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 - [Cargo lints](#cargo-lints)
15 - [Rustfix tests](#rustfix-tests)
16 - [Edition 2018 tests](#edition-2018-tests)
17 - [Testing manually](#testing-manually)
18 - [Lint declaration](#lint-declaration)
19 - [Lint registration](#lint-registration)
20 - [Lint passes](#lint-passes)
21 - [Emitting a lint](#emitting-a-lint)
22 - [Adding the lint logic](#adding-the-lint-logic)
23 - [Specifying the lint's minimum supported Rust version (MSRV)](#specifying-the-lints-minimum-supported-rust-version-msrv)
24 - [Author lint](#author-lint)
25 - [Documentation](#documentation)
26 - [Running rustfmt](#running-rustfmt)
27 - [Debugging](#debugging)
28 - [PR Checklist](#pr-checklist)
29 - [Adding configuration to a lint](#adding-configuration-to-a-lint)
30 - [Cheatsheet](#cheatsheet)
34 See the [Basics](basics.md#get-the-code) documentation.
38 There is a bit of boilerplate code that needs to be set up when creating a new
39 lint. Fortunately, you can use the clippy dev tools to handle this for you. We
40 are naming our new lint `foo_functions` (lints are generally written in snake
41 case), and we don't need type information so it will have an early pass type
42 (more on this later on). If you're not sure if the name you chose fits the lint,
43 take a look at our [lint naming guidelines][lint_naming]. To get started on this
44 lint you can run `cargo dev new_lint --name=foo_functions --pass=early
45 --category=pedantic` (category will default to nursery if not provided). This
46 command will create two files: `tests/ui/foo_functions.rs` and
47 `clippy_lints/src/foo_functions.rs`, as well as
48 [registering the lint](#lint-registration). For cargo lints, two project
49 hierarchies (fail/pass) will be created by default under `tests/ui-cargo`.
51 Next, we'll open up these files and add our lint!
55 Let's write some tests first that we can execute while we iterate on our lint.
57 Clippy uses UI tests for testing. UI tests check that the output of Clippy is
58 exactly as expected. Each test is just a plain Rust file that contains the code
59 we want to check. The output of Clippy is compared against a `.stderr` file.
60 Note that you don't have to create this file yourself, we'll get to
61 generating the `.stderr` files further down.
63 We start by opening the test file created at `tests/ui/foo_functions.rs`.
65 Update the file with some examples to get started:
68 #![warn(clippy::foo_functions)]
78 // Default trait methods
91 // We also don't want to lint method calls
98 Now we can run the test with `TESTNAME=foo_functions cargo uitest`,
99 currently this test is meaningless though.
101 While we are working on implementing our lint, we can keep running the UI
102 test. That allows us to check if the output is turning into what we want.
104 Once we are satisfied with the output, we need to run
105 `cargo dev bless` to update the `.stderr` file for our lint.
106 Please note that, we should run `TESTNAME=foo_functions cargo uitest`
107 every time before running `cargo dev bless`.
108 Running `TESTNAME=foo_functions cargo uitest` should pass then. When we commit
109 our lint, we need to commit the generated `.stderr` files, too. In general, you
110 should only commit files changed by `cargo dev bless` for the
111 specific lint you are creating/editing. Note that if the generated files are
112 empty, they should be removed.
114 Note that you can run multiple test files by specifying a comma separated list:
115 `TESTNAME=foo_functions,test2,test3`.
119 For cargo lints, the process of testing differs in that we are interested in
120 the `Cargo.toml` manifest file. We also need a minimal crate associated
123 If our new lint is named e.g. `foo_categories`, after running `cargo dev new_lint`
124 we will find by default two new crates, each with its manifest file:
126 * `tests/ui-cargo/foo_categories/fail/Cargo.toml`: this file should cause the new lint to raise an error.
127 * `tests/ui-cargo/foo_categories/pass/Cargo.toml`: this file should not trigger the lint.
129 If you need more cases, you can copy one of those crates (under `foo_categories`) and rename it.
131 The process of generating the `.stderr` file is the same, and prepending the `TESTNAME`
132 variable to `cargo uitest` works too.
