3 You are probably here because you want to add a new lint to Clippy. If this is
4 the first time you're contributing to Clippy, this document guides you through
5 creating an example lint from scratch.
7 To get started, we will create a lint that detects functions called `foo`,
8 because that's clearly a non-descriptive name.
12 * [Rustfix tests](#Rustfix-tests)
13 * [Lint declaration](#Lint-declaration)
14 * [Lint passes](#Lint-passes)
15 * [Emitting a lint](#Emitting-a-lint)
16 * [Adding the lint logic](#Adding-the-lint-logic)
17 * [Author lint](#Author-lint)
18 * [Documentation](#Documentation)
19 * [Running rustfmt](#Running-rustfmt)
20 * [Debugging](#Debugging)
21 * [PR Checklist](#PR-Checklist)
22 * [Cheatsheet](#Cheatsheet)
26 Clippy depends on the current git master version of rustc, which can change rapidly. Make sure you're
27 working near rust-clippy's master, and use the `setup-toolchain.sh` script to configure the appropriate
28 toolchain for this directory.
32 Let's write some tests first that we can execute while we iterate on our lint.
34 Clippy uses UI tests for testing. UI tests check that the output of Clippy is
35 exactly as expected. Each test is just a plain Rust file that contains the code
36 we want to check. The output of Clippy is compared against a `.stderr` file.
37 Note that you don't have to create this file yourself, we'll get to
38 generating the `.stderr` files further down.
40 Let's create the test file at `tests/ui/foo_functions.rs`. It doesn't really
41 matter what the file is called, but it's a good convention to name it after the
42 lint it is testing, so `foo_functions.rs` it is.
44 Inside we put some examples to get started:
47 #![warn(clippy::foo_functions)]
57 // Default trait methods
70 // We also don't want to lint method calls
78 Now we can run the test with `TESTNAME=ui/foo_functions cargo uitest`.
79 Currently this test will fail. If you go through the output you will see that we
80 have to add some missing imports to our lint file.
82 While you are working on implementing your lint, you can keep running the UI
83 test. That allows you to check if the output is turning into what you want.
85 Once you are satisfied with the output, you need to run
86 `tests/ui/update-all-references.sh` to update the `.stderr` file for your lint.
87 Running `TESTNAME=ui/foo_functions cargo uitest` should pass then. When you
88 commit your lint, be sure to commit the `.stderr` files, too.
92 If the lint you are working on is making use of structured suggestions, the
93 test file should include a `// run-rustfix` comment at the top. This will
94 additionally run [rustfix](https://github.com/rust-lang-nursery/rustfix) for
95 that test. Rustfix will apply the suggestions from the lint to the code of the
96 test file and compare that to the contents of a `.fixed` file.
98 Use `tests/ui/update-all-references.sh` to automatically generate the
99 `.fixed` file after running `cargo test`.
101 With tests in place, let's have a look at implementing our lint now.
105 Manually testing against an example file is useful if you have added some
106 `println!`s and test suite output becomes unreadable. To try Clippy with your
107 local modifications, run `env CLIPPY_TESTS=true cargo run --bin clippy-driver -- -L ./target/debug input.rs`
108 from the working copy root.
112 We start by creating a new file in the `clippy_lints` crate. That's the crate
113 where all the lint code is. We are going to call the file
114 `clippy_lints/src/foo_functions.rs` and import some initial things we need:
117 use rustc::lint::{LintArray, LintPass};
118 use rustc::{declare_tool_lint, lint_array};
121 The next step is to provide a lint declaration. Lints are declared using the
122 [`declare_clippy_lint!`][declare_clippy_lint] macro:
125 declare_clippy_lint! {
128 "function named `foo`, which is not a descriptive name"
132 * `FOO_FUNCTIONS` is the name of our lint. Be sure to follow the [lint naming
133 guidelines][lint_naming] here when naming your lint. In short, the name should
134 state the thing that is being checked for and read well when used with
135 `allow`/`warn`/`deny`.
136 * `pedantic` sets the lint level to `Allow`.
137 The exact mapping can be found [here][category_level_mapping]
138 * The last part should be a text that explains what exactly is wrong with the
141 With our lint declaration done, we will now make sure that our lint declaration
142 is assigned to a lint pass:
145 // clippy_lints/src/foo_functions.rs
147 // .. imports and lint declaration ..
149 #[derive(Copy, Clone)]
150 pub struct FooFunctionsPass;
152 impl LintPass for FooFunctionsPass {
153 fn get_lints(&self) -> LintArray {
159 fn name(&self) -> &'static str {
165 Don't worry about the `name` method here. As long as it includes the name of the
166 lint pass it should be fine.
