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 * [Documentation](#Documentation)
18 * [Running rustfmt](#Running-rustfmt)
19 * [Debugging](#Debugging)
20 * [PR Checklist](#PR-Checklist)
21 * [Cheatsheet](#Cheatsheet)
25 Clippy depends on the current git master version of rustc, which can change rapidly. Make sure you're
26 working near rust-clippy's master, and use the `setup-toolchain.sh` script to configure the appropriate
27 toolchain for this directory.
31 Let's write some tests first that we can execute while we iterate on our lint.
33 Clippy uses UI tests for testing. UI tests check that the output of Clippy is
34 exactly as expected. Each test is just a plain Rust file that contains the code
35 we want to check. The output of Clippy is compared against a `.stderr` file.
37 Let's create the test file at `tests/ui/foo_functions.rs`. It doesn't really
38 matter what the file is called, but it's a good convention to name it after the
39 lint it is testing, so `foo_functions.rs` it is.
41 Inside we put some examples to get started:
44 #![warn(clippy::foo_functions)]
54 // Default trait methods
67 // We also don't want to lint method calls
75 Now we can run the test with `TESTNAME=ui/foo_functions cargo uitest`.
76 Currently this test will fail. If you go through the output you will see that we
77 have to add some missing imports to our lint file.
79 While you are working on implementing your lint, you can keep running the UI
80 test. That allows you to check if the output is turning into what you want.
82 Once you are satisfied with the output, you need to run
83 `tests/ui/update-all-references.sh` to update the `stderr` file for your lint.
84 Running `TESTNAME=ui/foo_functions cargo uitest` should pass then. When you
85 commit your lint, be sure to commit the `*.stderr` files, too.
89 If the lint you are working on is making use of structured suggestions, the
90 test file should include a `// run-rustfix` comment at the top. This will
91 additionally run [rustfix](https://github.com/rust-lang-nursery/rustfix) for
92 that test. Rustfix will apply the suggestions from the lint to the code of the
93 test file and compare that to the contents of a `.fixed` file.
95 Use `tests/ui/update-all-references.sh` to automatically generate the
96 `.fixed` file after running `cargo test`.
98 With tests in place, let's have a look at implementing our lint now.
102 Manually testing against an example file is useful if you have added some
103 `println!`s and test suite output becomes unreadable. To try Clippy with your
104 local modifications, run `env CLIPPY_TESTS=true cargo run --bin clippy-driver -- -L ./target/debug input.rs`
105 from the working copy root.
109 We start by creating a new file in the `clippy_lints` crate. That's the crate
110 where all the lint code is. We are going to call the file
111 `clippy_lints/src/foo_functions.rs` and import some initial things we need:
114 use rustc::lint::{LintArray, LintPass};
115 use rustc::{declare_tool_lint, lint_array};
118 The next step is to provide a lint declaration. Lints are declared using the
119 [`declare_clippy_lint!`][declare_clippy_lint] macro:
122 declare_clippy_lint! {
125 "function named `foo`, which is not a descriptive name"
129 * `FOO_FUNCTIONS` is the name of our lint. Be sure to follow the [lint naming
130 guidelines][lint_naming] here when naming your lint. In short, the name should
131 state the thing that is being checked for and read well when used with
132 `allow`/`warn`/`deny`.
133 * `pedantic` sets the lint level to `Allow`.
134 The exact mapping can be found [here][category_level_mapping]
135 * The last part should be a text that explains what exactly is wrong with the
138 With our lint declaration done, we will now make sure that our lint declaration
139 is assigned to a lint pass:
142 // clippy_lints/src/foo_functions.rs
144 // .. imports and lint declaration ..
146 #[derive(Copy, Clone)]
147 pub struct FooFunctionsPass;
149 impl LintPass for FooFunctionsPass {
150 fn get_lints(&self) -> LintArray {
156 fn name(&self) -> &'static str {
162 Don't worry about the `name` method here. As long as it includes the name of the
163 lint pass it should be fine.
165 Next you should run `util/dev update_lints` to register the lint in various
166 places, mainly in `clippy_lints/src/lib.rs`.
168 While `update_lints` automates some things, it doesn't automate everything. We
169 will have to register our lint pass manually in the `register_plugins` function
170 in `clippy_lints/src/lib.rs`:
173 reg.register_early_lint_pass(box foo_functions::FooFunctionsPass);
176 Without that, running the UI tests would produce an error like `unknown clippy
177 lint: clippy::foo_functions`. The next decision we have to make is which lint
178 pass our lint is going to need.
