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 cargo dev lint 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?
192 #[clippy::version = "1.29.0"]
195 "function named `foo`, which is not a descriptive name"
199 * The section of lines prefixed with `///` constitutes the lint documentation
200 section. This is the default documentation style and will be displayed
201 [like this][example_lint_page]. To render and open this documentation locally
202 in a browser, run `cargo dev serve`.
203 * The `#[clippy::version]` attribute will be rendered as part of the lint documentation.
204 The value should be set to the current Rust version that the lint is developed in,
205 it can be retrieved by running `rustc -vV` in the rust-clippy directory. The version
206 is listed under *release*. (Use the version without the `-nightly`) suffix.
207 * `FOO_FUNCTIONS` is the name of our lint. Be sure to follow the
208 [lint naming guidelines][lint_naming] here when naming your lint.
209 In short, the name should state the thing that is being checked for and
210 read well when used with `allow`/`warn`/`deny`.
211 * `pedantic` sets the lint level to `Allow`.
212 The exact mapping can be found [here][category_level_mapping]
213 * The last part should be a text that explains what exactly is wrong with the
216 The rest of this file contains an empty implementation for our lint pass,
217 which in this case is `EarlyLintPass` and should look like this:
220 // clippy_lints/src/foo_functions.rs
222 // .. imports and lint declaration ..
224 declare_lint_pass!(FooFunctions => [FOO_FUNCTIONS]);
226 impl EarlyLintPass for FooFunctions {}
229 [declare_clippy_lint]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L60
230 [example_lint_page]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure
231 [lint_naming]: https://rust-lang.github.io/rfcs/0344-conventions-galore.html#lints
232 [category_level_mapping]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L110
236 When using `cargo dev new_lint`, the lint is automatically registered and
237 nothing more has to be done.
239 When declaring a new lint by hand and `cargo dev update_lints` is used, the lint
240 pass may have to be registered manually in the `register_plugins` function in
241 `clippy_lints/src/lib.rs`:
244 store.register_early_pass(|| Box::new(foo_functions::FooFunctions));
247 As one may expect, there is a corresponding `register_late_pass` method
248 available as well. Without a call to one of `register_early_pass` or
249 `register_late_pass`, the lint pass in question will not be run.
251 One reason that `cargo dev update_lints` does not automate this step is that
252 multiple lints can use the same lint pass, so registering the lint pass may
253 already be done when adding a new lint. Another reason that this step is not
254 automated is that the order that the passes are registered determines the order
255 the passes actually run, which in turn affects the order that any emitted lints
260 Writing a lint that only checks for the name of a function means that we only
261 have to deal with the AST and don't have to deal with the type system at all.
262 This is good, because it makes writing this particular lint less complicated.
264 We have to make this decision with every new Clippy lint. It boils down to using
265 either [`EarlyLintPass`][early_lint_pass] or [`LateLintPass`][late_lint_pass].
267 In short, the `LateLintPass` has access to type information while the
268 `EarlyLintPass` doesn't. If you don't need access to type information, use the
269 `EarlyLintPass`. The `EarlyLintPass` is also faster. However linting speed
270 hasn't really been a concern with Clippy so far.
272 Since we don't need type information for checking the function name, we used
273 `--pass=early` when running the new lint automation and all the imports were
276 [early_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html
277 [late_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.LateLintPass.html
281 With UI tests and the lint declaration in place, we can start working on the
282 implementation of the lint logic.
284 Let's start by implementing the `EarlyLintPass` for our `FooFunctions`:
287 impl EarlyLintPass for FooFunctions {
288 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
289 // TODO: Emit lint here
294 We implement the [`check_fn`][check_fn] method from the
295 [`EarlyLintPass`][early_lint_pass] trait. This gives us access to various
296 information about the function that is currently being checked. More on that in
297 the next section. Let's worry about the details later and emit our lint for
298 *every* function definition first.
