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 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 - [Documentation](#documentation)
25 - [Running rustfmt](#running-rustfmt)
26 - [Debugging](#debugging)
27 - [PR Checklist](#pr-checklist)
28 - [Adding configuration to a lint](#adding-configuration-to-a-lint)
29 - [Cheatsheet](#cheatsheet)
33 See the [Basics](basics.md#get-the-code) documentation.
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). If you're not sure if the name you chose fits the lint,
42 take a look at our [lint naming guidelines][lint_naming]. To get started on this
43 lint you can run `cargo dev new_lint --name=foo_functions --pass=early
44 --category=pedantic` (category will default to nursery if not provided). This
45 command will create two files: `tests/ui/foo_functions.rs` and
46 `clippy_lints/src/foo_functions.rs`, as well as run `cargo dev update_lints` to
47 register the new lint. For cargo lints, two project hierarchies (fail/pass) will
48 be created by default under `tests/ui-cargo`.
50 Next, we'll open up these files and add our lint!
54 Let's write some tests first that we can execute while we iterate on our lint.
56 Clippy uses UI tests for testing. UI tests check that the output of Clippy is
57 exactly as expected. Each test is just a plain Rust file that contains the code
58 we want to check. The output of Clippy is compared against a `.stderr` file.
59 Note that you don't have to create this file yourself, we'll get to
60 generating the `.stderr` files further down.
62 We start by opening the test file created at `tests/ui/foo_functions.rs`.
64 Update the file with some examples to get started:
67 #![warn(clippy::foo_functions)]
77 // Default trait methods
90 // We also don't want to lint method calls
97 Now we can run the test with `TESTNAME=foo_functions cargo uitest`,
98 currently this test is meaningless though.
100 While we are working on implementing our lint, we can keep running the UI
101 test. That allows us to check if the output is turning into what we want.
103 Once we are satisfied with the output, we need to run
104 `cargo dev bless` to update the `.stderr` file for our lint.
105 Please note that, we should run `TESTNAME=foo_functions cargo uitest`
106 every time before running `cargo dev bless`.
107 Running `TESTNAME=foo_functions cargo uitest` should pass then. When we commit
108 our lint, we need to commit the generated `.stderr` files, too. In general, you
109 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 that 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
119 the `Cargo.toml` manifest file. We also need a minimal crate associated
122 If our new lint is named e.g. `foo_categories`, after running `cargo dev new_lint`
123 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 new lint to raise an error.
126 * `tests/ui-cargo/foo_categories/pass/Cargo.toml`: this file should not trigger the lint.
128 If you need more cases, you can copy one of those crates (under `foo_categories`) and rename it.
130 The process of generating the `.stderr` file is the same, and prepending the `TESTNAME`
131 variable to `cargo uitest` works too.
135 If the lint you are working on is making use of structured suggestions, the
136 test file should include a `// run-rustfix` comment at the top. This will
137 additionally run [rustfix] for that test. Rustfix will apply the suggestions
138 from the lint to the code of the test file and compare that to the contents of
141 Use `cargo dev bless` to automatically generate the
142 `.fixed` file after running the tests.
144 [rustfix]: https://github.com/rust-lang/rustfix
146 ## Edition 2018 tests
148 Some features require the 2018 edition to work (e.g. `async_await`), but
149 compile-test tests run on the 2015 edition by default. To change this behavior
150 add `// edition:2018` at the top of the test file (note that it's space-sensitive).
154 Manually testing against an example file can be useful if you have added some
155 `println!`s and the test suite output becomes unreadable. To try Clippy with
156 your local modifications, run
159 env __CLIPPY_INTERNAL_TESTS=true cargo run --bin clippy-driver -- -L ./target/debug input.rs
162 from the working copy root. With tests in place, let's have a look at
163 implementing our lint now.
