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.
38 Let's create the test file at `tests/ui/foo_functions.rs`. It doesn't really
39 matter what the file is called, but it's a good convention to name it after the
40 lint it is testing, so `foo_functions.rs` it is.
42 Inside we put some examples to get started:
45 #![warn(clippy::foo_functions)]
55 // Default trait methods
68 // We also don't want to lint method calls
76 Now we can run the test with `TESTNAME=ui/foo_functions cargo uitest`.
77 Currently this test will fail. If you go through the output you will see that we
78 have to add some missing imports to our lint file.
80 While you are working on implementing your lint, you can keep running the UI
81 test. That allows you to check if the output is turning into what you want.
83 Once you are satisfied with the output, you need to run
84 `tests/ui/update-all-references.sh` to update the `stderr` file for your lint.
85 Running `TESTNAME=ui/foo_functions cargo uitest` should pass then. When you
86 commit your lint, be sure to commit the `*.stderr` files, too.
90 If the lint you are working on is making use of structured suggestions, the
91 test file should include a `// run-rustfix` comment at the top. This will
92 additionally run [rustfix](https://github.com/rust-lang-nursery/rustfix) for
93 that test. Rustfix will apply the suggestions from the lint to the code of the
94 test file and compare that to the contents of a `.fixed` file.
96 Use `tests/ui/update-all-references.sh` to automatically generate the
97 `.fixed` file after running `cargo test`.
99 With tests in place, let's have a look at implementing our lint now.
103 Manually testing against an example file is useful if you have added some
104 `println!`s and test suite output becomes unreadable. To try Clippy with your
105 local modifications, run `env CLIPPY_TESTS=true cargo run --bin clippy-driver -- -L ./target/debug input.rs`
106 from the working copy root.
110 We start by creating a new file in the `clippy_lints` crate. That's the crate
111 where all the lint code is. We are going to call the file
112 `clippy_lints/src/foo_functions.rs` and import some initial things we need:
115 use rustc::lint::{LintArray, LintPass};
116 use rustc::{declare_tool_lint, lint_array};
119 The next step is to provide a lint declaration. Lints are declared using the
120 [`declare_clippy_lint!`][declare_clippy_lint] macro:
123 declare_clippy_lint! {
126 "function named `foo`, which is not a descriptive name"
130 * `FOO_FUNCTIONS` is the name of our lint. Be sure to follow the [lint naming
131 guidelines][lint_naming] here when naming your lint. In short, the name should
132 state the thing that is being checked for and read well when used with
133 `allow`/`warn`/`deny`.
134 * `pedantic` sets the lint level to `Allow`.
135 The exact mapping can be found [here][category_level_mapping]
136 * The last part should be a text that explains what exactly is wrong with the
139 With our lint declaration done, we will now make sure that our lint declaration
140 is assigned to a lint pass:
143 // clippy_lints/src/foo_functions.rs
145 // .. imports and lint declaration ..
147 #[derive(Copy, Clone)]
148 pub struct FooFunctionsPass;
150 impl LintPass for FooFunctionsPass {
151 fn get_lints(&self) -> LintArray {
157 fn name(&self) -> &'static str {
163 Don't worry about the `name` method here. As long as it includes the name of the
164 lint pass it should be fine.
166 Next you should run `util/dev update_lints` to register the lint in various
167 places, mainly in `clippy_lints/src/lib.rs`.
169 While `update_lints` automates some things, it doesn't automate everything. You
170 will have to register our lint pass manually in the `register_plugins` function
171 in `clippy_lints/src/lib.rs`:
174 reg.register_early_lint_pass(box foo_functions::FooFunctionsPass);
177 Without that, running the UI tests would produce an error like `unknown clippy
178 lint: clippy::foo_functions`. The next decision we have to make is which lint
179 pass our lint is going to need.
183 Writing a lint that only checks for the name of a function means that we only
184 have to deal with the AST and don't have to deal with the type system at all.
185 This is good, because it makes writing this particular lint less complicated.
187 We have to make this decision with every new Clippy lint. It boils down to using
188 either [`EarlyLintPass`][early_lint_pass] or [`LateLintPass`][late_lint_pass].
