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. In general, you
105 should only commit files changed by `tests/ui/update-all-references.sh` for the
106 specific lint you are creating/editing.
110 If the lint you are working on is making use of structured suggestions, the
111 test file should include a `// run-rustfix` comment at the top. This will
112 additionally run [rustfix] for that test. Rustfix will apply the suggestions
113 from the lint to the code of the test file and compare that to the contents of
116 Use `tests/ui/update-all-references.sh` to automatically generate the
117 `.fixed` file after running the tests.
119 [rustfix]: https://github.com/rust-lang/rustfix
121 ## Edition 2018 tests
123 Some features require the 2018 edition to work (e.g. `async_await`), but
124 compile-test tests run on the 2015 edition by default. To change this behavior
125 add `// edition:2018` at the top of the test file (note that it's space-sensitive).
129 Manually testing against an example file can be useful if you have added some
130 `println!`s and the test suite output becomes unreadable. To try Clippy with
131 your local modifications, run `env CLIPPY_TESTS=true cargo run --bin
132 clippy-driver -- -L ./target/debug input.rs` from the working copy root.
134 With tests in place, let's have a look at implementing our lint now.
138 Let's start by opening the new file created in the `clippy_lints` crate
139 at `clippy_lints/src/foo_functions.rs`. That's the crate where all the
140 lint code is. This file has already imported some initial things we will need:
143 use rustc_lint::{EarlyLintPass, EarlyContext};
144 use rustc_session::{declare_lint_pass, declare_tool_lint};
145 use rustc_ast::ast::*;
148 The next step is to update the lint declaration. Lints are declared using the
149 [`declare_clippy_lint!`][declare_clippy_lint] macro, and we just need to update
150 the auto-generated lint declaration to have a real description, something like this:
153 declare_clippy_lint! {
154 /// **What it does:**
156 /// **Why is this bad?**
158 /// **Known problems:** None.
167 "function named `foo`, which is not a descriptive name"
171 * The section of lines prefixed with `///` constitutes the lint documentation
172 section. This is the default documentation style and will be displayed
173 [like this][example_lint_page].
174 * `FOO_FUNCTIONS` is the name of our lint. Be sure to follow the
175 [lint naming guidelines][lint_naming] here when naming your lint.
176 In short, the name should state the thing that is being checked for and
177 read well when used with `allow`/`warn`/`deny`.
178 * `pedantic` sets the lint level to `Allow`.
179 The exact mapping can be found [here][category_level_mapping]
180 * The last part should be a text that explains what exactly is wrong with the
183 The rest of this file contains an empty implementation for our lint pass,
184 which in this case is `EarlyLintPass` and should look like this:
187 // clippy_lints/src/foo_functions.rs
189 // .. imports and lint declaration ..
191 declare_lint_pass!(FooFunctions => [FOO_FUNCTIONS]);
193 impl EarlyLintPass for FooFunctions {}
196 Normally after declaring the lint, we have to run `cargo dev update_lints`,
197 which updates some files, so Clippy knows about the new lint. Since we used
198 `cargo dev new_lint ...` to generate the lint declaration, this was done
199 automatically. While `update_lints` automates most of the things, it doesn't
200 automate everything. We will have to register our lint pass manually in the
201 `register_plugins` function in `clippy_lints/src/lib.rs`:
204 store.register_early_pass(|| box foo_functions::FooFunctions);
207 [declare_clippy_lint]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L60
208 [example_lint_page]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure
209 [lint_naming]: https://rust-lang.github.io/rfcs/0344-conventions-galore.html#lints
210 [category_level_mapping]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L110
214 Writing a lint that only checks for the name of a function means that we only
215 have to deal with the AST and don't have to deal with the type system at all.
216 This is good, because it makes writing this particular lint less complicated.
218 We have to make this decision with every new Clippy lint. It boils down to using
219 either [`EarlyLintPass`][early_lint_pass] or [`LateLintPass`][late_lint_pass].
221 In short, the `LateLintPass` has access to type information while the
222 `EarlyLintPass` doesn't. If you don't need access to type information, use the
223 `EarlyLintPass`. The `EarlyLintPass` is also faster. However linting speed
224 hasn't really been a concern with Clippy so far.
226 Since we don't need type information for checking the function name, we used
227 `--pass=early` when running the new lint automation and all the imports were
230 [early_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html
231 [late_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.LateLintPass.html
235 With UI tests and the lint declaration in place, we can start working on the
236 implementation of the lint logic.
238 Let's start by implementing the `EarlyLintPass` for our `FooFunctions`:
241 impl EarlyLintPass for FooFunctions {
242 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
243 // TODO: Emit lint here
248 We implement the [`check_fn`][check_fn] method from the
249 [`EarlyLintPass`][early_lint_pass] trait. This gives us access to various
250 information about the function that is currently being checked. More on that in
251 the next section. Let's worry about the details later and emit our lint for
252 *every* function definition first.
254 Depending on how complex we want our lint message to be, we can choose from a
255 variety of lint emission functions. They can all be found in
256 [`clippy_lints/src/utils/diagnostics.rs`][diagnostics].
258 `span_lint_and_help` seems most appropriate in this case. It allows us to
259 provide an extra help message and we can't really suggest a better name
260 automatically. This is how it looks:
263 impl EarlyLintPass for FooFunctions {
264 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
269 "function named `foo`",
271 "consider using a more meaningful name"
277 Running our UI test should now produce output that contains the lint message.
279 [check_fn]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html#method.check_fn
280 [diagnostics]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/diagnostics.rs
282 ## Adding the lint logic
284 Writing the logic for your lint will most likely be different from our example,
285 so this section is kept rather short.
