FOO_FUNCTIONS,
span,
"function named `foo`",
"consider using a more meaningful name"
);
}
}
}
```
We separate the lint conditional from the lint emissions because it makes the
code a bit easier to read. In some cases this separation would also allow to
write some unit tests (as opposed to only UI tests) for the separate function.
In our example, `is_foo_fn` looks like:
```rust
// use statements, impl EarlyLintPass, check_fn, ..
fn is_foo_fn(fn_kind: FnKind<'_>) -> bool {
match fn_kind {
FnKind::ItemFn(ident, ..) | FnKind::Method(ident, ..) => {
ident.name == "foo"
},
FnKind::Closure(..) => false
}
}
```
Now we should also run the full test suite with `cargo test`. At this point
running `cargo test` should produce the expected output. Remember to run
`tests/ui/update-all-references.sh` to update the `.stderr` file.
`cargo test` (as opposed to `cargo uitest`) will also ensure that our lint
implementation is not violating any Clippy lints itself.
If you are still following the example, you will see that `FooFunctionsPass`
violates a Clippy lint. So we are going to rename that struct to just `Pass`:
```rust
#[derive(Copy, Clone)]
pub struct Pass;
impl LintPass for Pass { /* .. */ }
```
That should be it for the lint implementation. Running `cargo test` should now
pass.
### Author lint
If you have trouble implementing your lint, there is also the internal `author`
lint to generate Clippy code that detects the offending pattern. It does not
work for all of the Rust syntax, but can give a good starting point.
The quickest way to use it, is the [Rust playground][play].rust-lang.org).
Put the code you want to lint into the editor and add the `#[clippy::author]`
attribute above the item. Then run Clippy via `Tools -> Clippy` and you should
see the generated code in the output below.
[Here][author_example] is an example on the playground.
If the command was executed successfully, you can copy the code over to where
you are implementing your lint.
### Documentation
The final thing before submitting our PR is to add some documentation to our
lint declaration.
Please document your lint with a doc comment akin to the following:
```rust
declare_clippy_lint! {
/// **What it does:** Checks for ... (describe what the lint matches).
///
/// **Why is this bad?** Supply the reason for linting the code.
///
/// **Known problems:** None. (Or describe where it could go wrong.)
///
/// **Example:**
///
/// ```rust,ignore
/// // Bad
/// Insert a short example of code that triggers the lint
///
/// // Good
/// Insert a short example of improved code that doesn't trigger the lint
/// ```
pub FOO_FUNCTIONS,
pedantic,
"function named `foo`, which is not a descriptive name"
}
```
Once your lint is merged, this documentation will show up in the [lint
list][lint_list].
### Running rustfmt
[Rustfmt](https://github.com/rust-lang/rustfmt) is a tool for formatting Rust code according
to style guidelines. Your code has to be formatted by `rustfmt` before a PR can be merged.
It can be installed via `rustup`:
```bash
rustup component add rustfmt
```
Use `cargo fmt --all` to format the whole codebase.
### Debugging
If you want to debug parts of your lint implementation, you can use the `dbg!`
macro anywhere in your code. Running the tests should then include the debug
output in the `stdout` part.
### PR Checklist
Before submitting your PR make sure you followed all of the basic requirements:
- [ ] Followed [lint naming conventions][lint_naming]
- [ ] Added passing UI tests (including committed `.stderr` file)
- [ ] `cargo test` passes locally
- [ ] Executed `util/dev update_lints`
- [ ] Added lint documentation
### Cheatsheet
Here are some pointers to things you are likely going to need for every lint:
* [Clippy utils][utils] - Various helper functions. Maybe the function you need
is already in here (`implements_trait`, `match_path`, `snippet`, etc)
* [Clippy diagnostics][diagnostics]
* [The `if_chain` macro][if_chain]
* [`in_macro`][in_macro] and [`in_external_macro`][in_external_macro]
* [`Span`][span]
* [`Applicability`][applicability]
* [The rustc guide][rustc_guide] explains a lot of internal compiler concepts
* [The nightly rustc docs][nightly_docs] which has been linked to throughout
this guide
For `EarlyLintPass` lints:
* [`EarlyLintPass`][early_lint_pass]
* [`syntax::ast`][ast]
For `LateLintPass` lints:
* [`LateLintPass`][late_lint_pass]
* [`Ty::TyKind`][ty]
While most of Clippy's lint utils are documented, most of rustc's internals lack
documentation currently. This is unfortunate, but in most cases you can probably
get away with copying things from existing similar lints. If you are stuck,
don't hesitate to ask on Discord, IRC or in the issue/PR.
[lint_list]: https://rust-lang.github.io/rust-clippy/master/index.html
[lint_naming]: https://rust-lang.github.io/rfcs/0344-conventions-galore.html#lints
[category_level_mapping]: https://github.com/rust-lang/rust-clippy/blob/bd23cb89ec0ea63403a17d3fc5e50c88e38dd54f/clippy_lints/src/lib.rs#L43
[declare_clippy_lint]: https://github.com/rust-lang/rust-clippy/blob/a71acac1da7eaf667ab90a1d65d10e5cc4b80191/clippy_lints/src/lib.rs#L39
[compilation_stages]: https://rust-lang.github.io/rustc-guide/high-level-overview.html#the-main-stages-of-compilation
[check_fn]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/trait.EarlyLintPass.html#method.check_fn
[early_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/trait.EarlyLintPass.html
[late_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/trait.LateLintPass.html
[fn_kind]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/visit/enum.FnKind.html
[diagnostics]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/diagnostics.rs
[utils]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/mod.rs
[ident]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/source_map/symbol/struct.Ident.html
[span]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax_pos/struct.Span.html
[applicability]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/enum.Applicability.html
[if_chain]: https://docs.rs/if_chain/0.1.2/if_chain/
[ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/ty/sty/index.html
[ast]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/ast/index.html
[in_macro]: https://github.com/rust-lang/rust-clippy/blob/d0717d1f9531a03d154aaeb0cad94c243915a146/clippy_lints/src/utils/mod.rs#L94
[in_external_macro]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/fn.in_external_macro.html
[play]: https://play.rust-lang.org
[author_example]: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=f093b986e80ad62f3b67a1f24f5e66e2
[rustc_guide]: https://rust-lang.github.io/rustc-guide/
[nightly_docs]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/