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 - [Testing manually](#testing-manually)
17 - [Lint declaration](#lint-declaration)
18 - [Lint registration](#lint-registration)
19 - [Lint passes](#lint-passes)
20 - [Emitting a lint](#emitting-a-lint)
21 - [Adding the lint logic](#adding-the-lint-logic)
22 - [Specifying the lint's minimum supported Rust version (MSRV)](#specifying-the-lints-minimum-supported-rust-version-msrv)
23 - [Author lint](#author-lint)
24 - [Print HIR lint](#print-hir-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 - [Cheat Sheet](#cheat-sheet)
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 [registering the
48 lint](#lint-registration). For cargo lints, two project hierarchies (fail/pass)
49 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 generating
61 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`, currently
99 this test is meaningless though.
101 While we are working on implementing our lint, we can keep running the UI test.
102 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 `cargo dev bless` to
105 update the `.stderr` file for our lint. Please note that, we should run
106 `TESTNAME=foo_functions cargo uitest` every time before running `cargo dev
107 bless`. Running `TESTNAME=foo_functions cargo uitest` should pass then. When we
108 commit our lint, we need to commit the generated `.stderr` files, too. In
109 general, you should only commit files changed by `cargo dev bless` for the
110 specific lint you are creating/editing. Note that if the generated files are
111 empty, they should be removed.
113 > _Note:_ you can run multiple test files by specifying a comma separated list:
114 > `TESTNAME=foo_functions,test2,test3`.
118 For cargo lints, the process of testing differs in that we are interested in the
119 `Cargo.toml` manifest file. We also need a minimal crate associated with that
122 If our new lint is named e.g. `foo_categories`, after running `cargo dev
123 new_lint` we will find by default two new crates, each with its manifest file:
125 * `tests/ui-cargo/foo_categories/fail/Cargo.toml`: this file should cause the
126 new lint to raise an error.
127 * `tests/ui-cargo/foo_categories/pass/Cargo.toml`: this file should not trigger
130 If you need more cases, you can copy one of those crates (under
131 `foo_categories`) and rename it.
133 The process of generating the `.stderr` file is the same, and prepending the
134 `TESTNAME` variable to `cargo uitest` works too.
138 If the lint you are working on is making use of structured suggestions, the test
139 file should include a `// run-rustfix` comment at the top. This will
140 additionally run [rustfix] for that test. Rustfix will apply the suggestions
141 from the lint to the code of the test file and compare that to the contents of a
144 Use `cargo dev bless` to automatically generate the `.fixed` file after running
147 [rustfix]: https://github.com/rust-lang/rustfix
151 Manually testing against an example file can be useful if you have added some
152 `println!`s and the test suite output becomes unreadable. To try Clippy with
153 your local modifications, run
156 cargo dev lint input.rs
159 from the working copy root. With tests in place, let's have a look at
160 implementing our lint now.
164 Let's start by opening the new file created in the `clippy_lints` crate at
165 `clippy_lints/src/foo_functions.rs`. That's the crate where all the lint code
166 is. This file has already imported some initial things we will need:
169 use rustc_lint::{EarlyLintPass, EarlyContext};
170 use rustc_session::{declare_lint_pass, declare_tool_lint};
171 use rustc_ast::ast::*;
174 The next step is to update the lint declaration. Lints are declared using the
175 [`declare_clippy_lint!`][declare_clippy_lint] macro, and we just need to update
176 the auto-generated lint declaration to have a real description, something like
180 declare_clippy_lint! {
183 /// ### Why is this bad?
189 #[clippy::version = "1.29.0"]
192 "function named `foo`, which is not a descriptive name"
196 * The section of lines prefixed with `///` constitutes the lint documentation
197 section. This is the default documentation style and will be displayed [like
198 this][example_lint_page]. To render and open this documentation locally in a
199 browser, run `cargo dev serve`.
