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. For cargo lints,
46 two project hierarchies (fail/pass) will be created by default under `tests/ui-cargo`.
48 Next, we'll open up these files and add our lint!
52 Let's write some tests first that we can execute while we iterate on our lint.
54 Clippy uses UI tests for testing. UI tests check that the output of Clippy is
55 exactly as expected. Each test is just a plain Rust file that contains the code
56 we want to check. The output of Clippy is compared against a `.stderr` file.
57 Note that you don't have to create this file yourself, we'll get to
58 generating the `.stderr` files further down.
60 We start by opening the test file created at `tests/ui/foo_functions.rs`.
62 Update the file with some examples to get started:
65 #![warn(clippy::foo_functions)]
75 // Default trait methods
88 // We also don't want to lint method calls
95 Now we can run the test with `TESTNAME=foo_functions cargo uitest`,
96 currently this test is meaningless though.
98 While we are working on implementing our lint, we can keep running the UI
99 test. That allows us to check if the output is turning into what we want.
101 Once we are satisfied with the output, we need to run
102 `tests/ui/update-all-references.sh` to update the `.stderr` file for our lint.
103 Please note that, we should run `TESTNAME=foo_functions cargo uitest`
104 every time before running `tests/ui/update-all-references.sh`.
105 Running `TESTNAME=foo_functions cargo uitest` should pass then. When we commit
106 our lint, we need to commit the generated `.stderr` files, too. In general, you
107 should only commit files changed by `tests/ui/update-all-references.sh` for the
108 specific lint you are creating/editing.
112 For cargo lints, the process of testing differs in that we are interested in
113 the `Cargo.toml` manifest file. We also need a minimal crate associated
116 If our new lint is named e.g. `foo_categories`, after running `cargo dev new_lint`
117 we will find by default two new crates, each with its manifest file:
119 * `tests/ui-cargo/foo_categories/fail/Cargo.toml`: this file should cause the new lint to raise an error.
120 * `tests/ui-cargo/foo_categories/pass/Cargo.toml`: this file should not trigger the lint.
122 If you need more cases, you can copy one of those crates (under `foo_categories`) and rename it.
124 The process of generating the `.stderr` file is the same, and prepending the `TESTNAME`
125 variable to `cargo uitest` works too, but the script to update the references
126 is in another path: `tests/ui-cargo/update-all-references.sh`.
130 If the lint you are working on is making use of structured suggestions, the
131 test file should include a `// run-rustfix` comment at the top. This will
132 additionally run [rustfix] for that test. Rustfix will apply the suggestions
133 from the lint to the code of the test file and compare that to the contents of
136 Use `tests/ui/update-all-references.sh` to automatically generate the
137 `.fixed` file after running the tests.
139 [rustfix]: https://github.com/rust-lang/rustfix
141 ## Edition 2018 tests
143 Some features require the 2018 edition to work (e.g. `async_await`), but
144 compile-test tests run on the 2015 edition by default. To change this behavior
145 add `// edition:2018` at the top of the test file (note that it's space-sensitive).
149 Manually testing against an example file can be useful if you have added some
150 `println!`s and the test suite output becomes unreadable. To try Clippy with
151 your local modifications, run `env CLIPPY_TESTS=true cargo run --bin
152 clippy-driver -- -L ./target/debug input.rs` from the working copy root.
154 With tests in place, let's have a look at implementing our lint now.
158 Let's start by opening the new file created in the `clippy_lints` crate
159 at `clippy_lints/src/foo_functions.rs`. That's the crate where all the
160 lint code is. This file has already imported some initial things we will need:
163 use rustc_lint::{EarlyLintPass, EarlyContext};
164 use rustc_session::{declare_lint_pass, declare_tool_lint};
165 use rustc_ast::ast::*;
168 The next step is to update the lint declaration. Lints are declared using the
169 [`declare_clippy_lint!`][declare_clippy_lint] macro, and we just need to update
170 the auto-generated lint declaration to have a real description, something like this:
173 declare_clippy_lint! {
174 /// **What it does:**
176 /// **Why is this bad?**
178 /// **Known problems:** None.
187 "function named `foo`, which is not a descriptive name"
191 * The section of lines prefixed with `///` constitutes the lint documentation
192 section. This is the default documentation style and will be displayed
193 [like this][example_lint_page].
194 * `FOO_FUNCTIONS` is the name of our lint. Be sure to follow the
195 [lint naming guidelines][lint_naming] here when naming your lint.
196 In short, the name should state the thing that is being checked for and
197 read well when used with `allow`/`warn`/`deny`.