167 Let's start by opening the new file created in the `clippy_lints` crate
168 at `clippy_lints/src/foo_functions.rs`. That's the crate where all the
169 lint code is. This file has already imported some initial things we will need:
172 use rustc_lint::{EarlyLintPass, EarlyContext};
173 use rustc_session::{declare_lint_pass, declare_tool_lint};
174 use rustc_ast::ast::*;
177 The next step is to update the lint declaration. Lints are declared using the
178 [`declare_clippy_lint!`][declare_clippy_lint] macro, and we just need to update
179 the auto-generated lint declaration to have a real description, something like this:
182 declare_clippy_lint! {
185 /// ### Why is this bad?
193 "function named `foo`, which is not a descriptive name"
197 * The section of lines prefixed with `///` constitutes the lint documentation
198 section. This is the default documentation style and will be displayed
199 [like this][example_lint_page]. To render and open this documentation locally
200 in a browser, run `cargo dev serve`.
201 * `FOO_FUNCTIONS` is the name of our lint. Be sure to follow the
202 [lint naming guidelines][lint_naming] here when naming your lint.
203 In short, the name should state the thing that is being checked for and
204 read well when used with `allow`/`warn`/`deny`.
205 * `pedantic` sets the lint level to `Allow`.
206 The exact mapping can be found [here][category_level_mapping]
207 * The last part should be a text that explains what exactly is wrong with the
210 The rest of this file contains an empty implementation for our lint pass,
211 which in this case is `EarlyLintPass` and should look like this:
214 // clippy_lints/src/foo_functions.rs
216 // .. imports and lint declaration ..
218 declare_lint_pass!(FooFunctions => [FOO_FUNCTIONS]);
220 impl EarlyLintPass for FooFunctions {}
223 Normally after declaring the lint, we have to run `cargo dev update_lints`,
224 which updates some files, so Clippy knows about the new lint. Since we used
225 `cargo dev new_lint ...` to generate the lint declaration, this was done
226 automatically. While `update_lints` automates most of the things, it doesn't
227 automate everything. We will have to register our lint pass manually in the
228 `register_plugins` function in `clippy_lints/src/lib.rs`:
231 store.register_early_pass(|| box foo_functions::FooFunctions);
234 As one may expect, there is a corresponding `register_late_pass` method
235 available as well. Without a call to one of `register_early_pass` or
236 `register_late_pass`, the lint pass in question will not be run.
238 One reason that `cargo dev` does not automate this step is that multiple lints
239 can use the same lint pass, so registering the lint pass may already be done
240 when adding a new lint. Another reason that this step is not automated is that
241 the order that the passes are registered determines the order the passes
242 actually run, which in turn affects the order that any emitted lints are output
245 [declare_clippy_lint]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L60
246 [example_lint_page]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure
247 [lint_naming]: https://rust-lang.github.io/rfcs/0344-conventions-galore.html#lints
248 [category_level_mapping]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L110
252 Writing a lint that only checks for the name of a function means that we only
253 have to deal with the AST and don't have to deal with the type system at all.
254 This is good, because it makes writing this particular lint less complicated.
256 We have to make this decision with every new Clippy lint. It boils down to using
257 either [`EarlyLintPass`][early_lint_pass] or [`LateLintPass`][late_lint_pass].
259 In short, the `LateLintPass` has access to type information while the
260 `EarlyLintPass` doesn't. If you don't need access to type information, use the
261 `EarlyLintPass`. The `EarlyLintPass` is also faster. However linting speed
262 hasn't really been a concern with Clippy so far.
264 Since we don't need type information for checking the function name, we used
265 `--pass=early` when running the new lint automation and all the imports were
268 [early_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html
269 [late_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.LateLintPass.html
273 With UI tests and the lint declaration in place, we can start working on the
274 implementation of the lint logic.