200 * The `#[clippy::version]` attribute will be rendered as part of the lint
201 documentation. The value should be set to the current Rust version that the
202 lint is developed in, it can be retrieved by running `rustc -vV` in the
203 rust-clippy directory. The version is listed under *release*. (Use the version
204 without the `-nightly`) suffix.
205 * `FOO_FUNCTIONS` is the name of our lint. Be sure to follow the [lint naming
206 guidelines][lint_naming] here when naming your lint. In short, the name should
207 state the thing that is being checked for and read well when used with
208 `allow`/`warn`/`deny`.
209 * `pedantic` sets the lint level to `Allow`. The exact mapping can be found
210 [here][category_level_mapping]
211 * The last part should be a text that explains what exactly is wrong with the
214 The rest of this file contains an empty implementation for our lint pass, which
215 in this case is `EarlyLintPass` and should look like this:
218 // clippy_lints/src/foo_functions.rs
220 // .. imports and lint declaration ..
222 declare_lint_pass!(FooFunctions => [FOO_FUNCTIONS]);
224 impl EarlyLintPass for 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 When using `cargo dev new_lint`, the lint is automatically registered and
235 nothing more has to be done.
237 When declaring a new lint by hand and `cargo dev update_lints` is used, the lint
238 pass may have to be registered manually in the `register_plugins` function in
239 `clippy_lints/src/lib.rs`:
242 store.register_early_pass(|| Box::new(foo_functions::FooFunctions));
245 As one may expect, there is a corresponding `register_late_pass` method
246 available as well. Without a call to one of `register_early_pass` or
247 `register_late_pass`, the lint pass in question will not be run.
249 One reason that `cargo dev update_lints` does not automate this step is that
250 multiple lints can use the same lint pass, so registering the lint pass may
251 already be done when adding a new lint. Another reason that this step is not
252 automated is that the order that the passes are registered determines the order
253 the passes actually run, which in turn affects the order that any emitted lints
258 Writing a lint that only checks for the name of a function means that we only
259 have to deal with the AST and don't have to deal with the type system at all.
260 This is good, because it makes writing this particular lint less complicated.
262 We have to make this decision with every new Clippy lint. It boils down to using
263 either [`EarlyLintPass`][early_lint_pass] or [`LateLintPass`][late_lint_pass].
265 In short, the `LateLintPass` has access to type information while the
266 `EarlyLintPass` doesn't. If you don't need access to type information, use the
267 `EarlyLintPass`. The `EarlyLintPass` is also faster. However linting speed
268 hasn't really been a concern with Clippy so far.
270 Since we don't need type information for checking the function name, we used
271 `--pass=early` when running the new lint automation and all the imports were
274 [early_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html
275 [late_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.LateLintPass.html
279 With UI tests and the lint declaration in place, we can start working on the
280 implementation of the lint logic.
282 Let's start by implementing the `EarlyLintPass` for our `FooFunctions`:
285 impl EarlyLintPass for FooFunctions {
286 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
287 // TODO: Emit lint here
292 We implement the [`check_fn`][check_fn] method from the
293 [`EarlyLintPass`][early_lint_pass] trait. This gives us access to various
294 information about the function that is currently being checked. More on that in
295 the next section. Let's worry about the details later and emit our lint for
296 *every* function definition first.
298 Depending on how complex we want our lint message to be, we can choose from a
299 variety of lint emission functions. They can all be found in
300 [`clippy_utils/src/diagnostics.rs`][diagnostics].
302 `span_lint_and_help` seems most appropriate in this case. It allows us to
303 provide an extra help message and we can't really suggest a better name
304 automatically. This is how it looks:
307 impl EarlyLintPass for FooFunctions {
308 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
313 "function named `foo`",
315 "consider using a more meaningful name"
321 Running our UI test should now produce output that contains the lint message.
323 According to [the rustc-dev-guide], the text should be matter of fact and avoid
324 capitalization and periods, unless multiple sentences are needed. When code or
325 an identifier must appear in a message or label, it should be surrounded with
326 single grave accents \`.
