3 You are probably here because you want to add a new lint to Clippy. If this is
4 the first time you're contributing to Clippy, this document guides you through
5 creating an example lint from scratch.
7 To get started, we will create a lint that detects functions called `foo`,
8 because that's clearly a non-descriptive name.
10 - [Adding a new lint](#adding-a-new-lint)
12 - [Getting Started](#getting-started)
14 - [Rustfix tests](#rustfix-tests)
15 - [Edition 2018 tests](#edition-2018-tests)
16 - [Testing manually](#testing-manually)
17 - [Lint declaration](#lint-declaration)
18 - [Lint passes](#lint-passes)
19 - [Emitting a lint](#emitting-a-lint)
20 - [Adding the lint logic](#adding-the-lint-logic)
21 - [Author lint](#author-lint)
22 - [Documentation](#documentation)
23 - [Running rustfmt](#running-rustfmt)
24 - [Debugging](#debugging)
25 - [PR Checklist](#pr-checklist)
26 - [Adding configuration to a lint](#adding-configuration-to-a-lint)
27 - [Cheatsheet](#cheatsheet)
31 See the [Basics](basics.md#get-the-code) documentation.
35 There is a bit of boilerplate code that needs to be set up when creating a new
36 lint. Fortunately, you can use the clippy dev tools to handle this for you. We
37 are naming our new lint `foo_functions` (lints are generally written in snake
38 case), and we don't need type information so it will have an early pass type
39 (more on this later on). If you're not sure if the name you chose fits the lint,
40 take a look at our [lint naming guidelines][lint_naming]. To get started on this
41 lint you can run `cargo dev new_lint --name=foo_functions --pass=early
42 --category=pedantic` (category will default to nursery if not provided). This
43 command will create two files: `tests/ui/foo_functions.rs` and
44 `clippy_lints/src/foo_functions.rs`, as well as run `cargo dev update_lints` to
45 register the new lint. For cargo lints, two project hierarchies (fail/pass) will
46 be created by default under `tests/ui-cargo`.
48 Next, we'll open up these files and add our lint!
52 Let's write some tests first that we can execute while we iterate on our lint.
54 Clippy uses UI tests for testing. UI tests check that the output of Clippy is
55 exactly as expected. Each test is just a plain Rust file that contains the code
56 we want to check. The output of Clippy is compared against a `.stderr` file.
57 Note that you don't have to create this file yourself, we'll get to
58 generating the `.stderr` files further down.
60 We start by opening the test file created at `tests/ui/foo_functions.rs`.
62 Update the file with some examples to get started:
65 #![warn(clippy::foo_functions)]
75 // Default trait methods
88 // We also don't want to lint method calls
95 Now we can run the test with `TESTNAME=foo_functions cargo uitest`,
96 currently this test is meaningless though.
98 While we are working on implementing our lint, we can keep running the UI
99 test. That allows us to check if the output is turning into what we want.
101 Once we are satisfied with the output, we need to run
102 `cargo dev bless` to update the `.stderr` file for our lint.
103 Please note that, we should run `TESTNAME=foo_functions cargo uitest`
104 every time before running `cargo dev bless`.
105 Running `TESTNAME=foo_functions cargo uitest` should pass then. When we commit
106 our lint, we need to commit the generated `.stderr` files, too. In general, you
107 should only commit files changed by `cargo dev bless` for the
108 specific lint you are creating/editing. Note that if the generated files are
109 empty, they should be removed.
113 For cargo lints, the process of testing differs in that we are interested in
114 the `Cargo.toml` manifest file. We also need a minimal crate associated
117 If our new lint is named e.g. `foo_categories`, after running `cargo dev new_lint`
118 we will find by default two new crates, each with its manifest file:
120 * `tests/ui-cargo/foo_categories/fail/Cargo.toml`: this file should cause the new lint to raise an error.
121 * `tests/ui-cargo/foo_categories/pass/Cargo.toml`: this file should not trigger the lint.
123 If you need more cases, you can copy one of those crates (under `foo_categories`) and rename it.
125 The process of generating the `.stderr` file is the same, and prepending the `TESTNAME`
126 variable to `cargo uitest` works too.
130 If the lint you are working on is making use of structured suggestions, the
131 test file should include a `// run-rustfix` comment at the top. This will
132 additionally run [rustfix] for that test. Rustfix will apply the suggestions
133 from the lint to the code of the test file and compare that to the contents of
136 Use `cargo dev bless` to automatically generate the
137 `.fixed` file after running the tests.
139 [rustfix]: https://github.com/rust-lang/rustfix
141 ## Edition 2018 tests
143 Some features require the 2018 edition to work (e.g. `async_await`), but
144 compile-test tests run on the 2015 edition by default. To change this behavior
145 add `// edition:2018` at the top of the test file (note that it's space-sensitive).
149 Manually testing against an example file can be useful if you have added some
150 `println!`s and the test suite output becomes unreadable. To try Clippy with
151 your local modifications, run
154 env __CLIPPY_INTERNAL_TESTS=true cargo run --bin clippy-driver -- -L ./target/debug input.rs
157 from the working copy root. With tests in place, let's have a look at
158 implementing our lint now.
