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.
10 - [Adding a new lint](#adding-a-new-lint)
12 - [Getting Started](#getting-started)
14 - [Rustfix tests](#rustfix-tests)
15 - [Edition 2018 tests](#edition-2018-tests)
16 - [Testing manually](#testing-manually)
17 - [Lint declaration](#lint-declaration)
18 - [Lint passes](#lint-passes)
19 - [Emitting a lint](#emitting-a-lint)
20 - [Adding the lint logic](#adding-the-lint-logic)
21 - [Author lint](#author-lint)
22 - [Documentation](#documentation)
23 - [Running rustfmt](#running-rustfmt)
24 - [Debugging](#debugging)
25 - [PR Checklist](#pr-checklist)
26 - [Cheatsheet](#cheatsheet)
30 When working on Clippy, you will need the current git master version of rustc,
31 which can change rapidly. Make sure you're working near rust-clippy's master,
32 and use the `setup-toolchain.sh` script to configure the appropriate toolchain
33 for the Clippy directory.
37 There is a bit of boilerplate code that needs to be set up when creating a new
38 lint. Fortunately, you can use the clippy dev tools to handle this for you. We
39 are naming our new lint `foo_functions` (lints are generally written in snake
40 case), and we don't need type information so it will have an early pass type
41 (more on this later on). To get started on this lint you can run
42 `cargo dev new_lint --name=foo_functions --pass=early --category=pedantic`
43 (category will default to nursery if not provided). This command will create
44 two files: `tests/ui/foo_functions.rs` and `clippy_lints/src/foo_functions.rs`,
45 as well as run `cargo dev update_lints` to register the new lint. Next, we'll
46 open up these files and add our lint!
50 Let's write some tests first that we can execute while we iterate on our lint.
52 Clippy uses UI tests for testing. UI tests check that the output of Clippy is
53 exactly as expected. Each test is just a plain Rust file that contains the code
54 we want to check. The output of Clippy is compared against a `.stderr` file.
55 Note that you don't have to create this file yourself, we'll get to
56 generating the `.stderr` files further down.
58 We start by opening the test file created at `tests/ui/foo_functions.rs`.
60 Update the file with some examples to get started:
63 #![warn(clippy::foo_functions)]
73 // Default trait methods
86 // We also don't want to lint method calls
93 Now we can run the test with `TESTNAME=foo_functions cargo uitest`,
94 currently this test is meaningless though.
96 While we are working on implementing our lint, we can keep running the UI
97 test. That allows us to check if the output is turning into what we want.
99 Once we are satisfied with the output, we need to run
100 `tests/ui/update-all-references.sh` to update the `.stderr` file for our lint.
101 Please note that, we should run `TESTNAME=foo_functions cargo uitest`
102 every time before running `tests/ui/update-all-references.sh`.
103 Running `TESTNAME=foo_functions cargo uitest` should pass then. When we commit
104 our lint, we need to commit the generated `.stderr` files, too.
108 If the lint you are working on is making use of structured suggestions, the
109 test file should include a `// run-rustfix` comment at the top. This will
110 additionally run [rustfix] for that test. Rustfix will apply the suggestions
111 from the lint to the code of the test file and compare that to the contents of
114 Use `tests/ui/update-all-references.sh` to automatically generate the
115 `.fixed` file after running the tests.
117 [rustfix]: https://github.com/rust-lang/rustfix
119 ## Edition 2018 tests
121 Some features require the 2018 edition to work (e.g. `async_await`), but
122 compile-test tests run on the 2015 edition by default. To change this behavior
123 add `// edition:2018` at the top of the test file (note that it's space-sensitive).
127 Manually testing against an example file can be useful if you have added some
128 `println!`s and the test suite output becomes unreadable. To try Clippy with
129 your local modifications, run `env CLIPPY_TESTS=true cargo run --bin
130 clippy-driver -- -L ./target/debug input.rs` from the working copy root.
132 With tests in place, let's have a look at implementing our lint now.
136 Let's start by opening the new file created in the `clippy_lints` crate
137 at `clippy_lints/src/foo_functions.rs`. That's the crate where all the
138 lint code is. This file has already imported some initial things we will need:
141 use rustc_lint::{EarlyLintPass, EarlyContext};
142 use rustc_session::{declare_lint_pass, declare_tool_lint};
143 use rustc_ast::ast::*;
146 The next step is to update the lint declaration. Lints are declared using the
147 [`declare_clippy_lint!`][declare_clippy_lint] macro, and we just need to update
148 the auto-generated lint declaration to have a real description, something like this:
151 declare_clippy_lint! {
152 /// **What it does:**
154 /// **Why is this bad?**
156 /// **Known problems:** None.