198 * `pedantic` sets the lint level to `Allow`.
199 The exact mapping can be found [here][category_level_mapping]
200 * The last part should be a text that explains what exactly is wrong with the
203 The rest of this file contains an empty implementation for our lint pass,
204 which in this case is `EarlyLintPass` and should look like this:
207 // clippy_lints/src/foo_functions.rs
209 // .. imports and lint declaration ..
211 declare_lint_pass!(FooFunctions => [FOO_FUNCTIONS]);
213 impl EarlyLintPass for FooFunctions {}
216 Normally after declaring the lint, we have to run `cargo dev update_lints`,
217 which updates some files, so Clippy knows about the new lint. Since we used
218 `cargo dev new_lint ...` to generate the lint declaration, this was done
219 automatically. While `update_lints` automates most of the things, it doesn't
220 automate everything. We will have to register our lint pass manually in the
221 `register_plugins` function in `clippy_lints/src/lib.rs`:
224 store.register_early_pass(|| box foo_functions::FooFunctions);
227 [declare_clippy_lint]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L60
228 [example_lint_page]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure
229 [lint_naming]: https://rust-lang.github.io/rfcs/0344-conventions-galore.html#lints
230 [category_level_mapping]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L110
234 Writing a lint that only checks for the name of a function means that we only
235 have to deal with the AST and don't have to deal with the type system at all.
236 This is good, because it makes writing this particular lint less complicated.
238 We have to make this decision with every new Clippy lint. It boils down to using
239 either [`EarlyLintPass`][early_lint_pass] or [`LateLintPass`][late_lint_pass].
241 In short, the `LateLintPass` has access to type information while the
242 `EarlyLintPass` doesn't. If you don't need access to type information, use the
243 `EarlyLintPass`. The `EarlyLintPass` is also faster. However linting speed
244 hasn't really been a concern with Clippy so far.
246 Since we don't need type information for checking the function name, we used
247 `--pass=early` when running the new lint automation and all the imports were
250 [early_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html
251 [late_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.LateLintPass.html
255 With UI tests and the lint declaration in place, we can start working on the
256 implementation of the lint logic.
258 Let's start by implementing the `EarlyLintPass` for our `FooFunctions`:
261 impl EarlyLintPass for FooFunctions {
262 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
263 // TODO: Emit lint here
268 We implement the [`check_fn`][check_fn] method from the
269 [`EarlyLintPass`][early_lint_pass] trait. This gives us access to various
270 information about the function that is currently being checked. More on that in
271 the next section. Let's worry about the details later and emit our lint for
272 *every* function definition first.
274 Depending on how complex we want our lint message to be, we can choose from a
275 variety of lint emission functions. They can all be found in
276 [`clippy_lints/src/utils/diagnostics.rs`][diagnostics].
278 `span_lint_and_help` seems most appropriate in this case. It allows us to
279 provide an extra help message and we can't really suggest a better name
280 automatically. This is how it looks:
283 impl EarlyLintPass for FooFunctions {
284 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
289 "function named `foo`",
291 "consider using a more meaningful name"
297 Running our UI test should now produce output that contains the lint message.
299 [check_fn]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html#method.check_fn
300 [diagnostics]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/diagnostics.rs
302 ## Adding the lint logic
304 Writing the logic for your lint will most likely be different from our example,
305 so this section is kept rather short.
307 Using the [`check_fn`][check_fn] method gives us access to [`FnKind`][fn_kind]
308 that has the [`FnKind::Fn`] variant. It provides access to the name of the
309 function/method via an [`Ident`][ident].
311 With that we can expand our `check_fn` method to:
314 impl EarlyLintPass for FooFunctions {
315 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
316 if is_foo_fn(fn_kind) {
321 "function named `foo`",
323 "consider using a more meaningful name"
330 We separate the lint conditional from the lint emissions because it makes the
331 code a bit easier to read. In some cases this separation would also allow to
332 write some unit tests (as opposed to only UI tests) for the separate function.
334 In our example, `is_foo_fn` looks like:
337 // use statements, impl EarlyLintPass, check_fn, ..
339 fn is_foo_fn(fn_kind: FnKind<'_>) -> bool {
341 FnKind::Fn(_, ident, ..) => {
342 // check if `fn` name is `foo`
343 ident.name.as_str() == "foo"
346 FnKind::Closure(..) => false
351 Now we should also run the full test suite with `cargo test`. At this point
352 running `cargo test` should produce the expected output. Remember to run
353 `tests/ui/update-all-references.sh` to update the `.stderr` file.
