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 See the [Basics](basics.md#get-the-code) documentation.
34 There is a bit of boilerplate code that needs to be set up when creating a new
35 lint. Fortunately, you can use the clippy dev tools to handle this for you. We
36 are naming our new lint `foo_functions` (lints are generally written in snake
37 case), and we don't need type information so it will have an early pass type
38 (more on this later on). To get started on this lint you can run
39 `cargo dev new_lint --name=foo_functions --pass=early --category=pedantic`
40 (category will default to nursery if not provided). This command will create
41 two files: `tests/ui/foo_functions.rs` and `clippy_lints/src/foo_functions.rs`,
42 as well as run `cargo dev update_lints` to register the new lint. For cargo lints,
43 two project hierarchies (fail/pass) will be created by default under `tests/ui-cargo`.
45 Next, we'll open up these files and add our lint!
49 Let's write some tests first that we can execute while we iterate on our lint.
51 Clippy uses UI tests for testing. UI tests check that the output of Clippy is
52 exactly as expected. Each test is just a plain Rust file that contains the code
53 we want to check. The output of Clippy is compared against a `.stderr` file.
54 Note that you don't have to create this file yourself, we'll get to
55 generating the `.stderr` files further down.
57 We start by opening the test file created at `tests/ui/foo_functions.rs`.
59 Update the file with some examples to get started:
62 #![warn(clippy::foo_functions)]
72 // Default trait methods
85 // We also don't want to lint method calls
92 Now we can run the test with `TESTNAME=foo_functions cargo uitest`,
93 currently this test is meaningless though.
95 While we are working on implementing our lint, we can keep running the UI
96 test. That allows us to check if the output is turning into what we want.
98 Once we are satisfied with the output, we need to run
99 `tests/ui/update-all-references.sh` to update the `.stderr` file for our lint.
100 Please note that, we should run `TESTNAME=foo_functions cargo uitest`
101 every time before running `tests/ui/update-all-references.sh`.
102 Running `TESTNAME=foo_functions cargo uitest` should pass then. When we commit
103 our lint, we need to commit the generated `.stderr` files, too. In general, you
104 should only commit files changed by `tests/ui/update-all-references.sh` for the
105 specific lint you are creating/editing.
109 For cargo lints, the process of testing differs in that we are interested in
110 the `Cargo.toml` manifest file. We also need a minimal crate associated
113 If our new lint is named e.g. `foo_categories`, after running `cargo dev new_lint`
114 we will find by default two new crates, each with its manifest file:
116 * `tests/ui-cargo/foo_categories/fail/Cargo.toml`: this file should cause the new lint to raise an error.
117 * `tests/ui-cargo/foo_categories/pass/Cargo.toml`: this file should not trigger the lint.
119 If you need more cases, you can copy one of those crates (under `foo_categories`) and rename it.
121 The process of generating the `.stderr` file is the same, and prepending the `TESTNAME`
122 variable to `cargo uitest` works too, but the script to update the references
123 is in another path: `tests/ui-cargo/update-all-references.sh`.
127 If the lint you are working on is making use of structured suggestions, the
128 test file should include a `// run-rustfix` comment at the top. This will
129 additionally run [rustfix] for that test. Rustfix will apply the suggestions
130 from the lint to the code of the test file and compare that to the contents of
133 Use `tests/ui/update-all-references.sh` to automatically generate the
134 `.fixed` file after running the tests.
136 [rustfix]: https://github.com/rust-lang/rustfix
138 ## Edition 2018 tests
140 Some features require the 2018 edition to work (e.g. `async_await`), but
141 compile-test tests run on the 2015 edition by default. To change this behavior
142 add `// edition:2018` at the top of the test file (note that it's space-sensitive).
146 Manually testing against an example file can be useful if you have added some
147 `println!`s and the test suite output becomes unreadable. To try Clippy with
148 your local modifications, run `env CLIPPY_TESTS=true cargo run --bin
149 clippy-driver -- -L ./target/debug input.rs` from the working copy root.
151 With tests in place, let's have a look at implementing our lint now.
155 Let's start by opening the new file created in the `clippy_lints` crate
156 at `clippy_lints/src/foo_functions.rs`. That's the crate where all the
157 lint code is. This file has already imported some initial things we will need:
160 use rustc_lint::{EarlyLintPass, EarlyContext};
161 use rustc_session::{declare_lint_pass, declare_tool_lint};
162 use rustc_ast::ast::*;
165 The next step is to update the lint declaration. Lints are declared using the
166 [`declare_clippy_lint!`][declare_clippy_lint] macro, and we just need to update
167 the auto-generated lint declaration to have a real description, something like this:
170 declare_clippy_lint! {
171 /// **What it does:**
173 /// **Why is this bad?**
175 /// **Known problems:** None.