162 Let's start by opening the new file created in the `clippy_lints` crate
163 at `clippy_lints/src/foo_functions.rs`. That's the crate where all the
164 lint code is. This file has already imported some initial things we will need:
167 use rustc_lint::{EarlyLintPass, EarlyContext};
168 use rustc_session::{declare_lint_pass, declare_tool_lint};
169 use rustc_ast::ast::*;
172 The next step is to update the lint declaration. Lints are declared using the
173 [`declare_clippy_lint!`][declare_clippy_lint] macro, and we just need to update
174 the auto-generated lint declaration to have a real description, something like this:
177 declare_clippy_lint! {
178 /// **What it does:**
180 /// **Why is this bad?**
182 /// **Known problems:** None.
191 "function named `foo`, which is not a descriptive name"
195 * The section of lines prefixed with `///` constitutes the lint documentation
196 section. This is the default documentation style and will be displayed
197 [like this][example_lint_page]. To render and open this documentation locally
198 in a browser, run `cargo dev serve`.
199 * `FOO_FUNCTIONS` is the name of our lint. Be sure to follow the
200 [lint naming guidelines][lint_naming] here when naming your lint.
201 In short, the name should state the thing that is being checked for and
202 read well when used with `allow`/`warn`/`deny`.
203 * `pedantic` sets the lint level to `Allow`.
204 The exact mapping can be found [here][category_level_mapping]
205 * The last part should be a text that explains what exactly is wrong with the
208 The rest of this file contains an empty implementation for our lint pass,
209 which in this case is `EarlyLintPass` and should look like this:
212 // clippy_lints/src/foo_functions.rs
214 // .. imports and lint declaration ..
216 declare_lint_pass!(FooFunctions => [FOO_FUNCTIONS]);
218 impl EarlyLintPass for FooFunctions {}
221 Normally after declaring the lint, we have to run `cargo dev update_lints`,
222 which updates some files, so Clippy knows about the new lint. Since we used
223 `cargo dev new_lint ...` to generate the lint declaration, this was done
224 automatically. While `update_lints` automates most of the things, it doesn't
225 automate everything. We will have to register our lint pass manually in the
226 `register_plugins` function in `clippy_lints/src/lib.rs`:
229 store.register_early_pass(|| box foo_functions::FooFunctions);
232 As one may expect, there is a corresponding `register_late_pass` method
233 available as well. Without a call to one of `register_early_pass` or
234 `register_late_pass`, the lint pass in question will not be run.
236 One reason that `cargo dev` does not automate this step is that multiple lints
237 can use the same lint pass, so registering the lint pass may already be done
238 when adding a new lint. Another reason that this step is not automated is that
239 the order that the passes are registered determines the order the passes
240 actually run, which in turn affects the order that any emitted lints are output
243 [declare_clippy_lint]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L60
244 [example_lint_page]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure
245 [lint_naming]: https://rust-lang.github.io/rfcs/0344-conventions-galore.html#lints
246 [category_level_mapping]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L110
250 Writing a lint that only checks for the name of a function means that we only
251 have to deal with the AST and don't have to deal with the type system at all.
252 This is good, because it makes writing this particular lint less complicated.
254 We have to make this decision with every new Clippy lint. It boils down to using
255 either [`EarlyLintPass`][early_lint_pass] or [`LateLintPass`][late_lint_pass].
257 In short, the `LateLintPass` has access to type information while the
258 `EarlyLintPass` doesn't. If you don't need access to type information, use the
259 `EarlyLintPass`. The `EarlyLintPass` is also faster. However linting speed
260 hasn't really been a concern with Clippy so far.
262 Since we don't need type information for checking the function name, we used
263 `--pass=early` when running the new lint automation and all the imports were
266 [early_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html
267 [late_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.LateLintPass.html
271 With UI tests and the lint declaration in place, we can start working on the
272 implementation of the lint logic.
274 Let's start by implementing the `EarlyLintPass` for our `FooFunctions`:
277 impl EarlyLintPass for FooFunctions {
278 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
279 // TODO: Emit lint here
284 We implement the [`check_fn`][check_fn] method from the
285 [`EarlyLintPass`][early_lint_pass] trait. This gives us access to various
286 information about the function that is currently being checked. More on that in
287 the next section. Let's worry about the details later and emit our lint for
288 *every* function definition first.
290 Depending on how complex we want our lint message to be, we can choose from a
291 variety of lint emission functions. They can all be found in
292 [`clippy_lints/src/utils/diagnostics.rs`][diagnostics].
294 `span_lint_and_help` seems most appropriate in this case. It allows us to
295 provide an extra help message and we can't really suggest a better name
296 automatically. This is how it looks:
299 impl EarlyLintPass for FooFunctions {
300 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
305 "function named `foo`",
307 "consider using a more meaningful name"
313 Running our UI test should now produce output that contains the lint message.
315 According to [the rustc-dev-guide], the text should be matter of fact and avoid
316 capitalization and periods, unless multiple sentences are needed.