300 Depending on how complex we want our lint message to be, we can choose from a
301 variety of lint emission functions. They can all be found in
302 [`clippy_utils/src/diagnostics.rs`][diagnostics].
304 `span_lint_and_help` seems most appropriate in this case. It allows us to
305 provide an extra help message and we can't really suggest a better name
306 automatically. This is how it looks:
309 impl EarlyLintPass for FooFunctions {
310 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
315 "function named `foo`",
317 "consider using a more meaningful name"
323 Running our UI test should now produce output that contains the lint message.
325 According to [the rustc-dev-guide], the text should be matter of fact and avoid
326 capitalization and periods, unless multiple sentences are needed.
327 When code or an identifier must appear in a message or label, it should be
328 surrounded with single grave accents \`.
330 [check_fn]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html#method.check_fn
331 [diagnostics]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_utils/src/diagnostics.rs
332 [the rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/diagnostics.html
334 ## Adding the lint logic
336 Writing the logic for your lint will most likely be different from our example,
337 so this section is kept rather short.
339 Using the [`check_fn`][check_fn] method gives us access to [`FnKind`][fn_kind]
340 that has the [`FnKind::Fn`] variant. It provides access to the name of the
341 function/method via an [`Ident`][ident].
343 With that we can expand our `check_fn` method to:
346 impl EarlyLintPass for FooFunctions {
347 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
348 if is_foo_fn(fn_kind) {
353 "function named `foo`",
355 "consider using a more meaningful name"
362 We separate the lint conditional from the lint emissions because it makes the
363 code a bit easier to read. In some cases this separation would also allow to
364 write some unit tests (as opposed to only UI tests) for the separate function.
366 In our example, `is_foo_fn` looks like:
369 // use statements, impl EarlyLintPass, check_fn, ..
371 fn is_foo_fn(fn_kind: FnKind<'_>) -> bool {
373 FnKind::Fn(_, ident, ..) => {
374 // check if `fn` name is `foo`
375 ident.name.as_str() == "foo"
378 FnKind::Closure(..) => false
383 Now we should also run the full test suite with `cargo test`. At this point
384 running `cargo test` should produce the expected output. Remember to run
385 `cargo dev bless` to update the `.stderr` file.
387 `cargo test` (as opposed to `cargo uitest`) will also ensure that our lint
388 implementation is not violating any Clippy lints itself.
390 That should be it for the lint implementation. Running `cargo test` should now
393 [fn_kind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html
394 [`FnKind::Fn`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html#variant.Fn
395 [ident]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Ident.html
397 ## Specifying the lint's minimum supported Rust version (MSRV)
399 Sometimes a lint makes suggestions that require a certain version of Rust. For example, the `manual_strip` lint suggests
400 using `str::strip_prefix` and `str::strip_suffix` which is only available after Rust 1.45. In such cases, you need to
401 ensure that the MSRV configured for the project is >= the MSRV of the required Rust feature. If multiple features are
402 required, just use the one with a lower MSRV.
404 First, add an MSRV alias for the required feature in [`clippy_utils::msrvs`](/clippy_utils/src/msrvs.rs). This can be
405 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 in the LintPass struct, and a
415 constructor to initialize the field. The `msrv` value is passed to the constructor in `clippy_lints/lib.rs`.
418 pub struct ManualStrip {
419 msrv: Option<RustcVersion>,
424 pub fn new(msrv: Option<RustcVersion>) -> Self {
430 The project's MSRV can then be matched against the feature MSRV in the LintPass
431 using the `meets_msrv` utility function.
434 if !meets_msrv(self.msrv.as_ref(), &msrvs::STR_STRIP_PREFIX) {
439 The project's MSRV can also be specified as an inner attribute, which overrides
440 the value from `clippy.toml`. This can be accounted for using the
441 `extract_msrv_attr!(LintContext)` macro and passing
442 `LateContext`/`EarlyContext`.