328 [check_fn]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html#method.check_fn
329 [diagnostics]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_utils/src/diagnostics.rs
330 [the rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/diagnostics.html
332 ## Adding the lint logic
334 Writing the logic for your lint will most likely be different from our example,
335 so this section is kept rather short.
337 Using the [`check_fn`][check_fn] method gives us access to [`FnKind`][fn_kind]
338 that has the [`FnKind::Fn`] variant. It provides access to the name of the
339 function/method via an [`Ident`][ident].
341 With that we can expand our `check_fn` method to:
344 impl EarlyLintPass for FooFunctions {
345 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
346 if is_foo_fn(fn_kind) {
351 "function named `foo`",
353 "consider using a more meaningful name"
360 We separate the lint conditional from the lint emissions because it makes the
361 code a bit easier to read. In some cases this separation would also allow to
362 write some unit tests (as opposed to only UI tests) for the separate function.
364 In our example, `is_foo_fn` looks like:
367 // use statements, impl EarlyLintPass, check_fn, ..
369 fn is_foo_fn(fn_kind: FnKind<'_>) -> bool {
371 FnKind::Fn(_, ident, ..) => {
372 // check if `fn` name is `foo`
373 ident.name.as_str() == "foo"
376 FnKind::Closure(..) => false
381 Now we should also run the full test suite with `cargo test`. At this point
382 running `cargo test` should produce the expected output. Remember to run `cargo
383 dev bless` to update the `.stderr` file.
385 `cargo test` (as opposed to `cargo uitest`) will also ensure that our lint
386 implementation is not violating any Clippy lints itself.
388 That should be it for the lint implementation. Running `cargo test` should now
391 [fn_kind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html
392 [`FnKind::Fn`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html#variant.Fn
393 [ident]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Ident.html
395 ## Specifying the lint's minimum supported Rust version (MSRV)
397 Sometimes a lint makes suggestions that require a certain version of Rust. For
398 example, the `manual_strip` lint suggests using `str::strip_prefix` and
399 `str::strip_suffix` which is only available after Rust 1.45. In such cases, you
400 need to ensure that the MSRV configured for the project is >= the MSRV of the
401 required Rust feature. If multiple features are required, just use the one with
404 First, add an MSRV alias for the required feature in [`clippy_utils::msrvs`].
405 This can be accessed later as `msrvs::STR_STRIP_PREFIX`, for example.
410 1,45,0 { STR_STRIP_PREFIX }
414 In order to access the project-configured MSRV, you need to have an `msrv` field
415 in the LintPass struct, and a constructor to initialize the field. The `msrv`
416 value is passed to the constructor in `clippy_lints/lib.rs`.
419 pub struct ManualStrip {
420 msrv: Option<RustcVersion>,
425 pub fn new(msrv: Option<RustcVersion>) -> Self {
431 The project's MSRV can then be matched against the feature MSRV in the LintPass
432 using the `meets_msrv` utility function.
435 if !meets_msrv(self.msrv, msrvs::STR_STRIP_PREFIX) {
440 The project's MSRV can also be specified as an inner attribute, which overrides
441 the value from `clippy.toml`. This can be accounted for using the
442 `extract_msrv_attr!(LintContext)` macro and passing
443 `LateContext`/`EarlyContext`.
446 impl<'tcx> LateLintPass<'tcx> for ManualStrip {
447 fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
450 extract_msrv_attr!(LateContext);
454 Once the `msrv` is added to the lint, a relevant test case should be added to
455 `tests/ui/min_rust_version_attr.rs` which verifies that the lint isn't emitted
456 if the project's MSRV is lower.
458 As a last step, the lint should be added to the lint documentation. This is done
459 in `clippy_lints/src/utils/conf.rs`:
463 /// Lint: LIST, OF, LINTS, <THE_NEWLY_ADDED_LINT>. The minimum rust version that the project supports
464 (msrv: Option<String> = None),
469 [`clippy_utils::msrvs`]: https://doc.rust-lang.org/nightly/nightly-rustc/clippy_utils/msrvs/index.html
473 If you have trouble implementing your lint, there is also the internal `author`
474 lint to generate Clippy code that detects the offending pattern. It does not
475 work for all of the Rust syntax, but can give a good starting point.