136 If the lint you are working on is making use of structured suggestions, the
137 test file should include a `// run-rustfix` comment at the top. This will
138 additionally run [rustfix] for that test. Rustfix will apply the suggestions
139 from the lint to the code of the test file and compare that to the contents of
142 Use `cargo dev bless` to automatically generate the
143 `.fixed` file after running the tests.
145 [rustfix]: https://github.com/rust-lang/rustfix
147 ## Edition 2018 tests
149 Some features require the 2018 edition to work (e.g. `async_await`), but
150 compile-test tests run on the 2015 edition by default. To change this behavior
151 add `// edition:2018` at the top of the test file (note that it's space-sensitive).
155 Manually testing against an example file can be useful if you have added some
156 `println!`s and the test suite output becomes unreadable. To try Clippy with
157 your local modifications, run
160 env __CLIPPY_INTERNAL_TESTS=true cargo run --bin clippy-driver -- -L ./target/debug input.rs
163 from the working copy root. With tests in place, let's have a look at
164 implementing our lint now.
168 Let's start by opening the new file created in the `clippy_lints` crate
169 at `clippy_lints/src/foo_functions.rs`. That's the crate where all the
170 lint code is. This file has already imported some initial things we will need:
173 use rustc_lint::{EarlyLintPass, EarlyContext};
174 use rustc_session::{declare_lint_pass, declare_tool_lint};
175 use rustc_ast::ast::*;
178 The next step is to update the lint declaration. Lints are declared using the
179 [`declare_clippy_lint!`][declare_clippy_lint] macro, and we just need to update
180 the auto-generated lint declaration to have a real description, something like this:
183 declare_clippy_lint! {
186 /// ### Why is this bad?
194 "function named `foo`, which is not a descriptive name"
198 * The section of lines prefixed with `///` constitutes the lint documentation
199 section. This is the default documentation style and will be displayed
200 [like this][example_lint_page]. To render and open this documentation locally
201 in a browser, run `cargo dev serve`.
202 * `FOO_FUNCTIONS` is the name of our lint. Be sure to follow the
203 [lint naming guidelines][lint_naming] here when naming your lint.
204 In short, the name should state the thing that is being checked for and
205 read well when used with `allow`/`warn`/`deny`.
206 * `pedantic` sets the lint level to `Allow`.
207 The exact mapping can be found [here][category_level_mapping]
208 * The last part should be a text that explains what exactly is wrong with the
211 The rest of this file contains an empty implementation for our lint pass,
212 which in this case is `EarlyLintPass` and should look like this:
215 // clippy_lints/src/foo_functions.rs
217 // .. imports and lint declaration ..
219 declare_lint_pass!(FooFunctions => [FOO_FUNCTIONS]);
221 impl EarlyLintPass for 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 When using `cargo dev new_lint`, the lint is automatically registered and
232 nothing more has to be done.
234 When declaring a new lint by hand and `cargo dev update_lints` is used, the lint
235 pass may have to be registered manually in the `register_plugins` function in
236 `clippy_lints/src/lib.rs`:
239 store.register_early_pass(|| Box::new(foo_functions::FooFunctions));
242 As one may expect, there is a corresponding `register_late_pass` method
243 available as well. Without a call to one of `register_early_pass` or
244 `register_late_pass`, the lint pass in question will not be run.
246 One reason that `cargo dev update_lints` does not automate this step is that
247 multiple lints can use the same lint pass, so registering the lint pass may
248 already be done when adding a new lint. Another reason that this step is not
249 automated is that the order that the passes are registered determines the order
250 the passes actually run, which in turn affects the order that any emitted lints
255 Writing a lint that only checks for the name of a function means that we only
256 have to deal with the AST and don't have to deal with the type system at all.
257 This is good, because it makes writing this particular lint less complicated.
259 We have to make this decision with every new Clippy lint. It boils down to using
260 either [`EarlyLintPass`][early_lint_pass] or [`LateLintPass`][late_lint_pass].