184 "function named `foo`, which is not a descriptive name"
188 * The section of lines prefixed with `///` constitutes the lint documentation
189 section. This is the default documentation style and will be displayed
190 [like this][example_lint_page].
191 * `FOO_FUNCTIONS` is the name of our lint. Be sure to follow the
192 [lint naming guidelines][lint_naming] here when naming your lint.
193 In short, the name should state the thing that is being checked for and
194 read well when used with `allow`/`warn`/`deny`.
195 * `pedantic` sets the lint level to `Allow`.
196 The exact mapping can be found [here][category_level_mapping]
197 * The last part should be a text that explains what exactly is wrong with the
200 The rest of this file contains an empty implementation for our lint pass,
201 which in this case is `EarlyLintPass` and should look like this:
204 // clippy_lints/src/foo_functions.rs
206 // .. imports and lint declaration ..
208 declare_lint_pass!(FooFunctions => [FOO_FUNCTIONS]);
210 impl EarlyLintPass for FooFunctions {}
213 Normally after declaring the lint, we have to run `cargo dev update_lints`,
214 which updates some files, so Clippy knows about the new lint. Since we used
215 `cargo dev new_lint ...` to generate the lint declaration, this was done
216 automatically. While `update_lints` automates most of the things, it doesn't
217 automate everything. We will have to register our lint pass manually in the
218 `register_plugins` function in `clippy_lints/src/lib.rs`:
221 store.register_early_pass(|| box foo_functions::FooFunctions);
224 [declare_clippy_lint]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L60
225 [example_lint_page]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure
226 [lint_naming]: https://rust-lang.github.io/rfcs/0344-conventions-galore.html#lints
227 [category_level_mapping]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L110
231 Writing a lint that only checks for the name of a function means that we only
232 have to deal with the AST and don't have to deal with the type system at all.
233 This is good, because it makes writing this particular lint less complicated.
235 We have to make this decision with every new Clippy lint. It boils down to using
236 either [`EarlyLintPass`][early_lint_pass] or [`LateLintPass`][late_lint_pass].
238 In short, the `LateLintPass` has access to type information while the
239 `EarlyLintPass` doesn't. If you don't need access to type information, use the
240 `EarlyLintPass`. The `EarlyLintPass` is also faster. However linting speed
241 hasn't really been a concern with Clippy so far.
243 Since we don't need type information for checking the function name, we used
244 `--pass=early` when running the new lint automation and all the imports were
247 [early_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html
248 [late_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.LateLintPass.html
252 With UI tests and the lint declaration in place, we can start working on the
253 implementation of the lint logic.
255 Let's start by implementing the `EarlyLintPass` for our `FooFunctions`:
258 impl EarlyLintPass for FooFunctions {
259 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
260 // TODO: Emit lint here
265 We implement the [`check_fn`][check_fn] method from the
266 [`EarlyLintPass`][early_lint_pass] trait. This gives us access to various
267 information about the function that is currently being checked. More on that in
268 the next section. Let's worry about the details later and emit our lint for
269 *every* function definition first.
271 Depending on how complex we want our lint message to be, we can choose from a
272 variety of lint emission functions. They can all be found in
273 [`clippy_lints/src/utils/diagnostics.rs`][diagnostics].
275 `span_lint_and_help` seems most appropriate in this case. It allows us to
276 provide an extra help message and we can't really suggest a better name
277 automatically. This is how it looks:
280 impl EarlyLintPass for FooFunctions {
281 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
286 "function named `foo`",
288 "consider using a more meaningful name"
294 Running our UI test should now produce output that contains the lint message.
296 [check_fn]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html#method.check_fn
297 [diagnostics]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/diagnostics.rs
299 ## Adding the lint logic
301 Writing the logic for your lint will most likely be different from our example,
302 so this section is kept rather short.
304 Using the [`check_fn`][check_fn] method gives us access to [`FnKind`][fn_kind]
305 that has the [`FnKind::Fn`] variant. It provides access to the name of the
306 function/method via an [`Ident`][ident].
308 With that we can expand our `check_fn` method to:
311 impl EarlyLintPass for FooFunctions {
312 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
313 if is_foo_fn(fn_kind) {
318 "function named `foo`",
320 "consider using a more meaningful name"
327 We separate the lint conditional from the lint emissions because it makes the
328 code a bit easier to read. In some cases this separation would also allow to
329 write some unit tests (as opposed to only UI tests) for the separate function.
331 In our example, `is_foo_fn` looks like:
334 // use statements, impl EarlyLintPass, check_fn, ..
336 fn is_foo_fn(fn_kind: FnKind<'_>) -> bool {
338 FnKind::Fn(_, ident, ..) => {
339 // check if `fn` name is `foo`
340 ident.name.as_str() == "foo"
343 FnKind::Closure(..) => false
348 Now we should also run the full test suite with `cargo test`. At this point
349 running `cargo test` should produce the expected output. Remember to run
350 `tests/ui/update-all-references.sh` to update the `.stderr` file.