165 "function named `foo`, which is not a descriptive name"
169 * The section of lines prefixed with `///` constitutes the lint documentation
170 section. This is the default documentation style and will be displayed
171 [like this][example_lint_page].
172 * `FOO_FUNCTIONS` is the name of our lint. Be sure to follow the
173 [lint naming guidelines][lint_naming] here when naming your lint.
174 In short, the name should state the thing that is being checked for and
175 read well when used with `allow`/`warn`/`deny`.
176 * `pedantic` sets the lint level to `Allow`.
177 The exact mapping can be found [here][category_level_mapping]
178 * The last part should be a text that explains what exactly is wrong with the
181 The rest of this file contains an empty implementation for our lint pass,
182 which in this case is `EarlyLintPass` and should look like this:
185 // clippy_lints/src/foo_functions.rs
187 // .. imports and lint declaration ..
189 declare_lint_pass!(FooFunctions => [FOO_FUNCTIONS]);
191 impl EarlyLintPass for FooFunctions {}
194 Normally after declaring the lint, we have to run `cargo dev update_lints`,
195 which updates some files, so Clippy knows about the new lint. Since we used
196 `cargo dev new_lint ...` to generate the lint declaration, this was done
197 automatically. While `update_lints` automates most of the things, it doesn't
198 automate everything. We will have to register our lint pass manually in the
199 `register_plugins` function in `clippy_lints/src/lib.rs`:
202 store.register_early_pass(|| box foo_functions::FooFunctions);
205 [declare_clippy_lint]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L60
206 [example_lint_page]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure
207 [lint_naming]: https://rust-lang.github.io/rfcs/0344-conventions-galore.html#lints
208 [category_level_mapping]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L110
212 Writing a lint that only checks for the name of a function means that we only
213 have to deal with the AST and don't have to deal with the type system at all.
214 This is good, because it makes writing this particular lint less complicated.
216 We have to make this decision with every new Clippy lint. It boils down to using
217 either [`EarlyLintPass`][early_lint_pass] or [`LateLintPass`][late_lint_pass].
219 In short, the `LateLintPass` has access to type information while the
220 `EarlyLintPass` doesn't. If you don't need access to type information, use the
221 `EarlyLintPass`. The `EarlyLintPass` is also faster. However linting speed
222 hasn't really been a concern with Clippy so far.
224 Since we don't need type information for checking the function name, we used
225 `--pass=early` when running the new lint automation and all the imports were
228 [early_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html
229 [late_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.LateLintPass.html
233 With UI tests and the lint declaration in place, we can start working on the
234 implementation of the lint logic.
236 Let's start by implementing the `EarlyLintPass` for our `FooFunctions`:
239 impl EarlyLintPass for FooFunctions {
240 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
241 // TODO: Emit lint here
246 We implement the [`check_fn`][check_fn] method from the
247 [`EarlyLintPass`][early_lint_pass] trait. This gives us access to various
248 information about the function that is currently being checked. More on that in
249 the next section. Let's worry about the details later and emit our lint for
250 *every* function definition first.
252 Depending on how complex we want our lint message to be, we can choose from a
253 variety of lint emission functions. They can all be found in
254 [`clippy_lints/src/utils/diagnostics.rs`][diagnostics].
256 `span_lint_and_help` seems most appropriate in this case. It allows us to
257 provide an extra help message and we can't really suggest a better name
258 automatically. This is how it looks:
261 impl EarlyLintPass for FooFunctions {
262 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
267 "function named `foo`",
268 "consider using a more meaningful name"
274 Running our UI test should now produce output that contains the lint message.
276 [check_fn]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html#method.check_fn
277 [diagnostics]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/diagnostics.rs
279 ## Adding the lint logic
281 Writing the logic for your lint will most likely be different from our example,
282 so this section is kept rather short.
284 Using the [`check_fn`][check_fn] method gives us access to [`FnKind`][fn_kind]
285 that has the [`FnKind::Fn`] variant. It provides access to the name of the
286 function/method via an [`Ident`][ident].
288 With that we can expand our `check_fn` method to:
291 impl EarlyLintPass for FooFunctions {
292 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
293 if is_foo_fn(fn_kind) {
298 "function named `foo`",
299 "consider using a more meaningful name"
306 We separate the lint conditional from the lint emissions because it makes the
307 code a bit easier to read. In some cases this separation would also allow to
308 write some unit tests (as opposed to only UI tests) for the separate function.
310 In our example, `is_foo_fn` looks like:
313 // use statements, impl EarlyLintPass, check_fn, ..
315 fn is_foo_fn(fn_kind: FnKind<'_>) -> bool {
317 FnKind::Fn(_, ident, ..) => {
318 // check if `fn` name is `foo`
319 ident.name.as_str() == "foo"
322 FnKind::Closure(..) => false
327 Now we should also run the full test suite with `cargo test`. At this point
328 running `cargo test` should produce the expected output. Remember to run
329 `tests/ui/update-all-references.sh` to update the `.stderr` file.