262 In short, the `LateLintPass` has access to type information while the
263 `EarlyLintPass` doesn't. If you don't need access to type information, use the
264 `EarlyLintPass`. The `EarlyLintPass` is also faster. However linting speed
265 hasn't really been a concern with Clippy so far.
267 Since we don't need type information for checking the function name, we used
268 `--pass=early` when running the new lint automation and all the imports were
271 [early_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html
272 [late_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.LateLintPass.html
276 With UI tests and the lint declaration in place, we can start working on the
277 implementation of the lint logic.
279 Let's start by implementing the `EarlyLintPass` for our `FooFunctions`:
282 impl EarlyLintPass for FooFunctions {
283 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
284 // TODO: Emit lint here
289 We implement the [`check_fn`][check_fn] method from the
290 [`EarlyLintPass`][early_lint_pass] trait. This gives us access to various
291 information about the function that is currently being checked. More on that in
292 the next section. Let's worry about the details later and emit our lint for
293 *every* function definition first.
295 Depending on how complex we want our lint message to be, we can choose from a
296 variety of lint emission functions. They can all be found in
297 [`clippy_utils/src/diagnostics.rs`][diagnostics].
299 `span_lint_and_help` seems most appropriate in this case. It allows us to
300 provide an extra help message and we can't really suggest a better name
301 automatically. This is how it looks:
304 impl EarlyLintPass for FooFunctions {
305 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
310 "function named `foo`",
312 "consider using a more meaningful name"
318 Running our UI test should now produce output that contains the lint message.
320 According to [the rustc-dev-guide], the text should be matter of fact and avoid
321 capitalization and periods, unless multiple sentences are needed.
322 When code or an identifier must appear in a message or label, it should be
323 surrounded with single grave accents \`.
325 [check_fn]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html#method.check_fn
326 [diagnostics]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_utils/src/diagnostics.rs
327 [the rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/diagnostics.html
329 ## Adding the lint logic
331 Writing the logic for your lint will most likely be different from our example,
332 so this section is kept rather short.
334 Using the [`check_fn`][check_fn] method gives us access to [`FnKind`][fn_kind]
335 that has the [`FnKind::Fn`] variant. It provides access to the name of the
336 function/method via an [`Ident`][ident].
338 With that we can expand our `check_fn` method to:
341 impl EarlyLintPass for FooFunctions {
342 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
343 if is_foo_fn(fn_kind) {
348 "function named `foo`",
350 "consider using a more meaningful name"
357 We separate the lint conditional from the lint emissions because it makes the
358 code a bit easier to read. In some cases this separation would also allow to
359 write some unit tests (as opposed to only UI tests) for the separate function.
361 In our example, `is_foo_fn` looks like:
364 // use statements, impl EarlyLintPass, check_fn, ..
366 fn is_foo_fn(fn_kind: FnKind<'_>) -> bool {
368 FnKind::Fn(_, ident, ..) => {
369 // check if `fn` name is `foo`
370 ident.name.as_str() == "foo"
373 FnKind::Closure(..) => false
378 Now we should also run the full test suite with `cargo test`. At this point
379 running `cargo test` should produce the expected output. Remember to run
380 `cargo dev bless` to update the `.stderr` file.
382 `cargo test` (as opposed to `cargo uitest`) will also ensure that our lint
383 implementation is not violating any Clippy lints itself.
385 That should be it for the lint implementation. Running `cargo test` should now
388 [fn_kind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html
389 [`FnKind::Fn`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html#variant.Fn
390 [ident]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Ident.html
392 ## Specifying the lint's minimum supported Rust version (MSRV)
394 Sometimes a lint makes suggestions that require a certain version of Rust. For example, the `manual_strip` lint suggests
395 using `str::strip_prefix` and `str::strip_suffix` which is only available after Rust 1.45. In such cases, you need to
396 ensure that the MSRV configured for the project is >= the MSRV of the required Rust feature. If multiple features are
397 required, just use the one with a lower MSRV.
399 First, add an MSRV alias for the required feature in [`clippy_utils::msrvs`](/clippy_utils/src/msrvs.rs). This can be
400 accessed later as `msrvs::STR_STRIP_PREFIX`, for example.
