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 When working on Clippy, you will need the current git master version of rustc,
27 which can change rapidly. Make sure you're working near rust-clippy's master,
28 and use the `setup-toolchain.sh` script to configure the appropriate toolchain
29 for the Clippy directory.
33 Let's write some tests first that we can execute while we iterate on our lint.
35 Clippy uses UI tests for testing. UI tests check that the output of Clippy is
36 exactly as expected. Each test is just a plain Rust file that contains the code
37 we want to check. The output of Clippy is compared against a `.stderr` file.
38 Note that you don't have to create this file yourself, we'll get to
39 generating the `.stderr` files further down.
41 We start by creating the test file at `tests/ui/foo_functions.rs`. It doesn't
42 really matter what the file is called, but it's a good convention to name it
43 after the lint it is testing, so `foo_functions.rs` it is.
45 Inside the file we put some examples to get started:
48 #![warn(clippy::foo_functions)]
58 // Default trait methods
71 // We also don't want to lint method calls
79 Now we can run the test with `TESTNAME=ui/foo_functions cargo uitest`.
80 Currently this test will fail. If you go through the output you will see that we
81 are told that `clippy::foo_functions` is an unknown lint, which is expected.
83 While we are working on implementing our lint, we can keep running the UI
84 test. That allows us to check if the output is turning into what we want.
86 Once we are satisfied with the output, we need to run
87 `tests/ui/update-all-references.sh` to update the `.stderr` file for our lint.
88 Running `TESTNAME=ui/foo_functions cargo uitest` should pass then. When we
89 commit our lint, we need to commit the generated `.stderr` files, too.
93 If the lint you are working on is making use of structured suggestions, the
94 test file should include a `// run-rustfix` comment at the top. This will
95 additionally run [rustfix](https://github.com/rust-lang-nursery/rustfix) for
96 that test. Rustfix will apply the suggestions from the lint to the code of the
97 test file and compare that to the contents of a `.fixed` file.
99 Use `tests/ui/update-all-references.sh` to automatically generate the
100 `.fixed` file after running the tests.
102 With tests in place, let's have a look at implementing our lint now.
106 Manually testing against an example file can be useful if you have added some
107 `println!`s and the test suite output becomes unreadable. To try Clippy with
108 your local modifications, run `env CLIPPY_TESTS=true cargo run --bin
109 clippy-driver -- -L ./target/debug input.rs` from the working copy root.
113 We start by creating a new file in the `clippy_lints` crate. That's the crate
114 where all the lint code is. We are going to call the file
115 `clippy_lints/src/foo_functions.rs` and import some initial things we need:
118 use rustc::lint::{LintArray, LintPass};
119 use rustc::{declare_tool_lint, lint_array};
122 The next step is to provide a lint declaration. Lints are declared using the
123 [`declare_clippy_lint!`][declare_clippy_lint] macro:
126 declare_clippy_lint! {
129 "function named `foo`, which is not a descriptive name"
133 * `FOO_FUNCTIONS` is the name of our lint. Be sure to follow the [lint naming
134 guidelines][lint_naming] here when naming your lint. In short, the name should
135 state the thing that is being checked for and read well when used with
136 `allow`/`warn`/`deny`.
137 * `pedantic` sets the lint level to `Allow`.
138 The exact mapping can be found [here][category_level_mapping]
139 * The last part should be a text that explains what exactly is wrong with the
142 With our lint declaration done, we will now make sure that it is assigned to a
146 // clippy_lints/src/foo_functions.rs
148 // .. imports and lint declaration ..
150 #[derive(Copy, Clone)]
151 pub struct FooFunctionsPass;
153 impl LintPass for FooFunctionsPass {
154 fn get_lints(&self) -> LintArray {
160 fn name(&self) -> &'static str {
166 Don't worry about the `name` method here. As long as it includes the name of the
167 lint pass it should be fine.
169 Next we need to run `util/dev update_lints` to register the lint in various
170 places, mainly in `clippy_lints/src/lib.rs`.
172 While `update_lints` automates some things, it doesn't automate everything. We
173 will have to register our lint pass manually in the `register_plugins` function
174 in `clippy_lints/src/lib.rs`:
177 reg.register_early_lint_pass(box foo_functions::FooFunctionsPass);
180 This should fix the `unknown clippy lint: clippy::foo_functions` error that we
181 saw when we executed our tests the first time. The next decision we have to make
182 is which lint pass our lint is going to need.