352 `cargo test` (as opposed to `cargo uitest`) will also ensure that our lint
353 implementation is not violating any Clippy lints itself.
355 That should be it for the lint implementation. Running `cargo test` should now
358 [fn_kind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html
359 [`FnKind::Fn`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html#variant.Fn
360 [ident]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Ident.html
364 If you have trouble implementing your lint, there is also the internal `author`
365 lint to generate Clippy code that detects the offending pattern. It does not
366 work for all of the Rust syntax, but can give a good starting point.
368 The quickest way to use it, is the
369 [Rust playground: play.rust-lang.org][author_example].
370 Put the code you want to lint into the editor and add the `#[clippy::author]`
371 attribute above the item. Then run Clippy via `Tools -> Clippy` and you should
372 see the generated code in the output below.
374 [Here][author_example] is an example on the playground.
376 If the command was executed successfully, you can copy the code over to where
377 you are implementing your lint.
379 [author_example]: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=9a12cb60e5c6ad4e3003ac6d5e63cf55
383 The final thing before submitting our PR is to add some documentation to our
386 Please document your lint with a doc comment akin to the following:
389 declare_clippy_lint! {
390 /// **What it does:** Checks for ... (describe what the lint matches).
392 /// **Why is this bad?** Supply the reason for linting the code.
394 /// **Known problems:** None. (Or describe where it could go wrong.)
400 /// Insert a short example of code that triggers the lint
403 /// Insert a short example of improved code that doesn't trigger the lint
407 "function named `foo`, which is not a descriptive name"
411 Once your lint is merged, this documentation will show up in the [lint
414 [lint_list]: https://rust-lang.github.io/rust-clippy/master/index.html
418 [Rustfmt] is a tool for formatting Rust code according to style guidelines.
419 Your code has to be formatted by `rustfmt` before a PR can be merged.
420 Clippy uses nightly `rustfmt` in the CI.
422 It can be installed via `rustup`:
425 rustup component add rustfmt --toolchain=nightly
428 Use `cargo dev fmt` to format the whole codebase. Make sure that `rustfmt` is
429 installed for the nightly toolchain.
431 [Rustfmt]: https://github.com/rust-lang/rustfmt
435 If you want to debug parts of your lint implementation, you can use the [`dbg!`]
436 macro anywhere in your code. Running the tests should then include the debug
437 output in the `stdout` part.
439 [`dbg!`]: https://doc.rust-lang.org/std/macro.dbg.html
443 Before submitting your PR make sure you followed all of the basic requirements:
445 <!-- Sync this with `.github/PULL_REQUEST_TEMPLATE` -->
447 - [ ] Followed [lint naming conventions][lint_naming]
448 - [ ] Added passing UI tests (including committed `.stderr` file)
449 - [ ] `cargo test` passes locally
450 - [ ] Executed `cargo dev update_lints`
451 - [ ] Added lint documentation
452 - [ ] Run `cargo dev fmt`
456 Here are some pointers to things you are likely going to need for every lint:
458 * [Clippy utils][utils] - Various helper functions. Maybe the function you need
459 is already in here (`implements_trait`, `match_path`, `snippet`, etc)
460 * [Clippy diagnostics][diagnostics]
461 * [The `if_chain` macro][if_chain]
462 * [`from_expansion`][from_expansion] and [`in_external_macro`][in_external_macro]
464 * [`Applicability`][applicability]
465 * [Common tools for writing lints](common_tools_writing_lints.md) helps with common operations
466 * [The rustc-dev-guide][rustc-dev-guide] explains a lot of internal compiler concepts
467 * [The nightly rustc docs][nightly_docs] which has been linked to throughout
470 For `EarlyLintPass` lints:
472 * [`EarlyLintPass`][early_lint_pass]
473 * [`rustc_ast::ast`][ast]
475 For `LateLintPass` lints:
477 * [`LateLintPass`][late_lint_pass]
480 While most of Clippy's lint utils are documented, most of rustc's internals lack
481 documentation currently. This is unfortunate, but in most cases you can probably
482 get away with copying things from existing similar lints. If you are stuck,
483 don't hesitate to ask on [Discord] or in the issue/PR.
485 [utils]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/mod.rs
486 [if_chain]: https://docs.rs/if_chain/*/if_chain/
487 [from_expansion]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html#method.from_expansion
488 [in_external_macro]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/lint/fn.in_external_macro.html
489 [span]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html
490 [applicability]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/enum.Applicability.html
491 [rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/
492 [nightly_docs]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/
493 [ast]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/index.html
494 [ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/sty/index.html
495 [Discord]: https://discord.gg/rust-lang