405 1,45,0 { STR_STRIP_PREFIX }
409 In order to access the project-configured MSRV, you need to have an `msrv` field in the LintPass struct, and a
410 constructor to initialize the field. The `msrv` value is passed to the constructor in `clippy_lints/lib.rs`.
413 pub struct ManualStrip {
414 msrv: Option<RustcVersion>,
419 pub fn new(msrv: Option<RustcVersion>) -> Self {
425 The project's MSRV can then be matched against the feature MSRV in the LintPass
426 using the `meets_msrv` utility function.
429 if !meets_msrv(self.msrv.as_ref(), &msrvs::STR_STRIP_PREFIX) {
434 The project's MSRV can also be specified as an inner attribute, which overrides
435 the value from `clippy.toml`. This can be accounted for using the
436 `extract_msrv_attr!(LintContext)` macro and passing
437 `LateContext`/`EarlyContext`.
440 impl<'tcx> LateLintPass<'tcx> for ManualStrip {
441 fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
444 extract_msrv_attr!(LateContext);
448 Once the `msrv` is added to the lint, a relevant test case should be added to
449 `tests/ui/min_rust_version_attr.rs` which verifies that the lint isn't emitted
450 if the project's MSRV is lower.
452 As a last step, the lint should be added to the lint documentation. This is done
453 in `clippy_lints/src/utils/conf.rs`:
457 /// Lint: LIST, OF, LINTS, <THE_NEWLY_ADDED_LINT>. The minimum rust version that the project supports
458 (msrv: Option<String> = None),
465 If you have trouble implementing your lint, there is also the internal `author`
466 lint to generate Clippy code that detects the offending pattern. It does not
467 work for all of the Rust syntax, but can give a good starting point.
469 The quickest way to use it, is the
470 [Rust playground: play.rust-lang.org][author_example].
471 Put the code you want to lint into the editor and add the `#[clippy::author]`
472 attribute above the item. Then run Clippy via `Tools -> Clippy` and you should
473 see the generated code in the output below.
475 [Here][author_example] is an example on the playground.
477 If the command was executed successfully, you can copy the code over to where
478 you are implementing your lint.
480 [author_example]: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=9a12cb60e5c6ad4e3003ac6d5e63cf55
484 The final thing before submitting our PR is to add some documentation to our
487 Please document your lint with a doc comment akin to the following:
490 declare_clippy_lint! {
492 /// Checks for ... (describe what the lint matches).
494 /// ### Why is this bad?
495 /// Supply the reason for linting the code.
501 /// Insert a short example of code that triggers the lint
504 /// Insert a short example of improved code that doesn't trigger the lint
508 "function named `foo`, which is not a descriptive name"
512 Once your lint is merged, this documentation will show up in the [lint
515 [lint_list]: https://rust-lang.github.io/rust-clippy/master/index.html
519 [Rustfmt] is a tool for formatting Rust code according to style guidelines.
520 Your code has to be formatted by `rustfmt` before a PR can be merged.
521 Clippy uses nightly `rustfmt` in the CI.
523 It can be installed via `rustup`:
526 rustup component add rustfmt --toolchain=nightly
529 Use `cargo dev fmt` to format the whole codebase. Make sure that `rustfmt` is
530 installed for the nightly toolchain.
532 [Rustfmt]: https://github.com/rust-lang/rustfmt
536 If you want to debug parts of your lint implementation, you can use the [`dbg!`]
537 macro anywhere in your code. Running the tests should then include the debug
538 output in the `stdout` part.