186 Writing a lint that only checks for the name of a function means that we only
187 have to deal with the AST and don't have to deal with the type system at all.
188 This is good, because it makes writing this particular lint less complicated.
190 We have to make this decision with every new Clippy lint. It boils down to using
191 either [`EarlyLintPass`][early_lint_pass] or [`LateLintPass`][late_lint_pass].
193 In short, the `LateLintPass` has access to type information while the
194 `EarlyLintPass` doesn't. If you don't need access to type information, use the
195 `EarlyLintPass`. The `EarlyLintPass` is also faster. However linting speed
196 hasn't really been a concern with Clippy so far.
198 Since we don't need type information for checking the function name, we are
199 going to use the `EarlyLintPass`. It has to be imported as well, changing our
203 use rustc::lint::{LintArray, LintPass, EarlyLintPass, EarlyContext};
204 use rustc::{declare_tool_lint, lint_array};
209 With UI tests and the lint declaration in place, we can start working on the
210 implementation of the lint logic.
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].
232 `span_help_and_lint` seems most appropriate in this case. It allows us to
233 provide an extra help message and we can't really suggest a better name
234 automatically. This is how it looks:
237 impl EarlyLintPass for Pass {
238 fn check_fn(&mut self, cx: &EarlyContext<'_>, _: FnKind<'_>, _: &FnDecl, span: Span, _: NodeId) {
243 "function named `foo`",
244 "consider using a more meaningful name"
250 Running our UI test should now produce output that contains the lint message.
252 ### Adding the lint logic
254 Writing the logic for your lint will most likely be different from our example,
255 so this section is kept rather short.
257 Using the [`check_fn`][check_fn] method gives us access to [`FnKind`][fn_kind]
258 that has two relevant variants for us `FnKind::ItemFn` and `FnKind::Method`.
259 Both provide access to the name of the function/method via an [`Ident`][ident].
261 With that we can expand our `check_fn` method to:
264 impl EarlyLintPass for Pass {
265 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, _: &FnDecl, span: Span, _: NodeId) {
266 if is_foo_fn(fn_kind) {
271 "function named `foo`",
272 "consider using a more meaningful name"
279 We separate the lint conditional from the lint emissions because it makes the
280 code a bit easier to read. In some cases this separation would also allow to
281 write some unit tests (as opposed to only UI tests) for the separate function.
283 In our example, `is_foo_fn` looks like:
286 // use statements, impl EarlyLintPass, check_fn, ..
288 fn is_foo_fn(fn_kind: FnKind<'_>) -> bool {
290 FnKind::ItemFn(ident, ..) | FnKind::Method(ident, ..) => {
293 FnKind::Closure(..) => false
298 Now we should also run the full test suite with `cargo test`. At this point
299 running `cargo test` should produce the expected output. Remember to run
300 `tests/ui/update-all-references.sh` to update the `.stderr` file.
302 `cargo test` (as opposed to `cargo uitest`) will also ensure that our lint
303 implementation is not violating any Clippy lints itself.
305 If you are still following the example, you will see that `FooFunctionsPass`
306 violates a Clippy lint. So we are going to rename that struct to just `Pass`:
309 #[derive(Copy, Clone)]
312 impl LintPass for Pass { /* .. */ }
315 That should be it for the lint implementation. Running `cargo test` should now
320 If you have trouble implementing your lint, there is also the internal `author`
321 lint to generate Clippy code that detects the offending pattern. It does not
322 work for all of the Rust syntax, but can give a good starting point.
324 The quickest way to use it, is the [Rust playground][play].rust-lang.org).
325 Put the code you want to lint into the editor and add the `#[clippy::author]`
326 attribute above the item. Then run Clippy via `Tools -> Clippy` and you should
327 see the generated code in the output below.
329 [Here][author_example] is an example on the playground.
331 If the command was executed successfully, you can copy the code over to where
332 you are implementing your lint.
336 The final thing before submitting our PR is to add some documentation to our
339 Please document your lint with a doc comment akin to the following:
342 declare_clippy_lint! {
343 /// **What it does:** Checks for ... (describe what the lint matches).