331 `cargo test` (as opposed to `cargo uitest`) will also ensure that our lint
332 implementation is not violating any Clippy lints itself.
334 That should be it for the lint implementation. Running `cargo test` should now
337 [fn_kind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html
338 [`FnKind::Fn`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html#variant.Fn
339 [ident]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Ident.html
343 If you have trouble implementing your lint, there is also the internal `author`
344 lint to generate Clippy code that detects the offending pattern. It does not
345 work for all of the Rust syntax, but can give a good starting point.
347 The quickest way to use it, is the
348 [Rust playground: play.rust-lang.org][author_example].
349 Put the code you want to lint into the editor and add the `#[clippy::author]`
350 attribute above the item. Then run Clippy via `Tools -> Clippy` and you should
351 see the generated code in the output below.
353 [Here][author_example] is an example on the playground.
355 If the command was executed successfully, you can copy the code over to where
356 you are implementing your lint.
358 [author_example]: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=9a12cb60e5c6ad4e3003ac6d5e63cf55
362 The final thing before submitting our PR is to add some documentation to our
365 Please document your lint with a doc comment akin to the following:
368 declare_clippy_lint! {
369 /// **What it does:** Checks for ... (describe what the lint matches).
371 /// **Why is this bad?** Supply the reason for linting the code.
373 /// **Known problems:** None. (Or describe where it could go wrong.)
379 /// Insert a short example of code that triggers the lint
382 /// Insert a short example of improved code that doesn't trigger the lint
386 "function named `foo`, which is not a descriptive name"
390 Once your lint is merged, this documentation will show up in the [lint
393 [lint_list]: https://rust-lang.github.io/rust-clippy/master/index.html
397 [Rustfmt] is a tool for formatting Rust code according to style guidelines.
398 Your code has to be formatted by `rustfmt` before a PR can be merged.
399 Clippy uses nightly `rustfmt` in the CI.
401 It can be installed via `rustup`:
404 rustup component add rustfmt --toolchain=nightly
407 Use `cargo dev fmt` to format the whole codebase. Make sure that `rustfmt` is
408 installed for the nightly toolchain.
410 [Rustfmt]: https://github.com/rust-lang/rustfmt
414 If you want to debug parts of your lint implementation, you can use the [`dbg!`]
415 macro anywhere in your code. Running the tests should then include the debug
416 output in the `stdout` part.
418 [`dbg!`]: https://doc.rust-lang.org/std/macro.dbg.html
422 Before submitting your PR make sure you followed all of the basic requirements:
424 <!-- Sync this with `.github/PULL_REQUEST_TEMPLATE` -->
426 - [ ] Followed [lint naming conventions][lint_naming]
427 - [ ] Added passing UI tests (including committed `.stderr` file)
428 - [ ] `cargo test` passes locally
429 - [ ] Executed `cargo dev update_lints`
430 - [ ] Added lint documentation
431 - [ ] Run `cargo dev fmt`
435 Here are some pointers to things you are likely going to need for every lint:
437 * [Clippy utils][utils] - Various helper functions. Maybe the function you need
438 is already in here (`implements_trait`, `match_path`, `snippet`, etc)
439 * [Clippy diagnostics][diagnostics]
440 * [The `if_chain` macro][if_chain]
441 * [`from_expansion`][from_expansion] and [`in_external_macro`][in_external_macro]
443 * [`Applicability`][applicability]
444 * [The rustc-dev-guide][rustc-dev-guide] explains a lot of internal compiler concepts
445 * [The nightly rustc docs][nightly_docs] which has been linked to throughout
448 For `EarlyLintPass` lints:
450 * [`EarlyLintPass`][early_lint_pass]
451 * [`rustc_ast::ast`][ast]
453 For `LateLintPass` lints:
455 * [`LateLintPass`][late_lint_pass]
458 While most of Clippy's lint utils are documented, most of rustc's internals lack
459 documentation currently. This is unfortunate, but in most cases you can probably
460 get away with copying things from existing similar lints. If you are stuck,
461 don't hesitate to ask on [Discord] or in the issue/PR.
463 [utils]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/mod.rs
464 [if_chain]: https://docs.rs/if_chain/*/if_chain/
465 [from_expansion]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html#method.from_expansion
466 [in_external_macro]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/lint/fn.in_external_macro.html
467 [span]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html
468 [applicability]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/enum.Applicability.html
469 [rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/
470 [nightly_docs]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/
471 [ast]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/index.html
472 [ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/sty/index.html
473 [Discord]: https://discord.gg/rust-lang