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 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 contents of the `Cargo.toml` files. If our new lint is named e.g. `foo_categories`,
114 after running `cargo dev new_lint` we will find two new manifest files:
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 The process of generating the `.stderr` file is the same, and prepending the `TESTNAME`
120 variable to `cargo uitest` works too, but the script to update the references
121 is in another path: `tests/ui-cargo/update-all-references.sh`.
125 If the lint you are working on is making use of structured suggestions, the
126 test file should include a `// run-rustfix` comment at the top. This will
127 additionally run [rustfix] for that test. Rustfix will apply the suggestions
128 from the lint to the code of the test file and compare that to the contents of
131 Use `tests/ui/update-all-references.sh` to automatically generate the
132 `.fixed` file after running the tests.
134 [rustfix]: https://github.com/rust-lang/rustfix
136 ## Edition 2018 tests
138 Some features require the 2018 edition to work (e.g. `async_await`), but
139 compile-test tests run on the 2015 edition by default. To change this behavior
140 add `// edition:2018` at the top of the test file (note that it's space-sensitive).
144 Manually testing against an example file can be useful if you have added some
145 `println!`s and the test suite output becomes unreadable. To try Clippy with
146 your local modifications, run `env CLIPPY_TESTS=true cargo run --bin
147 clippy-driver -- -L ./target/debug input.rs` from the working copy root.
149 With tests in place, let's have a look at implementing our lint now.
153 Let's start by opening the new file created in the `clippy_lints` crate
154 at `clippy_lints/src/foo_functions.rs`. That's the crate where all the
155 lint code is. This file has already imported some initial things we will need:
158 use rustc_lint::{EarlyLintPass, EarlyContext};
159 use rustc_session::{declare_lint_pass, declare_tool_lint};
160 use rustc_ast::ast::*;
163 The next step is to update the lint declaration. Lints are declared using the
164 [`declare_clippy_lint!`][declare_clippy_lint] macro, and we just need to update
165 the auto-generated lint declaration to have a real description, something like this:
168 declare_clippy_lint! {
169 /// **What it does:**
171 /// **Why is this bad?**
173 /// **Known problems:** None.
182 "function named `foo`, which is not a descriptive name"
186 * The section of lines prefixed with `///` constitutes the lint documentation
187 section. This is the default documentation style and will be displayed
188 [like this][example_lint_page].
189 * `FOO_FUNCTIONS` is the name of our lint. Be sure to follow the
190 [lint naming guidelines][lint_naming] here when naming your lint.
191 In short, the name should state the thing that is being checked for and
192 read well when used with `allow`/`warn`/`deny`.
193 * `pedantic` sets the lint level to `Allow`.
194 The exact mapping can be found [here][category_level_mapping]
195 * The last part should be a text that explains what exactly is wrong with the
198 The rest of this file contains an empty implementation for our lint pass,
199 which in this case is `EarlyLintPass` and should look like this:
202 // clippy_lints/src/foo_functions.rs
204 // .. imports and lint declaration ..
206 declare_lint_pass!(FooFunctions => [FOO_FUNCTIONS]);
208 impl EarlyLintPass for FooFunctions {}
211 Normally after declaring the lint, we have to run `cargo dev update_lints`,
212 which updates some files, so Clippy knows about the new lint. Since we used
213 `cargo dev new_lint ...` to generate the lint declaration, this was done
214 automatically. While `update_lints` automates most of the things, it doesn't
215 automate everything. We will have to register our lint pass manually in the
216 `register_plugins` function in `clippy_lints/src/lib.rs`:
219 store.register_early_pass(|| box foo_functions::FooFunctions);
222 [declare_clippy_lint]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L60
223 [example_lint_page]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure
224 [lint_naming]: https://rust-lang.github.io/rfcs/0344-conventions-galore.html#lints
225 [category_level_mapping]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L110
229 Writing a lint that only checks for the name of a function means that we only
230 have to deal with the AST and don't have to deal with the type system at all.