168 Next you should run `util/dev update_lints` to register the lint in various
169 places, mainly in `clippy_lints/src/lib.rs`.
171 While `update_lints` automates some things, it doesn't automate everything. You
172 will have to register our lint pass manually in the `register_plugins` function
173 in `clippy_lints/src/lib.rs`:
176 reg.register_early_lint_pass(box foo_functions::FooFunctionsPass);
179 Without that, running the UI tests would produce an error like `unknown clippy
180 lint: clippy::foo_functions`. The next decision we have to make is which lint
181 pass our lint is going to need.
185 Writing a lint that only checks for the name of a function means that we only
186 have to deal with the AST and don't have to deal with the type system at all.
187 This is good, because it makes writing this particular lint less complicated.
189 We have to make this decision with every new Clippy lint. It boils down to using
190 either [`EarlyLintPass`][early_lint_pass] or [`LateLintPass`][late_lint_pass].
191 This is a result of Rust's compilation process. You can read more about it [in
192 the rustc guide][compilation_stages].
194 In short, the `LateLintPass` has access to type information while the
195 `EarlyLintPass` doesn't. If you don't need access to type information, use the
196 `EarlyLintPass`. The `EarlyLintPass` is also faster. However linting speed
197 hasn't really been a concern with Clippy so far.
199 Since we don't need type information for checking the function name, we are
200 going to use the `EarlyLintPass`. It has to be imported as well, changing our
204 use rustc::lint::{LintArray, LintPass, EarlyLintPass, EarlyContext};
205 use rustc::{declare_tool_lint, lint_array};
210 With UI tests and the lint declaration in place, we can start working on the
211 implementation of the lint logic. We can keep executing the tests until we make
214 Let's start by implementing the `EarlyLintPass` for our `FooFunctionsPass`:
217 impl EarlyLintPass for FooFunctionsPass {
218 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, _: &FnDecl, span: Span, _: NodeId) {
219 // TODO: Emit lint here
224 We implement the [`check_fn`][check_fn] method from the
225 [`EarlyLintPass`][early_lint_pass] trait. This gives us access to various
226 information about the function that is currently being checked. More on that in
227 the next section. Let's worry about the details later and emit our lint for
228 *every* function definition first.
230 Depending on how complex we want our lint message to be, we can choose from a
231 variety of lint emission functions. They can all be found in
232 [`clippy_lints/src/utils/diagnostics.rs`][diagnostics].
236 impl EarlyLintPass for Pass {
237 fn check_fn(&mut self, cx: &EarlyContext<'_>, _: FnKind<'_>, _: &FnDecl, span: Span, _: NodeId) {
242 "function named `foo`",
243 "consider using a more meaningful name"
249 ### Adding the lint logic
251 Writing the logic for your lint will most likely be different from this example,
252 so this section is kept rather short.
254 Using the [`check_fn`][check_fn] method gives us access to [`FnKind`][fn_kind]
255 that has two relevant variants for us `FnKind::ItemFn` and `FnKind::Method`.
256 Both provide access to the name of the function/method via an [`Ident`][ident].
258 With that we can expand our `check_fn` method to:
261 impl EarlyLintPass for Pass {
262 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, _: &FnDecl, span: Span, _: NodeId) {
263 if is_foo_fn(fn_kind) {
268 "function named `foo`",
269 "consider using a more meaningful name"
276 We separate the lint conditional from the lint emissions because it makes the
277 code a bit easier to read. In some cases this separation would also allow to
278 write some unit tests (as opposed to only UI tests) for the separate function.
280 In our example, `is_foo_fn` looks like:
283 // use statements, impl EarlyLintPass, check_fn, ..
285 fn is_foo_fn(fn_kind: FnKind<'_>) -> bool {
287 FnKind::ItemFn(ident, ..) | FnKind::Method(ident, ..) => {
290 FnKind::Closure(..) => false
295 Now you'll want to also run the full test suite with `cargo test`. At this point
296 running `cargo test` should produce the expected output. Remember to run
297 `tests/ui/update-all-references.sh` to update the `.stderr` file.
299 `cargo test` (as opposed to `cargo uitest`) will also ensure that our lint
300 implementation is not violating any Clippy lints itself.
302 If you are still following the example, you will see that `FooFunctionsPass`
303 violates a Clippy lint. So we are going to rename that struct to just `Pass`:
306 #[derive(Copy, Clone)]
309 impl LintPass for Pass { /* .. */ }
312 That should be it for the lint implementation. Running `cargo test` should now
317 If you have trouble implementing your lint, there is also the internal `author`
318 lint to generate Clippy code that detects the offending pattern. It does not
319 work for all of the Rust syntax, but can give a good starting point.