276 Let's start by implementing the `EarlyLintPass` for our `FooFunctions`:
279 impl EarlyLintPass for FooFunctions {
280 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
281 // TODO: Emit lint here
286 We implement the [`check_fn`][check_fn] method from the
287 [`EarlyLintPass`][early_lint_pass] trait. This gives us access to various
288 information about the function that is currently being checked. More on that in
289 the next section. Let's worry about the details later and emit our lint for
290 *every* function definition first.
292 Depending on how complex we want our lint message to be, we can choose from a
293 variety of lint emission functions. They can all be found in
294 [`clippy_utils/src/diagnostics.rs`][diagnostics].
296 `span_lint_and_help` seems most appropriate in this case. It allows us to
297 provide an extra help message and we can't really suggest a better name
298 automatically. This is how it looks:
301 impl EarlyLintPass for FooFunctions {
302 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
307 "function named `foo`",
309 "consider using a more meaningful name"
315 Running our UI test should now produce output that contains the lint message.
317 According to [the rustc-dev-guide], the text should be matter of fact and avoid
318 capitalization and periods, unless multiple sentences are needed.
319 When code or an identifier must appear in a message or label, it should be
320 surrounded with single grave accents \`.
322 [check_fn]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html#method.check_fn
323 [diagnostics]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_utils/src/diagnostics.rs
324 [the rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/diagnostics.html
326 ## Adding the lint logic
328 Writing the logic for your lint will most likely be different from our example,
329 so this section is kept rather short.
331 Using the [`check_fn`][check_fn] method gives us access to [`FnKind`][fn_kind]
332 that has the [`FnKind::Fn`] variant. It provides access to the name of the
333 function/method via an [`Ident`][ident].
335 With that we can expand our `check_fn` method to:
338 impl EarlyLintPass for FooFunctions {
339 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
340 if is_foo_fn(fn_kind) {
345 "function named `foo`",
347 "consider using a more meaningful name"
354 We separate the lint conditional from the lint emissions because it makes the
355 code a bit easier to read. In some cases this separation would also allow to
356 write some unit tests (as opposed to only UI tests) for the separate function.
358 In our example, `is_foo_fn` looks like:
361 // use statements, impl EarlyLintPass, check_fn, ..
363 fn is_foo_fn(fn_kind: FnKind<'_>) -> bool {
365 FnKind::Fn(_, ident, ..) => {
366 // check if `fn` name is `foo`
367 ident.name.as_str() == "foo"
370 FnKind::Closure(..) => false
375 Now we should also run the full test suite with `cargo test`. At this point
376 running `cargo test` should produce the expected output. Remember to run
377 `cargo dev bless` to update the `.stderr` file.
379 `cargo test` (as opposed to `cargo uitest`) will also ensure that our lint
380 implementation is not violating any Clippy lints itself.
382 That should be it for the lint implementation. Running `cargo test` should now
385 [fn_kind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html
386 [`FnKind::Fn`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html#variant.Fn
387 [ident]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Ident.html
389 ## Specifying the lint's minimum supported Rust version (MSRV)
391 Sometimes a lint makes suggestions that require a certain version of Rust. For example, the `manual_strip` lint suggests
392 using `str::strip_prefix` and `str::strip_suffix` which is only available after Rust 1.45. In such cases, you need to
393 ensure that the MSRV configured for the project is >= the MSRV of the required Rust feature. If multiple features are
394 required, just use the one with a lower MSRV.
396 First, add an MSRV alias for the required feature in [`clippy_utils::msrvs`](/clippy_utils/src/msrvs.rs). This can be
397 accessed later as `msrvs::STR_STRIP_PREFIX`, for example.
402 1,45,0 { STR_STRIP_PREFIX }
406 In order to access the project-configured MSRV, you need to have an `msrv` field in the LintPass struct, and a
407 constructor to initialize the field. The `msrv` value is passed to the constructor in `clippy_lints/lib.rs`.