231 This is good, because it makes writing this particular lint less complicated.
233 We have to make this decision with every new Clippy lint. It boils down to using
234 either [`EarlyLintPass`][early_lint_pass] or [`LateLintPass`][late_lint_pass].
236 In short, the `LateLintPass` has access to type information while the
237 `EarlyLintPass` doesn't. If you don't need access to type information, use the
238 `EarlyLintPass`. The `EarlyLintPass` is also faster. However linting speed
239 hasn't really been a concern with Clippy so far.
241 Since we don't need type information for checking the function name, we used
242 `--pass=early` when running the new lint automation and all the imports were
245 [early_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html
246 [late_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.LateLintPass.html
250 With UI tests and the lint declaration in place, we can start working on the
251 implementation of the lint logic.
253 Let's start by implementing the `EarlyLintPass` for our `FooFunctions`:
256 impl EarlyLintPass for FooFunctions {
257 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
258 // TODO: Emit lint here
263 We implement the [`check_fn`][check_fn] method from the
264 [`EarlyLintPass`][early_lint_pass] trait. This gives us access to various
265 information about the function that is currently being checked. More on that in
266 the next section. Let's worry about the details later and emit our lint for
267 *every* function definition first.
269 Depending on how complex we want our lint message to be, we can choose from a
270 variety of lint emission functions. They can all be found in
271 [`clippy_lints/src/utils/diagnostics.rs`][diagnostics].
273 `span_lint_and_help` seems most appropriate in this case. It allows us to
274 provide an extra help message and we can't really suggest a better name
275 automatically. This is how it looks:
278 impl EarlyLintPass for FooFunctions {
279 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
284 "function named `foo`",
286 "consider using a more meaningful name"
292 Running our UI test should now produce output that contains the lint message.
294 [check_fn]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html#method.check_fn
295 [diagnostics]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/diagnostics.rs
297 ## Adding the lint logic
299 Writing the logic for your lint will most likely be different from our example,
300 so this section is kept rather short.
302 Using the [`check_fn`][check_fn] method gives us access to [`FnKind`][fn_kind]
303 that has the [`FnKind::Fn`] variant. It provides access to the name of the
304 function/method via an [`Ident`][ident].
306 With that we can expand our `check_fn` method to:
309 impl EarlyLintPass for FooFunctions {
310 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
311 if is_foo_fn(fn_kind) {
316 "function named `foo`",
318 "consider using a more meaningful name"
325 We separate the lint conditional from the lint emissions because it makes the
326 code a bit easier to read. In some cases this separation would also allow to
327 write some unit tests (as opposed to only UI tests) for the separate function.
329 In our example, `is_foo_fn` looks like:
332 // use statements, impl EarlyLintPass, check_fn, ..
334 fn is_foo_fn(fn_kind: FnKind<'_>) -> bool {
336 FnKind::Fn(_, ident, ..) => {
337 // check if `fn` name is `foo`
338 ident.name.as_str() == "foo"
341 FnKind::Closure(..) => false
346 Now we should also run the full test suite with `cargo test`. At this point
347 running `cargo test` should produce the expected output. Remember to run
348 `tests/ui/update-all-references.sh` to update the `.stderr` file.
350 `cargo test` (as opposed to `cargo uitest`) will also ensure that our lint
351 implementation is not violating any Clippy lints itself.
353 That should be it for the lint implementation. Running `cargo test` should now
356 [fn_kind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html
357 [`FnKind::Fn`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html#variant.Fn
358 [ident]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Ident.html
362 If you have trouble implementing your lint, there is also the internal `author`
363 lint to generate Clippy code that detects the offending pattern. It does not
364 work for all of the Rust syntax, but can give a good starting point.
366 The quickest way to use it, is the
367 [Rust playground: play.rust-lang.org][author_example].