287 Using the [`check_fn`][check_fn] method gives us access to [`FnKind`][fn_kind]
288 that has the [`FnKind::Fn`] variant. It provides access to the name of the
289 function/method via an [`Ident`][ident].
291 With that we can expand our `check_fn` method to:
294 impl EarlyLintPass for FooFunctions {
295 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
296 if is_foo_fn(fn_kind) {
301 "function named `foo`",
303 "consider using a more meaningful name"
310 We separate the lint conditional from the lint emissions because it makes the
311 code a bit easier to read. In some cases this separation would also allow to
312 write some unit tests (as opposed to only UI tests) for the separate function.
314 In our example, `is_foo_fn` looks like:
317 // use statements, impl EarlyLintPass, check_fn, ..
319 fn is_foo_fn(fn_kind: FnKind<'_>) -> bool {
321 FnKind::Fn(_, ident, ..) => {
322 // check if `fn` name is `foo`
323 ident.name.as_str() == "foo"
326 FnKind::Closure(..) => false
331 Now we should also run the full test suite with `cargo test`. At this point
332 running `cargo test` should produce the expected output. Remember to run
333 `tests/ui/update-all-references.sh` to update the `.stderr` file.
335 `cargo test` (as opposed to `cargo uitest`) will also ensure that our lint
336 implementation is not violating any Clippy lints itself.
338 That should be it for the lint implementation. Running `cargo test` should now
341 [fn_kind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html
342 [`FnKind::Fn`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html#variant.Fn
343 [ident]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Ident.html
347 If you have trouble implementing your lint, there is also the internal `author`
348 lint to generate Clippy code that detects the offending pattern. It does not
349 work for all of the Rust syntax, but can give a good starting point.
351 The quickest way to use it, is the
352 [Rust playground: play.rust-lang.org][author_example].
353 Put the code you want to lint into the editor and add the `#[clippy::author]`
354 attribute above the item. Then run Clippy via `Tools -> Clippy` and you should
355 see the generated code in the output below.
357 [Here][author_example] is an example on the playground.
359 If the command was executed successfully, you can copy the code over to where
360 you are implementing your lint.
362 [author_example]: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=9a12cb60e5c6ad4e3003ac6d5e63cf55
366 The final thing before submitting our PR is to add some documentation to our
369 Please document your lint with a doc comment akin to the following:
372 declare_clippy_lint! {
373 /// **What it does:** Checks for ... (describe what the lint matches).
375 /// **Why is this bad?** Supply the reason for linting the code.
377 /// **Known problems:** None. (Or describe where it could go wrong.)
383 /// Insert a short example of code that triggers the lint
386 /// Insert a short example of improved code that doesn't trigger the lint
390 "function named `foo`, which is not a descriptive name"
394 Once your lint is merged, this documentation will show up in the [lint
397 [lint_list]: https://rust-lang.github.io/rust-clippy/master/index.html
401 [Rustfmt] is a tool for formatting Rust code according to style guidelines.
402 Your code has to be formatted by `rustfmt` before a PR can be merged.
403 Clippy uses nightly `rustfmt` in the CI.
405 It can be installed via `rustup`:
408 rustup component add rustfmt --toolchain=nightly
411 Use `cargo dev fmt` to format the whole codebase. Make sure that `rustfmt` is
412 installed for the nightly toolchain.
414 [Rustfmt]: https://github.com/rust-lang/rustfmt
418 If you want to debug parts of your lint implementation, you can use the [`dbg!`]
419 macro anywhere in your code. Running the tests should then include the debug
420 output in the `stdout` part.
422 [`dbg!`]: https://doc.rust-lang.org/std/macro.dbg.html
426 Before submitting your PR make sure you followed all of the basic requirements:
428 <!-- Sync this with `.github/PULL_REQUEST_TEMPLATE` -->
430 - [ ] Followed [lint naming conventions][lint_naming]
431 - [ ] Added passing UI tests (including committed `.stderr` file)
432 - [ ] `cargo test` passes locally
433 - [ ] Executed `cargo dev update_lints`
434 - [ ] Added lint documentation
435 - [ ] Run `cargo dev fmt`
439 Here are some pointers to things you are likely going to need for every lint:
441 * [Clippy utils][utils] - Various helper functions. Maybe the function you need
442 is already in here (`implements_trait`, `match_path`, `snippet`, etc)
443 * [Clippy diagnostics][diagnostics]
444 * [The `if_chain` macro][if_chain]
445 * [`from_expansion`][from_expansion] and [`in_external_macro`][in_external_macro]
447 * [`Applicability`][applicability]
448 * [The rustc-dev-guide][rustc-dev-guide] explains a lot of internal compiler concepts
449 * [The nightly rustc docs][nightly_docs] which has been linked to throughout
452 For `EarlyLintPass` lints:
454 * [`EarlyLintPass`][early_lint_pass]
455 * [`rustc_ast::ast`][ast]
457 For `LateLintPass` lints:
459 * [`LateLintPass`][late_lint_pass]
462 While most of Clippy's lint utils are documented, most of rustc's internals lack
463 documentation currently. This is unfortunate, but in most cases you can probably
464 get away with copying things from existing similar lints. If you are stuck,
465 don't hesitate to ask on [Discord] or in the issue/PR.
467 [utils]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/mod.rs
468 [if_chain]: https://docs.rs/if_chain/*/if_chain/
469 [from_expansion]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html#method.from_expansion
470 [in_external_macro]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/lint/fn.in_external_macro.html
471 [span]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html
472 [applicability]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/enum.Applicability.html
473 [rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/
474 [nightly_docs]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/
475 [ast]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/index.html
476 [ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/sty/index.html
477 [Discord]: https://discord.gg/rust-lang