540 [`dbg!`]: https://doc.rust-lang.org/std/macro.dbg.html
544 Before submitting your PR make sure you followed all of the basic requirements:
546 <!-- Sync this with `.github/PULL_REQUEST_TEMPLATE` -->
548 - \[ ] Followed [lint naming conventions][lint_naming]
549 - \[ ] Added passing UI tests (including committed `.stderr` file)
550 - \[ ] `cargo test` passes locally
551 - \[ ] Executed `cargo dev update_lints`
552 - \[ ] Added lint documentation
553 - \[ ] Run `cargo dev fmt`
555 ## Adding configuration to a lint
557 Clippy supports the configuration of lints values using a `clippy.toml` file in the workspace
558 directory. Adding a configuration to a lint can be useful for thresholds or to constrain some
559 behavior that can be seen as a false positive for some users. Adding a configuration is done
560 in the following steps:
562 1. Adding a new configuration entry to [clippy_lints::utils::conf](/clippy_lints/src/utils/conf.rs)
567 /// <The configuration field doc comment>
568 (configuration_ident: Type = DefaultValue),
570 The doc comment is automatically added to the documentation of the listed lints. The default
571 value will be formatted using the `Debug` implementation of the type.
572 2. Adding the configuration value to the lint impl struct:
573 1. This first requires the definition of a lint impl struct. Lint impl structs are usually
574 generated with the `declare_lint_pass!` macro. This struct needs to be defined manually
575 to add some kind of metadata to it:
577 // Generated struct definition
578 declare_lint_pass!(StructName => [
582 // New manual definition struct
583 #[derive(Copy, Clone)]
584 pub struct StructName {}
586 impl_lint_pass!(StructName => [
591 2. Next add the configuration value and a corresponding creation method like this:
593 #[derive(Copy, Clone)]
594 pub struct StructName {
595 configuration_ident: Type,
601 pub fn new(configuration_ident: Type) -> Self {
608 3. Passing the configuration value to the lint impl struct:
610 First find the struct construction in the [clippy_lints lib file](/clippy_lints/src/lib.rs).
611 The configuration value is now cloned or copied into a local value that is then passed to the
612 impl struct like this:
614 // Default generated registration:
615 store.register_*_pass(|| box module::StructName);
617 // New registration with configuration value
618 let configuration_ident = conf.configuration_ident.clone();
619 store.register_*_pass(move || box module::StructName::new(configuration_ident));
622 Congratulations the work is almost done. The configuration value can now be accessed
623 in the linting code via `self.configuration_ident`.
626 1. The default configured value can be tested like any normal lint in [`tests/ui`](/tests/ui).
627 2. The configuration itself will be tested separately in [`tests/ui-toml`](/tests/ui-toml).
628 Simply add a new subfolder with a fitting name. This folder contains a `clippy.toml` file
629 with the configuration value and a rust file that should be linted by Clippy. The test can
630 otherwise be written as usual.
634 Here are some pointers to things you are likely going to need for every lint:
636 * [Clippy utils][utils] - Various helper functions. Maybe the function you need
637 is already in here (`implements_trait`, `match_def_path`, `snippet`, etc)
638 * [Clippy diagnostics][diagnostics]
639 * [The `if_chain` macro][if_chain]
640 * [`from_expansion`][from_expansion] and [`in_external_macro`][in_external_macro]
642 * [`Applicability`][applicability]
643 * [Common tools for writing lints](common_tools_writing_lints.md) helps with common operations
644 * [The rustc-dev-guide][rustc-dev-guide] explains a lot of internal compiler concepts
645 * [The nightly rustc docs][nightly_docs] which has been linked to throughout
648 For `EarlyLintPass` lints:
650 * [`EarlyLintPass`][early_lint_pass]
651 * [`rustc_ast::ast`][ast]
653 For `LateLintPass` lints:
655 * [`LateLintPass`][late_lint_pass]
658 While most of Clippy's lint utils are documented, most of rustc's internals lack
659 documentation currently. This is unfortunate, but in most cases you can probably
660 get away with copying things from existing similar lints. If you are stuck,
661 don't hesitate to ask on [Zulip] or in the issue/PR.
663 [utils]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_utils/src/lib.rs
664 [if_chain]: https://docs.rs/if_chain/*/if_chain/
665 [from_expansion]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html#method.from_expansion
666 [in_external_macro]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/lint/fn.in_external_macro.html
667 [span]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html
668 [applicability]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/enum.Applicability.html
669 [rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/
670 [nightly_docs]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/
671 [ast]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/index.html
672 [ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/sty/index.html
673 [Zulip]: https://rust-lang.zulipchat.com/#narrow/stream/clippy