410 pub struct ManualStrip {
411 msrv: Option<RustcVersion>,
416 pub fn new(msrv: Option<RustcVersion>) -> Self {
422 The project's MSRV can then be matched against the feature MSRV in the LintPass
423 using the `meets_msrv` utility function.
426 if !meets_msrv(self.msrv.as_ref(), &msrvs::STR_STRIP_PREFIX) {
431 The project's MSRV can also be specified as an inner attribute, which overrides
432 the value from `clippy.toml`. This can be accounted for using the
433 `extract_msrv_attr!(LintContext)` macro and passing
434 `LateContext`/`EarlyContext`.
437 impl<'tcx> LateLintPass<'tcx> for ManualStrip {
438 fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
441 extract_msrv_attr!(LateContext);
445 Once the `msrv` is added to the lint, a relevant test case should be added to
446 `tests/ui/min_rust_version_attr.rs` which verifies that the lint isn't emitted
447 if the project's MSRV is lower.
449 As a last step, the lint should be added to the lint documentation. This is done
450 in `clippy_lints/src/utils/conf.rs`:
454 /// Lint: LIST, OF, LINTS, <THE_NEWLY_ADDED_LINT>. The minimum rust version that the project supports
455 (msrv: Option<String> = None),
462 If you have trouble implementing your lint, there is also the internal `author`
463 lint to generate Clippy code that detects the offending pattern. It does not
464 work for all of the Rust syntax, but can give a good starting point.
466 The quickest way to use it, is the
467 [Rust playground: play.rust-lang.org][author_example].
468 Put the code you want to lint into the editor and add the `#[clippy::author]`
469 attribute above the item. Then run Clippy via `Tools -> Clippy` and you should
470 see the generated code in the output below.
472 [Here][author_example] is an example on the playground.
474 If the command was executed successfully, you can copy the code over to where
475 you are implementing your lint.
477 [author_example]: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=9a12cb60e5c6ad4e3003ac6d5e63cf55
481 The final thing before submitting our PR is to add some documentation to our
484 Please document your lint with a doc comment akin to the following:
487 declare_clippy_lint! {
489 /// Checks for ... (describe what the lint matches).
491 /// ### Why is this bad?
492 /// Supply the reason for linting the code.
498 /// Insert a short example of code that triggers the lint
501 /// Insert a short example of improved code that doesn't trigger the lint
505 "function named `foo`, which is not a descriptive name"
509 Once your lint is merged, this documentation will show up in the [lint
512 [lint_list]: https://rust-lang.github.io/rust-clippy/master/index.html
516 [Rustfmt] is a tool for formatting Rust code according to style guidelines.
517 Your code has to be formatted by `rustfmt` before a PR can be merged.
518 Clippy uses nightly `rustfmt` in the CI.
520 It can be installed via `rustup`:
523 rustup component add rustfmt --toolchain=nightly
526 Use `cargo dev fmt` to format the whole codebase. Make sure that `rustfmt` is
527 installed for the nightly toolchain.
529 [Rustfmt]: https://github.com/rust-lang/rustfmt
533 If you want to debug parts of your lint implementation, you can use the [`dbg!`]
534 macro anywhere in your code. Running the tests should then include the debug
535 output in the `stdout` part.
537 [`dbg!`]: https://doc.rust-lang.org/std/macro.dbg.html
541 Before submitting your PR make sure you followed all of the basic requirements:
543 <!-- Sync this with `.github/PULL_REQUEST_TEMPLATE` -->
545 - \[ ] Followed [lint naming conventions][lint_naming]
546 - \[ ] Added passing UI tests (including committed `.stderr` file)
547 - \[ ] `cargo test` passes locally
548 - \[ ] Executed `cargo dev update_lints`
549 - \[ ] Added lint documentation
550 - \[ ] Run `cargo dev fmt`
552 ## Adding configuration to a lint
554 Clippy supports the configuration of lints values using a `clippy.toml` file in the workspace
555 directory. Adding a configuration to a lint can be useful for thresholds or to constrain some
556 behavior that can be seen as a false positive for some users. Adding a configuration is done
557 in the following steps:
559 1. Adding a new configuration entry to [clippy_lints::utils::conf](/clippy_lints/src/utils/conf.rs)