321 The quickest way to use it, is the [Rust playground][play].rust-lang.org).
322 Put the code you want to lint into the editor and add the `#[clippy::author]`
323 attribute above the item. Then run Clippy via `Tools -> Clippy` and you should
324 see the generated code in the output below.
326 [Here][author_example] is an example on the playground.
328 If the command was executed successfully, you can copy the code over to where
329 you are implementing your lint.
333 The final thing before submitting our PR is to add some documentation to our
336 Please document your lint with a doc comment akin to the following:
339 /// **What it does:** Checks for ... (describe what the lint matches).
341 /// **Why is this bad?** Supply the reason for linting the code.
343 /// **Known problems:** None. (Or describe where it could go wrong.)
349 /// Insert a short example of code that triggers the lint
352 /// Insert a short example of improved code that doesn't trigger the lint
354 declare_clippy_lint! { /* ... */ }
357 Once your lint is merged, this documentation will show up in the [lint
362 [Rustfmt](https://github.com/rust-lang/rustfmt) is a tool for formatting Rust code according
363 to style guidelines. Your code has to be formatted by `rustfmt` before a PR can be merged.
365 It can be installed via `rustup`:
368 rustup component add rustfmt
371 Use `cargo fmt --all` to format the whole codebase.
375 If you want to debug parts of your lint implementation, you can use the `dbg!`
376 macro anywhere in your code. Running the tests should then include the debug
377 output in the `stdout` part.
381 Before submitting your PR make sure you followed all of the basic requirements:
383 - [ ] Followed [lint naming conventions][lint_naming]
384 - [ ] Added passing UI tests (including committed `.stderr` file)
385 - [ ] `cargo test` passes locally
386 - [ ] Added lint documentation
390 Here are some pointers to things you are likely going to need for every lint:
392 * [Clippy utils][utils] - Various helper functions. Maybe the function you need
393 is already in here (`implements_trait`, `match_path`, `snippet`, etc)
394 * [Clippy diagnostics][diagnostics]
395 * [The `if_chain` macro][if_chain]
396 * [`in_macro`][in_macro] and [`in_external_macro`][in_external_macro]
398 * [`Applicability`][applicability]
400 For `EarlyLintPass` lints:
402 * [`EarlyLintPass`][early_lint_pass]
403 * [`syntax::ast`][ast]
405 For `LateLintPass` lints:
407 * [`LateLintPass`][late_lint_pass]
411 While most of Clippy's lint utils are documented, most of rustc's internals lack
412 documentation currently. This is unfortunate, but in most cases you can probably
413 get away with copying things from existing similar lints. If you are stuck,
414 don't hesitate to ask on Discord, IRC or in the issue/PR.
416 [lint_list]: https://rust-lang.github.io/rust-clippy/master/index.html
417 [lint_naming]: https://rust-lang.github.io/rfcs/0344-conventions-galore.html#lints
418 [category_level_mapping]: https://github.com/rust-lang/rust-clippy/blob/bd23cb89ec0ea63403a17d3fc5e50c88e38dd54f/clippy_lints/src/lib.rs#L43
419 [declare_clippy_lint]: https://github.com/rust-lang/rust-clippy/blob/a71acac1da7eaf667ab90a1d65d10e5cc4b80191/clippy_lints/src/lib.rs#L39
420 [compilation_stages]: https://rust-lang.github.io/rustc-guide/high-level-overview.html#the-main-stages-of-compilation
421 [check_fn]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/trait.EarlyLintPass.html#method.check_fn
422 [early_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/trait.EarlyLintPass.html
423 [late_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/trait.LateLintPass.html
424 [fn_kind]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/visit/enum.FnKind.html
425 [diagnostics]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/diagnostics.rs
426 [utils]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/mod.rs
427 [ident]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/source_map/symbol/struct.Ident.html
428 [span]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax_pos/struct.Span.html
429 [applicability]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/enum.Applicability.html
430 [if_chain]: https://docs.rs/if_chain/0.1.2/if_chain/
431 [ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/ty/sty/index.html
432 [ast]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/ast/index.html
433 [in_macro]: https://github.com/rust-lang/rust-clippy/blob/d0717d1f9531a03d154aaeb0cad94c243915a146/clippy_lints/src/utils/mod.rs#L94
434 [in_external_macro]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/fn.in_external_macro.html
435 [play]: https://play.rust-lang.org
436 [author_example]: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=f093b986e80ad62f3b67a1f24f5e66e2