345 /// **Why is this bad?** Supply the reason for linting the code.
347 /// **Known problems:** None. (Or describe where it could go wrong.)
353 /// Insert a short example of code that triggers the lint
356 /// Insert a short example of improved code that doesn't trigger the lint
360 "function named `foo`, which is not a descriptive name"
364 Once your lint is merged, this documentation will show up in the [lint
369 [Rustfmt](https://github.com/rust-lang/rustfmt) is a tool for formatting Rust code according
370 to style guidelines. Your code has to be formatted by `rustfmt` before a PR can be merged.
372 It can be installed via `rustup`:
375 rustup component add rustfmt
378 Use `cargo fmt --all` to format the whole codebase.
382 If you want to debug parts of your lint implementation, you can use the `dbg!`
383 macro anywhere in your code. Running the tests should then include the debug
384 output in the `stdout` part.
388 Before submitting your PR make sure you followed all of the basic requirements:
390 - [ ] Followed [lint naming conventions][lint_naming]
391 - [ ] Added passing UI tests (including committed `.stderr` file)
392 - [ ] `cargo test` passes locally
393 - [ ] Executed `util/dev update_lints`
394 - [ ] Added lint documentation
398 Here are some pointers to things you are likely going to need for every lint:
400 * [Clippy utils][utils] - Various helper functions. Maybe the function you need
401 is already in here (`implements_trait`, `match_path`, `snippet`, etc)
402 * [Clippy diagnostics][diagnostics]
403 * [The `if_chain` macro][if_chain]
404 * [`in_macro`][in_macro] and [`in_external_macro`][in_external_macro]
406 * [`Applicability`][applicability]
407 * [The rustc guide][rustc_guide] explains a lot of internal compiler concepts
408 * [The nightly rustc docs][nightly_docs] which has been linked to throughout
411 For `EarlyLintPass` lints:
413 * [`EarlyLintPass`][early_lint_pass]
414 * [`syntax::ast`][ast]
416 For `LateLintPass` lints:
418 * [`LateLintPass`][late_lint_pass]
422 While most of Clippy's lint utils are documented, most of rustc's internals lack
423 documentation currently. This is unfortunate, but in most cases you can probably
424 get away with copying things from existing similar lints. If you are stuck,
425 don't hesitate to ask on Discord, IRC or in the issue/PR.
427 [lint_list]: https://rust-lang.github.io/rust-clippy/master/index.html
428 [lint_naming]: https://rust-lang.github.io/rfcs/0344-conventions-galore.html#lints
429 [category_level_mapping]: https://github.com/rust-lang/rust-clippy/blob/bd23cb89ec0ea63403a17d3fc5e50c88e38dd54f/clippy_lints/src/lib.rs#L43
430 [declare_clippy_lint]: https://github.com/rust-lang/rust-clippy/blob/a71acac1da7eaf667ab90a1d65d10e5cc4b80191/clippy_lints/src/lib.rs#L39
431 [compilation_stages]: https://rust-lang.github.io/rustc-guide/high-level-overview.html#the-main-stages-of-compilation
432 [check_fn]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/trait.EarlyLintPass.html#method.check_fn
433 [early_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/trait.EarlyLintPass.html
434 [late_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/trait.LateLintPass.html
435 [fn_kind]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/visit/enum.FnKind.html
436 [diagnostics]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/diagnostics.rs
437 [utils]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/mod.rs
438 [ident]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/source_map/symbol/struct.Ident.html
439 [span]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax_pos/struct.Span.html
440 [applicability]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/enum.Applicability.html
441 [if_chain]: https://docs.rs/if_chain/0.1.2/if_chain/
442 [ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/ty/sty/index.html
443 [ast]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/ast/index.html
444 [in_macro]: https://github.com/rust-lang/rust-clippy/blob/d0717d1f9531a03d154aaeb0cad94c243915a146/clippy_lints/src/utils/mod.rs#L94
445 [in_external_macro]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/fn.in_external_macro.html
446 [play]: https://play.rust-lang.org
447 [author_example]: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=f093b986e80ad62f3b67a1f24f5e66e2
448 [rustc_guide]: https://rust-lang.github.io/rustc-guide/
449 [nightly_docs]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/