564 /// <The configuration field doc comment>
565 (configuration_ident: Type = DefaultValue),
567 The doc comment will be automatically added to the lint documentation.
568 2. Adding the configuration value to the lint impl struct:
569 1. This first requires the definition of a lint impl struct. Lint impl structs are usually
570 generated with the `declare_lint_pass!` macro. This struct needs to be defined manually
571 to add some kind of metadata to it:
573 // Generated struct definition
574 declare_lint_pass!(StructName => [
578 // New manual definition struct
579 #[derive(Copy, Clone)]
580 pub struct StructName {}
582 impl_lint_pass!(StructName => [
587 2. Next add the configuration value and a corresponding creation method like this:
589 #[derive(Copy, Clone)]
590 pub struct StructName {
591 configuration_ident: Type,
597 pub fn new(configuration_ident: Type) -> Self {
604 3. Passing the configuration value to the lint impl struct:
606 First find the struct construction in the [clippy_lints lib file](/clippy_lints/src/lib.rs).
607 The configuration value is now cloned or copied into a local value that is then passed to the
608 impl struct like this:
610 // Default generated registration:
611 store.register_*_pass(|| box module::StructName);
613 // New registration with configuration value
614 let configuration_ident = conf.configuration_ident.clone();
615 store.register_*_pass(move || box module::StructName::new(configuration_ident));
618 Congratulations the work is almost done. The configuration value can now be accessed
619 in the linting code via `self.configuration_ident`.
622 1. The default configured value can be tested like any normal lint in [`tests/ui`](/tests/ui).
623 2. The configuration itself will be tested separately in [`tests/ui-toml`](/tests/ui-toml).
624 Simply add a new subfolder with a fitting name. This folder contains a `clippy.toml` file
625 with the configuration value and a rust file that should be linted by Clippy. The test can
626 otherwise be written as usual.
630 Here are some pointers to things you are likely going to need for every lint:
632 * [Clippy utils][utils] - Various helper functions. Maybe the function you need
633 is already in here (`implements_trait`, `match_def_path`, `snippet`, etc)
634 * [Clippy diagnostics][diagnostics]
635 * [The `if_chain` macro][if_chain]
636 * [`from_expansion`][from_expansion] and [`in_external_macro`][in_external_macro]
638 * [`Applicability`][applicability]
639 * [Common tools for writing lints](common_tools_writing_lints.md) helps with common operations
640 * [The rustc-dev-guide][rustc-dev-guide] explains a lot of internal compiler concepts
641 * [The nightly rustc docs][nightly_docs] which has been linked to throughout
644 For `EarlyLintPass` lints:
646 * [`EarlyLintPass`][early_lint_pass]
647 * [`rustc_ast::ast`][ast]
649 For `LateLintPass` lints:
651 * [`LateLintPass`][late_lint_pass]
654 While most of Clippy's lint utils are documented, most of rustc's internals lack
655 documentation currently. This is unfortunate, but in most cases you can probably
656 get away with copying things from existing similar lints. If you are stuck,
657 don't hesitate to ask on [Zulip] or in the issue/PR.
659 [utils]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_utils/src/lib.rs
660 [if_chain]: https://docs.rs/if_chain/*/if_chain/
661 [from_expansion]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html#method.from_expansion
662 [in_external_macro]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/lint/fn.in_external_macro.html
663 [span]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html
664 [applicability]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/enum.Applicability.html
665 [rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/
666 [nightly_docs]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/
667 [ast]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/index.html
668 [ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/sty/index.html
669 [Zulip]: https://rust-lang.zulipchat.com/#narrow/stream/clippy