477 The quickest way to use it, is the [Rust playground:
478 play.rust-lang.org][author_example]. Put the code you want to lint into the
479 editor and add the `#[clippy::author]` attribute above the item. Then run Clippy
480 via `Tools -> Clippy` and you should see the generated code in the output below.
482 [Here][author_example] is an example on the playground.
484 If the command was executed successfully, you can copy the code over to where
485 you are implementing your lint.
487 [author_example]: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=9a12cb60e5c6ad4e3003ac6d5e63cf55
491 To implement a lint, it's helpful to first understand the internal
492 representation that rustc uses. Clippy has the `#[clippy::dump]` attribute that
493 prints the [_High-Level Intermediate Representation (HIR)_] of the item,
494 statement, or expression that the attribute is attached to. To attach the
495 attribute to expressions you often need to enable
496 `#![feature(stmt_expr_attributes)]`.
498 [Here][print_hir_example] you can find an example, just select _Tools_ and run
501 [_High-Level Intermediate Representation (HIR)_]: https://rustc-dev-guide.rust-lang.org/hir.html
502 [print_hir_example]: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=daf14db3a7f39ca467cd1b86c34b9afb
506 The final thing before submitting our PR is to add some documentation to our
509 Please document your lint with a doc comment akin to the following:
512 declare_clippy_lint! {
514 /// Checks for ... (describe what the lint matches).
516 /// ### Why is this bad?
517 /// Supply the reason for linting the code.
522 /// // A short example of code that triggers the lint
527 /// // A short example of improved code that doesn't trigger the lint
529 #[clippy::version = "1.29.0"]
532 "function named `foo`, which is not a descriptive name"
536 Once your lint is merged, this documentation will show up in the [lint
539 [lint_list]: https://rust-lang.github.io/rust-clippy/master/index.html
543 [Rustfmt] is a tool for formatting Rust code according to style guidelines. Your
544 code has to be formatted by `rustfmt` before a PR can be merged. Clippy uses
545 nightly `rustfmt` in the CI.
547 It can be installed via `rustup`:
550 rustup component add rustfmt --toolchain=nightly
553 Use `cargo dev fmt` to format the whole codebase. Make sure that `rustfmt` is
554 installed for the nightly toolchain.
556 [Rustfmt]: https://github.com/rust-lang/rustfmt
560 If you want to debug parts of your lint implementation, you can use the [`dbg!`]
561 macro anywhere in your code. Running the tests should then include the debug
562 output in the `stdout` part.
564 [`dbg!`]: https://doc.rust-lang.org/std/macro.dbg.html
568 Before submitting your PR make sure you followed all of the basic requirements:
570 <!-- Sync this with `.github/PULL_REQUEST_TEMPLATE` -->
572 - \[ ] Followed [lint naming conventions][lint_naming]
573 - \[ ] Added passing UI tests (including committed `.stderr` file)
574 - \[ ] `cargo test` passes locally
575 - \[ ] Executed `cargo dev update_lints`
576 - \[ ] Added lint documentation
577 - \[ ] Run `cargo dev fmt`
579 ## Adding configuration to a lint
581 Clippy supports the configuration of lints values using a `clippy.toml` file in
582 the workspace directory. Adding a configuration to a lint can be useful for
583 thresholds or to constrain some behavior that can be seen as a false positive
584 for some users. Adding a configuration is done in the following steps:
586 1. Adding a new configuration entry to [`clippy_lints::utils::conf`] like this:
591 /// <The configuration field doc comment>
592 (configuration_ident: Type = DefaultValue),
595 The doc comment is automatically added to the documentation of the listed
596 lints. The default value will be formatted using the `Debug` implementation
598 2. Adding the configuration value to the lint impl struct:
599 1. This first requires the definition of a lint impl struct. Lint impl
600 structs are usually generated with the `declare_lint_pass!` macro. This
601 struct needs to be defined manually to add some kind of metadata to it:
603 // Generated struct definition
604 declare_lint_pass!(StructName => [
608 // New manual definition struct
609 #[derive(Copy, Clone)]
610 pub struct StructName {}
612 impl_lint_pass!(StructName => [
617 2. Next add the configuration value and a corresponding creation method like
620 #[derive(Copy, Clone)]
621 pub struct StructName {
622 configuration_ident: Type,
628 pub fn new(configuration_ident: Type) -> Self {
635 3. Passing the configuration value to the lint impl struct:
637 First find the struct construction in the [`clippy_lints` lib file]. The
638 configuration value is now cloned or copied into a local value that is then
639 passed to the impl struct like this:
642 // Default generated registration:
643 store.register_*_pass(|| box module::StructName);
645 // New registration with configuration value
646 let configuration_ident = conf.configuration_ident.clone();
647 store.register_*_pass(move || box module::StructName::new(configuration_ident));
650 Congratulations the work is almost done. The configuration value can now be
651 accessed in the linting code via `self.configuration_ident`.
