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). If you're not sure if the name you chose fits the lint,
39 take a look at our [lint naming guidelines][lint_naming]. To get started on this
40 lint you can run `cargo dev new_lint --name=foo_functions --pass=early
41 --category=pedantic` (category will default to nursery if not provided). This
42 command will create two files: `tests/ui/foo_functions.rs` and
43 `clippy_lints/src/foo_functions.rs`, as well as run `cargo dev update_lints` to
44 register the new lint. For cargo lints, two project hierarchies (fail/pass) will
45 be created by default under `tests/ui-cargo`.
47 Next, we'll open up these files and add our lint!
51 Let's write some tests first that we can execute while we iterate on our lint.
53 Clippy uses UI tests for testing. UI tests check that the output of Clippy is
54 exactly as expected. Each test is just a plain Rust file that contains the code
55 we want to check. The output of Clippy is compared against a `.stderr` file.
56 Note that you don't have to create this file yourself, we'll get to
57 generating the `.stderr` files further down.
59 We start by opening the test file created at `tests/ui/foo_functions.rs`.
61 Update the file with some examples to get started:
64 #![warn(clippy::foo_functions)]
74 // Default trait methods
87 // We also don't want to lint method calls
94 Now we can run the test with `TESTNAME=foo_functions cargo uitest`,
95 currently this test is meaningless though.
97 While we are working on implementing our lint, we can keep running the UI
98 test. That allows us to check if the output is turning into what we want.
100 Once we are satisfied with the output, we need to run
101 `tests/ui/update-all-references.sh` to update the `.stderr` file for our lint.
102 Please note that, we should run `TESTNAME=foo_functions cargo uitest`
103 every time before running `tests/ui/update-all-references.sh`.
104 Running `TESTNAME=foo_functions cargo uitest` should pass then. When we commit
105 our lint, we need to commit the generated `.stderr` files, too. In general, you
106 should only commit files changed by `tests/ui/update-all-references.sh` for the
107 specific lint you are creating/editing.
111 For cargo lints, the process of testing differs in that we are interested in
112 the `Cargo.toml` manifest file. We also need a minimal crate associated
115 If our new lint is named e.g. `foo_categories`, after running `cargo dev new_lint`
116 we will find by default two new crates, each with its manifest file:
118 * `tests/ui-cargo/foo_categories/fail/Cargo.toml`: this file should cause the new lint to raise an error.
119 * `tests/ui-cargo/foo_categories/pass/Cargo.toml`: this file should not trigger the lint.
121 If you need more cases, you can copy one of those crates (under `foo_categories`) and rename it.
123 The process of generating the `.stderr` file is the same, and prepending the `TESTNAME`
124 variable to `cargo uitest` works too, but the script to update the references
125 is in another path: `tests/ui-cargo/update-all-references.sh`.
129 If the lint you are working on is making use of structured suggestions, the
130 test file should include a `// run-rustfix` comment at the top. This will
131 additionally run [rustfix] for that test. Rustfix will apply the suggestions
132 from the lint to the code of the test file and compare that to the contents of
135 Use `tests/ui/update-all-references.sh` to automatically generate the
136 `.fixed` file after running the tests.
138 [rustfix]: https://github.com/rust-lang/rustfix
140 ## Edition 2018 tests
142 Some features require the 2018 edition to work (e.g. `async_await`), but
143 compile-test tests run on the 2015 edition by default. To change this behavior
144 add `// edition:2018` at the top of the test file (note that it's space-sensitive).
148 Manually testing against an example file can be useful if you have added some
149 `println!`s and the test suite output becomes unreadable. To try Clippy with
150 your local modifications, run `env CLIPPY_TESTS=true cargo run --bin
151 clippy-driver -- -L ./target/debug input.rs` from the working copy root.
153 With tests in place, let's have a look at implementing our lint now.
157 Let's start by opening the new file created in the `clippy_lints` crate
158 at `clippy_lints/src/foo_functions.rs`. That's the crate where all the
159 lint code is. This file has already imported some initial things we will need:
162 use rustc_lint::{EarlyLintPass, EarlyContext};
163 use rustc_session::{declare_lint_pass, declare_tool_lint};
164 use rustc_ast::ast::*;
167 The next step is to update the lint declaration. Lints are declared using the
168 [`declare_clippy_lint!`][declare_clippy_lint] macro, and we just need to update
169 the auto-generated lint declaration to have a real description, something like this:
172 declare_clippy_lint! {
173 /// **What it does:**
175 /// **Why is this bad?**
177 /// **Known problems:** None.
186 "function named `foo`, which is not a descriptive name"
190 * The section of lines prefixed with `///` constitutes the lint documentation
191 section. This is the default documentation style and will be displayed
192 [like this][example_lint_page]. To render and open this documentation locally
193 in a browser, run `cargo dev serve`.
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 According to [the rustc-dev-guide], the text should be matter of fact and avoid
300 capitalization and periods, unless multiple sentences are needed.
301 When code or an identifier must appear in a message or label, it should be
302 surrounded with single acute accents \`.
304 [check_fn]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html#method.check_fn
305 [diagnostics]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/diagnostics.rs
306 [the rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/diagnostics.html
308 ## Adding the lint logic
310 Writing the logic for your lint will most likely be different from our example,
311 so this section is kept rather short.
313 Using the [`check_fn`][check_fn] method gives us access to [`FnKind`][fn_kind]
314 that has the [`FnKind::Fn`] variant. It provides access to the name of the
315 function/method via an [`Ident`][ident].