182 Writing a lint that just checks for the name of a function means that we just
183 have to deal with the AST and don't have to deal with the type system at all.
184 This is good, because it makes writing this particular lint less complicated.
186 We have to make this decision with every new Clippy lint. It boils down to using
187 either [`EarlyLintPass`][early_lint_pass] or [`LateLintPass`][late_lint_pass].
188 This is a result of Rust's compilation process. You can read more about it [in
189 the rustc guide][compilation_stages].
191 In short, the `LateLintPass` has access to type information while the
192 `EarlyLintPass` doesn't. If you don't need access to type information, use the
193 `EarlyLintPass`. The `EarlyLintPass` is also faster. However linting speed
194 hasn't really been a concern with Clippy so far.
196 Since we don't need type information for checking the function name, we are
197 going to use the `EarlyLintPass`. It has to be imported as well, changing our
201 use rustc::lint::{LintArray, LintPass, EarlyLintPass, EarlyContext};
202 use rustc::{declare_tool_lint, lint_array};
207 With UI tests in place, we can start working on the implementation of the lint logic. We can keep executing the tests until we make them pass.
209 Let's start by implementing the `EarlyLintPass` for our `FooFunctionsPass`:
212 impl EarlyLintPass for FooFunctionsPass {
213 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, _: &FnDecl, span: Span, _: NodeId) {
214 // TODO: Emit lint here
219 We implement the [`check_fn`][check_fn] method from the
220 [`EarlyLintPass`][early_lint_pass] trait. This gives us access to various
221 information about the function that is currently being checked. More on that in
222 the next section. Let's worry about the details later and emit our lint for
223 *every* function definition first.
225 Depending on how complex we want our lint message to be, we can choose from a
226 variety of lint emission functions. They can all be found in
227 [`clippy_lints/src/utils/diagnostics.rs`][diagnostics].
231 impl EarlyLintPass for Pass {
232 fn check_fn(&mut self, cx: &EarlyContext<'_>, _: FnKind<'_>, _: &FnDecl, span: Span, _: NodeId) {
237 "function named `foo`",
238 "consider using a more meaningful name"
244 ### Adding the lint logic
246 Writing the logic for your lint will most likely be different from this example,
247 so this section is kept rather short.
249 Using the [`check_fn`][check_fn] method gives us access to [`FnKind`][fn_kind]
250 that has two relevant variants for us `FnKind::ItemFn` and `FnKind::Method`.
251 Both provide access to the name of the function/method via an [`Ident`][ident].
253 With that we can expand our `check_fn` method to:
256 impl EarlyLintPass for Pass {
257 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, _: &FnDecl, span: Span, _: NodeId) {
258 if is_foo_fn(fn_kind) {
263 "function named `foo`",
264 "consider using a more meaningful name"
271 We separate the lint conditional from the lint emissions because it makes the
272 code a bit easier to read. In some cases this separation would also allow to
273 write some unit tests (as opposed to UI tests) for the separate function.
275 In our example, `is_foo_fn` looks like:
278 // use statements, impl EarlyLintPass, check_fn, ..
280 fn is_foo_fn(fn_kind: FnKind<'_>) -> bool {
282 FnKind::ItemFn(ident, ..) | FnKind::Method(ident, ..) => {
285 FnKind::Closure(..) => false
290 Now you'll want to also run the full test suite with `cargo test`. At this point
291 running `cargo test` should produce the expected output. Remember to run
292 `tests/ui/update-all-references.sh` to update the `.stderr` file.
294 `cargo test` (as opposed to `cargo uitest`) will also ensure that our lint
295 implementation is not violating any Clippy lints itself.
297 If you are still following the example, you'll see that the `FooFunctionsPass`
298 violates a Clippy lint. So we are going to rename that struct to just `Pass`:
301 #[derive(Copy, Clone)]
304 impl LintPass for Pass { /* .. */ }
307 That should be it for the lint implementation. Running `cargo test` should now
312 If you have trouble implementing your lint, there is also the internal `author`
313 lint to generate Clippy code that detects the offending pattern. It does not
314 work for all of the Rust syntax, but can give a good starting point.