654 1. The default configured value can be tested like any normal lint in
656 2. The configuration itself will be tested separately in [`tests/ui-toml`].
657 Simply add a new subfolder with a fitting name. This folder contains a
658 `clippy.toml` file with the configuration value and a rust file that
659 should be linted by Clippy. The test can otherwise be written as usual.
661 [`clippy_lints::utils::conf`]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/conf.rs
662 [`clippy_lints` lib file]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/lib.rs
663 [`tests/ui`]: https://github.com/rust-lang/rust-clippy/blob/master/tests/ui
664 [`tests/ui-toml`]: https://github.com/rust-lang/rust-clippy/blob/master/tests/ui-toml
668 Here are some pointers to things you are likely going to need for every lint:
670 * [Clippy utils][utils] - Various helper functions. Maybe the function you need
671 is already in here ([`is_type_diagnostic_item`], [`implements_trait`],
673 * [Clippy diagnostics][diagnostics]
674 * [Let chains][let-chains]
675 * [`from_expansion`][from_expansion] and
676 [`in_external_macro`][in_external_macro]
678 * [`Applicability`][applicability]
679 * [Common tools for writing lints](common_tools_writing_lints.md) helps with
681 * [The rustc-dev-guide][rustc-dev-guide] explains a lot of internal compiler
683 * [The nightly rustc docs][nightly_docs] which has been linked to throughout
686 For `EarlyLintPass` lints:
688 * [`EarlyLintPass`][early_lint_pass]
689 * [`rustc_ast::ast`][ast]
691 For `LateLintPass` lints:
693 * [`LateLintPass`][late_lint_pass]
696 While most of Clippy's lint utils are documented, most of rustc's internals lack
697 documentation currently. This is unfortunate, but in most cases you can probably
698 get away with copying things from existing similar lints. If you are stuck,
699 don't hesitate to ask on [Zulip] or in the issue/PR.
701 [utils]: https://doc.rust-lang.org/nightly/nightly-rustc/clippy_utils/index.html
702 [`is_type_diagnostic_item`]: https://doc.rust-lang.org/nightly/nightly-rustc/clippy_utils/ty/fn.is_type_diagnostic_item.html
703 [`implements_trait`]: https://doc.rust-lang.org/nightly/nightly-rustc/clippy_utils/ty/fn.implements_trait.html
704 [`snippet`]: https://doc.rust-lang.org/nightly/nightly-rustc/clippy_utils/source/fn.snippet.html
705 [let-chains]: https://github.com/rust-lang/rust/pull/94927
706 [from_expansion]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html#method.from_expansion
707 [in_external_macro]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/lint/fn.in_external_macro.html
708 [span]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html
709 [applicability]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/enum.Applicability.html
710 [rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/
711 [nightly_docs]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/
712 [ast]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/index.html
713 [ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/sty/index.html
714 [Zulip]: https://rust-lang.zulipchat.com/#narrow/stream/clippy