317 With that we can expand our `check_fn` method to:
320 impl EarlyLintPass for FooFunctions {
321 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
322 if is_foo_fn(fn_kind) {
327 "function named `foo`",
329 "consider using a more meaningful name"
336 We separate the lint conditional from the lint emissions because it makes the
337 code a bit easier to read. In some cases this separation would also allow to
338 write some unit tests (as opposed to only UI tests) for the separate function.
340 In our example, `is_foo_fn` looks like:
343 // use statements, impl EarlyLintPass, check_fn, ..
345 fn is_foo_fn(fn_kind: FnKind<'_>) -> bool {
347 FnKind::Fn(_, ident, ..) => {
348 // check if `fn` name is `foo`
349 ident.name.as_str() == "foo"
352 FnKind::Closure(..) => false
357 Now we should also run the full test suite with `cargo test`. At this point
358 running `cargo test` should produce the expected output. Remember to run
359 `tests/ui/update-all-references.sh` to update the `.stderr` file.
361 `cargo test` (as opposed to `cargo uitest`) will also ensure that our lint
362 implementation is not violating any Clippy lints itself.
364 That should be it for the lint implementation. Running `cargo test` should now
367 [fn_kind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html
368 [`FnKind::Fn`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html#variant.Fn
369 [ident]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Ident.html
373 If you have trouble implementing your lint, there is also the internal `author`
374 lint to generate Clippy code that detects the offending pattern. It does not
375 work for all of the Rust syntax, but can give a good starting point.
377 The quickest way to use it, is the
378 [Rust playground: play.rust-lang.org][author_example].
379 Put the code you want to lint into the editor and add the `#[clippy::author]`
380 attribute above the item. Then run Clippy via `Tools -> Clippy` and you should
381 see the generated code in the output below.
383 [Here][author_example] is an example on the playground.
385 If the command was executed successfully, you can copy the code over to where
386 you are implementing your lint.
388 [author_example]: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=9a12cb60e5c6ad4e3003ac6d5e63cf55
392 The final thing before submitting our PR is to add some documentation to our
395 Please document your lint with a doc comment akin to the following:
398 declare_clippy_lint! {
399 /// **What it does:** Checks for ... (describe what the lint matches).
401 /// **Why is this bad?** Supply the reason for linting the code.
403 /// **Known problems:** None. (Or describe where it could go wrong.)
409 /// Insert a short example of code that triggers the lint
412 /// Insert a short example of improved code that doesn't trigger the lint
416 "function named `foo`, which is not a descriptive name"
420 Once your lint is merged, this documentation will show up in the [lint
423 [lint_list]: https://rust-lang.github.io/rust-clippy/master/index.html
427 [Rustfmt] is a tool for formatting Rust code according to style guidelines.
428 Your code has to be formatted by `rustfmt` before a PR can be merged.
429 Clippy uses nightly `rustfmt` in the CI.
431 It can be installed via `rustup`:
434 rustup component add rustfmt --toolchain=nightly
437 Use `cargo dev fmt` to format the whole codebase. Make sure that `rustfmt` is
438 installed for the nightly toolchain.
440 [Rustfmt]: https://github.com/rust-lang/rustfmt
444 If you want to debug parts of your lint implementation, you can use the [`dbg!`]
445 macro anywhere in your code. Running the tests should then include the debug
446 output in the `stdout` part.
448 [`dbg!`]: https://doc.rust-lang.org/std/macro.dbg.html
452 Before submitting your PR make sure you followed all of the basic requirements:
454 <!-- Sync this with `.github/PULL_REQUEST_TEMPLATE` -->
456 - [ ] Followed [lint naming conventions][lint_naming]
457 - [ ] Added passing UI tests (including committed `.stderr` file)
458 - [ ] `cargo test` passes locally
459 - [ ] Executed `cargo dev update_lints`
460 - [ ] Added lint documentation
461 - [ ] Run `cargo dev fmt`
465 Here are some pointers to things you are likely going to need for every lint:
467 * [Clippy utils][utils] - Various helper functions. Maybe the function you need
468 is already in here (`implements_trait`, `match_path`, `snippet`, etc)
469 * [Clippy diagnostics][diagnostics]
470 * [The `if_chain` macro][if_chain]
471 * [`from_expansion`][from_expansion] and [`in_external_macro`][in_external_macro]
473 * [`Applicability`][applicability]
474 * [Common tools for writing lints](common_tools_writing_lints.md) helps with common operations
475 * [The rustc-dev-guide][rustc-dev-guide] explains a lot of internal compiler concepts
476 * [The nightly rustc docs][nightly_docs] which has been linked to throughout
479 For `EarlyLintPass` lints:
481 * [`EarlyLintPass`][early_lint_pass]
482 * [`rustc_ast::ast`][ast]
484 For `LateLintPass` lints:
486 * [`LateLintPass`][late_lint_pass]
489 While most of Clippy's lint utils are documented, most of rustc's internals lack
490 documentation currently. This is unfortunate, but in most cases you can probably
491 get away with copying things from existing similar lints. If you are stuck,
492 don't hesitate to ask on [Zulip] or in the issue/PR.
494 [utils]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/mod.rs
495 [if_chain]: https://docs.rs/if_chain/*/if_chain/
496 [from_expansion]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html#method.from_expansion
497 [in_external_macro]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/lint/fn.in_external_macro.html
498 [span]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html
499 [applicability]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/enum.Applicability.html
500 [rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/
501 [nightly_docs]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/
502 [ast]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/index.html
503 [ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/sty/index.html
504 [Zulip]: https://rust-lang.zulipchat.com/#narrow/stream/clippy