189 This is a result of Rust's compilation process. You can read more about it [in
190 the rustc guide][compilation_stages].
192 In short, the `LateLintPass` has access to type information while the
193 `EarlyLintPass` doesn't. If you don't need access to type information, use the
194 `EarlyLintPass`. The `EarlyLintPass` is also faster. However linting speed
195 hasn't really been a concern with Clippy so far.
197 Since we don't need type information for checking the function name, we are
198 going to use the `EarlyLintPass`. It has to be imported as well, changing our
202 use rustc::lint::{LintArray, LintPass, EarlyLintPass, EarlyContext};
203 use rustc::{declare_tool_lint, lint_array};
208 With UI tests and the lint declaration in place, we can start working on the
209 implementation of the lint logic. We can keep executing the tests until we make
212 Let's start by implementing the `EarlyLintPass` for our `FooFunctionsPass`:
215 impl EarlyLintPass for FooFunctionsPass {
216 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, _: &FnDecl, span: Span, _: NodeId) {
217 // TODO: Emit lint here
222 We implement the [`check_fn`][check_fn] method from the
223 [`EarlyLintPass`][early_lint_pass] trait. This gives us access to various
224 information about the function that is currently being checked. More on that in
225 the next section. Let's worry about the details later and emit our lint for
226 *every* function definition first.
228 Depending on how complex we want our lint message to be, we can choose from a
229 variety of lint emission functions. They can all be found in
230 [`clippy_lints/src/utils/diagnostics.rs`][diagnostics].
234 impl EarlyLintPass for Pass {
235 fn check_fn(&mut self, cx: &EarlyContext<'_>, _: FnKind<'_>, _: &FnDecl, span: Span, _: NodeId) {
240 "function named `foo`",
241 "consider using a more meaningful name"
247 ### Adding the lint logic
249 Writing the logic for your lint will most likely be different from this example,
250 so this section is kept rather short.
252 Using the [`check_fn`][check_fn] method gives us access to [`FnKind`][fn_kind]
253 that has two relevant variants for us `FnKind::ItemFn` and `FnKind::Method`.
254 Both provide access to the name of the function/method via an [`Ident`][ident].
256 With that we can expand our `check_fn` method to:
259 impl EarlyLintPass for Pass {
260 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, _: &FnDecl, span: Span, _: NodeId) {
261 if is_foo_fn(fn_kind) {
266 "function named `foo`",
267 "consider using a more meaningful name"
274 We separate the lint conditional from the lint emissions because it makes the
275 code a bit easier to read. In some cases this separation would also allow to
276 write some unit tests (as opposed to only UI tests) for the separate function.
278 In our example, `is_foo_fn` looks like:
281 // use statements, impl EarlyLintPass, check_fn, ..
283 fn is_foo_fn(fn_kind: FnKind<'_>) -> bool {
285 FnKind::ItemFn(ident, ..) | FnKind::Method(ident, ..) => {
288 FnKind::Closure(..) => false
293 Now you'll want to also run the full test suite with `cargo test`. At this point
294 running `cargo test` should produce the expected output. Remember to run
295 `tests/ui/update-all-references.sh` to update the `.stderr` file.
297 `cargo test` (as opposed to `cargo uitest`) will also ensure that our lint
298 implementation is not violating any Clippy lints itself.
300 If you are still following the example, you will see that `FooFunctionsPass`
301 violates a Clippy lint. So we are going to rename that struct to just `Pass`:
304 #[derive(Copy, Clone)]
307 impl LintPass for Pass { /* .. */ }
310 That should be it for the lint implementation. Running `cargo test` should now
315 If you have trouble implementing your lint, there is also the internal `author`
316 lint to generate Clippy code that detects the offending pattern. It does not
317 work for all of the Rust syntax, but can give a good starting point.
319 The quickest way to use it, is the [Rust playground][play].rust-lang.org).
320 Put the code you want to lint into the editor and add the `#[clippy::author]`
321 attribute above the item. Then run Clippy via `Tools -> Clippy` and you should
322 see the generated code in the output below.
324 [Here][author_example] is an example on the playground.