355 `cargo test` (as opposed to `cargo uitest`) will also ensure that our lint
356 implementation is not violating any Clippy lints itself.
358 That should be it for the lint implementation. Running `cargo test` should now
361 [fn_kind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html
362 [`FnKind::Fn`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html#variant.Fn
363 [ident]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Ident.html
367 If you have trouble implementing your lint, there is also the internal `author`
368 lint to generate Clippy code that detects the offending pattern. It does not
369 work for all of the Rust syntax, but can give a good starting point.
371 The quickest way to use it, is the
372 [Rust playground: play.rust-lang.org][author_example].
373 Put the code you want to lint into the editor and add the `#[clippy::author]`
374 attribute above the item. Then run Clippy via `Tools -> Clippy` and you should
375 see the generated code in the output below.
377 [Here][author_example] is an example on the playground.
379 If the command was executed successfully, you can copy the code over to where
380 you are implementing your lint.
382 [author_example]: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=9a12cb60e5c6ad4e3003ac6d5e63cf55
386 The final thing before submitting our PR is to add some documentation to our
389 Please document your lint with a doc comment akin to the following:
392 declare_clippy_lint! {
393 /// **What it does:** Checks for ... (describe what the lint matches).
395 /// **Why is this bad?** Supply the reason for linting the code.
397 /// **Known problems:** None. (Or describe where it could go wrong.)
403 /// Insert a short example of code that triggers the lint
406 /// Insert a short example of improved code that doesn't trigger the lint
410 "function named `foo`, which is not a descriptive name"
414 Once your lint is merged, this documentation will show up in the [lint
417 [lint_list]: https://rust-lang.github.io/rust-clippy/master/index.html
421 [Rustfmt] is a tool for formatting Rust code according to style guidelines.
422 Your code has to be formatted by `rustfmt` before a PR can be merged.
423 Clippy uses nightly `rustfmt` in the CI.
425 It can be installed via `rustup`:
428 rustup component add rustfmt --toolchain=nightly
431 Use `cargo dev fmt` to format the whole codebase. Make sure that `rustfmt` is
432 installed for the nightly toolchain.
434 [Rustfmt]: https://github.com/rust-lang/rustfmt
438 If you want to debug parts of your lint implementation, you can use the [`dbg!`]
439 macro anywhere in your code. Running the tests should then include the debug
440 output in the `stdout` part.
442 [`dbg!`]: https://doc.rust-lang.org/std/macro.dbg.html
446 Before submitting your PR make sure you followed all of the basic requirements:
448 <!-- Sync this with `.github/PULL_REQUEST_TEMPLATE` -->
450 - [ ] Followed [lint naming conventions][lint_naming]
451 - [ ] Added passing UI tests (including committed `.stderr` file)
452 - [ ] `cargo test` passes locally
453 - [ ] Executed `cargo dev update_lints`
454 - [ ] Added lint documentation
455 - [ ] Run `cargo dev fmt`
459 Here are some pointers to things you are likely going to need for every lint:
461 * [Clippy utils][utils] - Various helper functions. Maybe the function you need
462 is already in here (`implements_trait`, `match_path`, `snippet`, etc)
463 * [Clippy diagnostics][diagnostics]
464 * [The `if_chain` macro][if_chain]
465 * [`from_expansion`][from_expansion] and [`in_external_macro`][in_external_macro]
467 * [`Applicability`][applicability]
468 * [Common tools for writing lints](common_tools_writing_lints.md) helps with common operations
469 * [The rustc-dev-guide][rustc-dev-guide] explains a lot of internal compiler concepts
470 * [The nightly rustc docs][nightly_docs] which has been linked to throughout
473 For `EarlyLintPass` lints:
475 * [`EarlyLintPass`][early_lint_pass]
476 * [`rustc_ast::ast`][ast]
478 For `LateLintPass` lints:
480 * [`LateLintPass`][late_lint_pass]
483 While most of Clippy's lint utils are documented, most of rustc's internals lack
484 documentation currently. This is unfortunate, but in most cases you can probably
485 get away with copying things from existing similar lints. If you are stuck,
486 don't hesitate to ask on [Discord] or in the issue/PR.
488 [utils]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/mod.rs
489 [if_chain]: https://docs.rs/if_chain/*/if_chain/
490 [from_expansion]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html#method.from_expansion
491 [in_external_macro]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/lint/fn.in_external_macro.html
492 [span]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html
493 [applicability]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/enum.Applicability.html
494 [rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/
495 [nightly_docs]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/
496 [ast]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/index.html
497 [ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/sty/index.html
498 [Discord]: https://discord.gg/rust-lang