316 First, create a new UI test file in the `tests/ui/` directory with the pattern
320 // ./tests/ui/my_lint.rs
323 let arr: [i32; 1] = [7]; // Replace line with the code you want to match
327 Now you run `TESTNAME=ui/my_lint cargo uitest` to produce a `.stdout` file with
331 // ./tests/ui/my_lint.stdout
334 if let ExprKind::Array(ref elements) = stmt.node;
335 if elements.len() == 1;
336 if let ExprKind::Lit(ref lit) = elements[0].node;
337 if let LitKind::Int(7, _) = lit.node;
339 // report your lint here
344 If the command was executed successfully, you can copy the code over to where
345 you are implementing your lint.
349 The final thing before submitting our PR is to add some documentation to our
352 Please document your lint with a doc comment akin to the following:
355 /// **What it does:** Checks for ... (describe what the lint matches).
357 /// **Why is this bad?** Supply the reason for linting the code.
359 /// **Known problems:** None. (Or describe where it could go wrong.)
365 /// Insert a short example of code that triggers the lint
368 /// Insert a short example of improved code that doesn't trigger the lint
370 declare_clippy_lint! { /* ... */ }
373 Once your lint is merged, this documentation will show up in the [lint
378 [Rustfmt](https://github.com/rust-lang/rustfmt) is a tool for formatting Rust code according
379 to style guidelines. Your code has to be formatted by `rustfmt` before a PR can be merged.
381 It can be installed via `rustup`:
384 rustup component add rustfmt
387 Use `cargo fmt --all` to format the whole codebase.
391 If you want to debug parts of your lint implementation, you can use the `dbg!`
392 macro anywhere in your code. Running the tests should then include the debug
393 output in the `stdout` part.
397 Before submitting your PR make sure you followed all of the basic requirements:
399 - [ ] Followed [lint naming conventions][lint_naming]
400 - [ ] Added passing UI tests (including committed `.stderr` file)
401 - [ ] `cargo test` passes locally
402 - [ ] Added lint documentation
406 Here are some pointers to things you are likely going to need for every lint:
408 * [Clippy utils][utils] - Various helper functions. Maybe the function you need
409 is already in here (`implements_trait`, `match_path`, `snippet`, etc)
410 * [Clippy diagnostics][diagnostics]
411 * [The `if_chain` macro][if_chain]
412 * [`in_macro`][in_macro] and [`in_external_macro`][in_external_macro]
414 * [`Applicability`][applicability]
416 For `EarlyLintPass` lints:
418 * [`EarlyLintPass`][early_lint_pass]
419 * [`syntax::ast`][ast]
421 For `LateLintPass` lints:
423 * [`LateLintPass`][late_lint_pass]
427 While most of Clippy's lint utils are documented, most of rustc's internals lack
428 documentation currently. This is unfortunate, but in most cases you can probably
429 get away with copying things from existing similar lints. If you are stuck,
430 don't hesitate to ask on Discord, IRC or in the issue/PR.
432 [lint_list]: https://rust-lang.github.io/rust-clippy/master/index.html
433 [lint_naming]: https://rust-lang.github.io/rfcs/0344-conventions-galore.html#lints
434 [category_level_mapping]: https://github.com/rust-lang/rust-clippy/blob/bd23cb89ec0ea63403a17d3fc5e50c88e38dd54f/clippy_lints/src/lib.rs#L43
435 [declare_clippy_lint]: https://github.com/rust-lang/rust-clippy/blob/a71acac1da7eaf667ab90a1d65d10e5cc4b80191/clippy_lints/src/lib.rs#L39
436 [compilation_stages]: https://rust-lang.github.io/rustc-guide/high-level-overview.html#the-main-stages-of-compilation
437 [check_fn]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/trait.EarlyLintPass.html#method.check_fn
438 [early_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/trait.EarlyLintPass.html
439 [late_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/trait.LateLintPass.html
440 [fn_kind]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/visit/enum.FnKind.html
441 [diagnostics]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/diagnostics.rs
442 [utils]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/mod.rs
443 [ident]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/source_map/symbol/struct.Ident.html
444 [span]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax_pos/struct.Span.html
445 [applicability]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/enum.Applicability.html
446 [if_chain]: https://docs.rs/if_chain/0.1.2/if_chain/
447 [ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/ty/sty/index.html
448 [ast]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/ast/index.html
449 [in_macro]: https://github.com/rust-lang/rust-clippy/blob/d0717d1f9531a03d154aaeb0cad94c243915a146/clippy_lints/src/utils/mod.rs#L94
450 [in_external_macro]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/fn.in_external_macro.html