445 impl<'tcx> LateLintPass<'tcx> for ManualStrip {
446 fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
449 extract_msrv_attr!(LateContext);
453 Once the `msrv` is added to the lint, a relevant test case should be added to
454 `tests/ui/min_rust_version_attr.rs` which verifies that the lint isn't emitted
455 if the project's MSRV is lower.
457 As a last step, the lint should be added to the lint documentation. This is done
458 in `clippy_lints/src/utils/conf.rs`:
462 /// Lint: LIST, OF, LINTS, <THE_NEWLY_ADDED_LINT>. The minimum rust version that the project supports
463 (msrv: Option<String> = None),
470 If you have trouble implementing your lint, there is also the internal `author`
471 lint to generate Clippy code that detects the offending pattern. It does not
472 work for all of the Rust syntax, but can give a good starting point.
474 The quickest way to use it, is the
475 [Rust playground: play.rust-lang.org][author_example].
476 Put the code you want to lint into the editor and add the `#[clippy::author]`
477 attribute above the item. Then run Clippy via `Tools -> Clippy` and you should
478 see the generated code in the output below.
480 [Here][author_example] is an example on the playground.
482 If the command was executed successfully, you can copy the code over to where
483 you are implementing your lint.
485 [author_example]: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=9a12cb60e5c6ad4e3003ac6d5e63cf55
489 The final thing before submitting our PR is to add some documentation to our
492 Please document your lint with a doc comment akin to the following:
495 declare_clippy_lint! {
497 /// Checks for ... (describe what the lint matches).
499 /// ### Why is this bad?
500 /// Supply the reason for linting the code.
506 /// Insert a short example of code that triggers the lint
509 /// Insert a short example of improved code that doesn't trigger the lint
511 #[clippy::version = "1.29.0"]
514 "function named `foo`, which is not a descriptive name"
518 Once your lint is merged, this documentation will show up in the [lint
521 [lint_list]: https://rust-lang.github.io/rust-clippy/master/index.html
525 [Rustfmt] is a tool for formatting Rust code according to style guidelines.
526 Your code has to be formatted by `rustfmt` before a PR can be merged.
527 Clippy uses nightly `rustfmt` in the CI.
529 It can be installed via `rustup`:
532 rustup component add rustfmt --toolchain=nightly
535 Use `cargo dev fmt` to format the whole codebase. Make sure that `rustfmt` is
536 installed for the nightly toolchain.
538 [Rustfmt]: https://github.com/rust-lang/rustfmt
542 If you want to debug parts of your lint implementation, you can use the [`dbg!`]
543 macro anywhere in your code. Running the tests should then include the debug
544 output in the `stdout` part.
546 [`dbg!`]: https://doc.rust-lang.org/std/macro.dbg.html
550 Before submitting your PR make sure you followed all of the basic requirements:
552 <!-- Sync this with `.github/PULL_REQUEST_TEMPLATE` -->
554 - \[ ] Followed [lint naming conventions][lint_naming]
555 - \[ ] Added passing UI tests (including committed `.stderr` file)
556 - \[ ] `cargo test` passes locally
557 - \[ ] Executed `cargo dev update_lints`
558 - \[ ] Added lint documentation
559 - \[ ] Run `cargo dev fmt`
561 ## Adding configuration to a lint
563 Clippy supports the configuration of lints values using a `clippy.toml` file in the workspace
564 directory. Adding a configuration to a lint can be useful for thresholds or to constrain some
565 behavior that can be seen as a false positive for some users. Adding a configuration is done
566 in the following steps:
568 1. Adding a new configuration entry to [clippy_lints::utils::conf](/clippy_lints/src/utils/conf.rs)
573 /// <The configuration field doc comment>
574 (configuration_ident: Type = DefaultValue),
576 The doc comment is automatically added to the documentation of the listed lints. The default
577 value will be formatted using the `Debug` implementation of the type.