368 Put the code you want to lint into the editor and add the `#[clippy::author]`
369 attribute above the item. Then run Clippy via `Tools -> Clippy` and you should
370 see the generated code in the output below.
372 [Here][author_example] is an example on the playground.
374 If the command was executed successfully, you can copy the code over to where
375 you are implementing your lint.
377 [author_example]: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=9a12cb60e5c6ad4e3003ac6d5e63cf55
381 The final thing before submitting our PR is to add some documentation to our
384 Please document your lint with a doc comment akin to the following:
387 declare_clippy_lint! {
388 /// **What it does:** Checks for ... (describe what the lint matches).
390 /// **Why is this bad?** Supply the reason for linting the code.
392 /// **Known problems:** None. (Or describe where it could go wrong.)
398 /// Insert a short example of code that triggers the lint
401 /// Insert a short example of improved code that doesn't trigger the lint
405 "function named `foo`, which is not a descriptive name"
409 Once your lint is merged, this documentation will show up in the [lint
412 [lint_list]: https://rust-lang.github.io/rust-clippy/master/index.html
416 [Rustfmt] is a tool for formatting Rust code according to style guidelines.
417 Your code has to be formatted by `rustfmt` before a PR can be merged.
418 Clippy uses nightly `rustfmt` in the CI.
420 It can be installed via `rustup`:
423 rustup component add rustfmt --toolchain=nightly
426 Use `cargo dev fmt` to format the whole codebase. Make sure that `rustfmt` is
427 installed for the nightly toolchain.
429 [Rustfmt]: https://github.com/rust-lang/rustfmt
433 If you want to debug parts of your lint implementation, you can use the [`dbg!`]
434 macro anywhere in your code. Running the tests should then include the debug
435 output in the `stdout` part.
437 [`dbg!`]: https://doc.rust-lang.org/std/macro.dbg.html
441 Before submitting your PR make sure you followed all of the basic requirements:
443 <!-- Sync this with `.github/PULL_REQUEST_TEMPLATE` -->
445 - [ ] Followed [lint naming conventions][lint_naming]
446 - [ ] Added passing UI tests (including committed `.stderr` file)
447 - [ ] `cargo test` passes locally
448 - [ ] Executed `cargo dev update_lints`
449 - [ ] Added lint documentation
450 - [ ] Run `cargo dev fmt`
454 Here are some pointers to things you are likely going to need for every lint:
456 * [Clippy utils][utils] - Various helper functions. Maybe the function you need
457 is already in here (`implements_trait`, `match_path`, `snippet`, etc)
458 * [Clippy diagnostics][diagnostics]
459 * [The `if_chain` macro][if_chain]
460 * [`from_expansion`][from_expansion] and [`in_external_macro`][in_external_macro]
462 * [`Applicability`][applicability]
463 * [The rustc-dev-guide][rustc-dev-guide] explains a lot of internal compiler concepts
464 * [The nightly rustc docs][nightly_docs] which has been linked to throughout
467 For `EarlyLintPass` lints:
469 * [`EarlyLintPass`][early_lint_pass]
470 * [`rustc_ast::ast`][ast]
472 For `LateLintPass` lints:
474 * [`LateLintPass`][late_lint_pass]
477 While most of Clippy's lint utils are documented, most of rustc's internals lack
478 documentation currently. This is unfortunate, but in most cases you can probably
479 get away with copying things from existing similar lints. If you are stuck,
480 don't hesitate to ask on [Discord] or in the issue/PR.
482 [utils]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/mod.rs
483 [if_chain]: https://docs.rs/if_chain/*/if_chain/
484 [from_expansion]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html#method.from_expansion
485 [in_external_macro]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/lint/fn.in_external_macro.html
486 [span]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html
487 [applicability]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/enum.Applicability.html
488 [rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/
489 [nightly_docs]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/
490 [ast]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/index.html
491 [ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/sty/index.html
492 [Discord]: https://discord.gg/rust-lang