326 If the command was executed successfully, you can copy the code over to where
327 you are implementing your lint.
331 The final thing before submitting our PR is to add some documentation to our
334 Please document your lint with a doc comment akin to the following:
337 /// **What it does:** Checks for ... (describe what the lint matches).
339 /// **Why is this bad?** Supply the reason for linting the code.
341 /// **Known problems:** None. (Or describe where it could go wrong.)
347 /// Insert a short example of code that triggers the lint
350 /// Insert a short example of improved code that doesn't trigger the lint
352 declare_clippy_lint! { /* ... */ }
355 Once your lint is merged, this documentation will show up in the [lint
360 [Rustfmt](https://github.com/rust-lang/rustfmt) is a tool for formatting Rust code according
361 to style guidelines. Your code has to be formatted by `rustfmt` before a PR can be merged.
363 It can be installed via `rustup`:
366 rustup component add rustfmt
369 Use `cargo fmt --all` to format the whole codebase.
373 If you want to debug parts of your lint implementation, you can use the `dbg!`
374 macro anywhere in your code. Running the tests should then include the debug
375 output in the `stdout` part.
379 Before submitting your PR make sure you followed all of the basic requirements:
381 - [ ] Followed [lint naming conventions][lint_naming]
382 - [ ] Added passing UI tests (including committed `.stderr` file)
383 - [ ] `cargo test` passes locally
384 - [ ] Added lint documentation
388 Here are some pointers to things you are likely going to need for every lint:
390 * [Clippy utils][utils] - Various helper functions. Maybe the function you need
391 is already in here (`implements_trait`, `match_path`, `snippet`, etc)
392 * [Clippy diagnostics][diagnostics]
393 * [The `if_chain` macro][if_chain]
394 * [`in_macro`][in_macro] and [`in_external_macro`][in_external_macro]
396 * [`Applicability`][applicability]
398 For `EarlyLintPass` lints:
400 * [`EarlyLintPass`][early_lint_pass]
401 * [`syntax::ast`][ast]
403 For `LateLintPass` lints:
405 * [`LateLintPass`][late_lint_pass]
409 While most of Clippy's lint utils are documented, most of rustc's internals lack
410 documentation currently. This is unfortunate, but in most cases you can probably
411 get away with copying things from existing similar lints. If you are stuck,
412 don't hesitate to ask on Discord, IRC or in the issue/PR.
414 [lint_list]: https://rust-lang.github.io/rust-clippy/master/index.html
415 [lint_naming]: https://rust-lang.github.io/rfcs/0344-conventions-galore.html#lints
416 [category_level_mapping]: https://github.com/rust-lang/rust-clippy/blob/bd23cb89ec0ea63403a17d3fc5e50c88e38dd54f/clippy_lints/src/lib.rs#L43
417 [declare_clippy_lint]: https://github.com/rust-lang/rust-clippy/blob/a71acac1da7eaf667ab90a1d65d10e5cc4b80191/clippy_lints/src/lib.rs#L39
418 [compilation_stages]: https://rust-lang.github.io/rustc-guide/high-level-overview.html#the-main-stages-of-compilation
419 [check_fn]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/trait.EarlyLintPass.html#method.check_fn
420 [early_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/trait.EarlyLintPass.html
421 [late_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/trait.LateLintPass.html
422 [fn_kind]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/visit/enum.FnKind.html
423 [diagnostics]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/diagnostics.rs
424 [utils]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/mod.rs
425 [ident]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/source_map/symbol/struct.Ident.html
426 [span]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax_pos/struct.Span.html
427 [applicability]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/enum.Applicability.html
428 [if_chain]: https://docs.rs/if_chain/0.1.2/if_chain/
429 [ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/ty/sty/index.html
430 [ast]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/ast/index.html
431 [in_macro]: https://github.com/rust-lang/rust-clippy/blob/d0717d1f9531a03d154aaeb0cad94c243915a146/clippy_lints/src/utils/mod.rs#L94
432 [in_external_macro]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/fn.in_external_macro.html
433 [play]: https://play.rust-lang.org
434 [author_example]: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=f093b986e80ad62f3b67a1f24f5e66e2