578 2. Adding the configuration value to the lint impl struct:
579 1. This first requires the definition of a lint impl struct. Lint impl structs are usually
580 generated with the `declare_lint_pass!` macro. This struct needs to be defined manually
581 to add some kind of metadata to it:
583 // Generated struct definition
584 declare_lint_pass!(StructName => [
588 // New manual definition struct
589 #[derive(Copy, Clone)]
590 pub struct StructName {}
592 impl_lint_pass!(StructName => [
597 2. Next add the configuration value and a corresponding creation method like this:
599 #[derive(Copy, Clone)]
600 pub struct StructName {
601 configuration_ident: Type,
607 pub fn new(configuration_ident: Type) -> Self {
614 3. Passing the configuration value to the lint impl struct:
616 First find the struct construction in the [clippy_lints lib file](/clippy_lints/src/lib.rs).
617 The configuration value is now cloned or copied into a local value that is then passed to the
618 impl struct like this:
620 // Default generated registration:
621 store.register_*_pass(|| box module::StructName);
623 // New registration with configuration value
624 let configuration_ident = conf.configuration_ident.clone();
625 store.register_*_pass(move || box module::StructName::new(configuration_ident));
628 Congratulations the work is almost done. The configuration value can now be accessed
629 in the linting code via `self.configuration_ident`.
632 1. The default configured value can be tested like any normal lint in [`tests/ui`](/tests/ui).
633 2. The configuration itself will be tested separately in [`tests/ui-toml`](/tests/ui-toml).
634 Simply add a new subfolder with a fitting name. This folder contains a `clippy.toml` file
635 with the configuration value and a rust file that should be linted by Clippy. The test can
636 otherwise be written as usual.
640 Here are some pointers to things you are likely going to need for every lint:
642 * [Clippy utils][utils] - Various helper functions. Maybe the function you need
643 is already in here ([`is_type_diagnostic_item`], [`implements_trait`], [`snippet`], etc)
644 * [Clippy diagnostics][diagnostics]
645 * [The `if_chain` macro][if_chain]
646 * [`from_expansion`][from_expansion] and [`in_external_macro`][in_external_macro]
648 * [`Applicability`][applicability]
649 * [Common tools for writing lints](common_tools_writing_lints.md) helps with common operations
650 * [The rustc-dev-guide][rustc-dev-guide] explains a lot of internal compiler concepts
651 * [The nightly rustc docs][nightly_docs] which has been linked to throughout
654 For `EarlyLintPass` lints:
656 * [`EarlyLintPass`][early_lint_pass]
657 * [`rustc_ast::ast`][ast]
659 For `LateLintPass` lints:
661 * [`LateLintPass`][late_lint_pass]
664 While most of Clippy's lint utils are documented, most of rustc's internals lack
665 documentation currently. This is unfortunate, but in most cases you can probably
666 get away with copying things from existing similar lints. If you are stuck,
667 don't hesitate to ask on [Zulip] or in the issue/PR.
669 [utils]: https://doc.rust-lang.org/nightly/nightly-rustc/clippy_utils/index.html
670 [`is_type_diagnostic_item`]: https://doc.rust-lang.org/nightly/nightly-rustc/clippy_utils/ty/fn.is_type_diagnostic_item.html
671 [`implements_trait`]: https://doc.rust-lang.org/nightly/nightly-rustc/clippy_utils/ty/fn.implements_trait.html
672 [`snippet`]: https://doc.rust-lang.org/nightly/nightly-rustc/clippy_utils/source/fn.snippet.html
673 [if_chain]: https://docs.rs/if_chain/*/if_chain/
674 [from_expansion]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html#method.from_expansion
675 [in_external_macro]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/lint/fn.in_external_macro.html
676 [span]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html
677 [applicability]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/enum.Applicability.html
678 [rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/
679 [nightly_docs]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/
680 [ast]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/index.html
681 [ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/sty/index.html
682 [Zulip]: https://rust-lang.zulipchat.com/#narrow/stream/clippy