317 When code or an identifier must appear in a message or label, it should be
318 surrounded with single grave accents \`.
320 [check_fn]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html#method.check_fn
321 [diagnostics]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/diagnostics.rs
322 [the rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/diagnostics.html
324 ## Adding the lint logic
326 Writing the logic for your lint will most likely be different from our example,
327 so this section is kept rather short.
329 Using the [`check_fn`][check_fn] method gives us access to [`FnKind`][fn_kind]
330 that has the [`FnKind::Fn`] variant. It provides access to the name of the
331 function/method via an [`Ident`][ident].
333 With that we can expand our `check_fn` method to:
336 impl EarlyLintPass for FooFunctions {
337 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
338 if is_foo_fn(fn_kind) {
343 "function named `foo`",
345 "consider using a more meaningful name"
352 We separate the lint conditional from the lint emissions because it makes the
353 code a bit easier to read. In some cases this separation would also allow to
354 write some unit tests (as opposed to only UI tests) for the separate function.
356 In our example, `is_foo_fn` looks like:
359 // use statements, impl EarlyLintPass, check_fn, ..
361 fn is_foo_fn(fn_kind: FnKind<'_>) -> bool {
363 FnKind::Fn(_, ident, ..) => {
364 // check if `fn` name is `foo`
365 ident.name.as_str() == "foo"
368 FnKind::Closure(..) => false
373 Now we should also run the full test suite with `cargo test`. At this point
374 running `cargo test` should produce the expected output. Remember to run
375 `cargo dev bless` to update the `.stderr` file.
377 `cargo test` (as opposed to `cargo uitest`) will also ensure that our lint
378 implementation is not violating any Clippy lints itself.
380 That should be it for the lint implementation. Running `cargo test` should now
383 [fn_kind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html
384 [`FnKind::Fn`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html#variant.Fn
385 [ident]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Ident.html
387 ## Specifying the lint's minimum supported Rust version (msrv)
389 Projects supporting older versions of Rust would need to disable a lint if it targets features
390 present in later versions. Support for this can be added by specifying an msrv in your lint like so,
393 const MANUAL_STRIP_MSRV: RustcVersion = RustcVersion::new(1, 45, 0);
396 The project's msrv will also have to be an attribute in the lint so you'll have to add a struct
397 and constructor for your lint. The project's msrv needs to be passed when the lint is registered
401 pub struct ManualStrip {
402 msrv: Option<RustcVersion>,
407 pub fn new(msrv: Option<RustcVersion>) -> Self {
413 The project's msrv can then be matched against the lint's msrv in the LintPass using the `meets_msrv` utility
417 if !meets_msrv(self.msrv.as_ref(), &MANUAL_STRIP_MSRV) {
422 The project's msrv can also be specified as an inner attribute, which overrides the value from
423 `clippy.toml`. This can be accounted for using the `extract_msrv_attr!(LintContext)` macro and passing
424 LateContext/EarlyContext.
427 impl<'tcx> LateLintPass<'tcx> for ManualStrip {
428 fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
431 extract_msrv_attr!(LateContext);
435 Once the msrv is added to the lint, a relevant test case should be added to `tests/ui/min_rust_version_attr.rs`
436 which verifies that the lint isn't emitted if the project's msrv is lower.
440 If you have trouble implementing your lint, there is also the internal `author`
441 lint to generate Clippy code that detects the offending pattern. It does not
442 work for all of the Rust syntax, but can give a good starting point.
444 The quickest way to use it, is the
445 [Rust playground: play.rust-lang.org][author_example].
446 Put the code you want to lint into the editor and add the `#[clippy::author]`
447 attribute above the item. Then run Clippy via `Tools -> Clippy` and you should
448 see the generated code in the output below.
450 [Here][author_example] is an example on the playground.
452 If the command was executed successfully, you can copy the code over to where
453 you are implementing your lint.
455 [author_example]: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=9a12cb60e5c6ad4e3003ac6d5e63cf55
459 The final thing before submitting our PR is to add some documentation to our
462 Please document your lint with a doc comment akin to the following:
465 declare_clippy_lint! {
466 /// **What it does:** Checks for ... (describe what the lint matches).
468 /// **Why is this bad?** Supply the reason for linting the code.
470 /// **Known problems:** None. (Or describe where it could go wrong.)
476 /// Insert a short example of code that triggers the lint
479 /// Insert a short example of improved code that doesn't trigger the lint
483 "function named `foo`, which is not a descriptive name"
487 Once your lint is merged, this documentation will show up in the [lint
490 [lint_list]: https://rust-lang.github.io/rust-clippy/master/index.html
494 [Rustfmt] is a tool for formatting Rust code according to style guidelines.
495 Your code has to be formatted by `rustfmt` before a PR can be merged.
496 Clippy uses nightly `rustfmt` in the CI.
498 It can be installed via `rustup`:
501 rustup component add rustfmt --toolchain=nightly
504 Use `cargo dev fmt` to format the whole codebase. Make sure that `rustfmt` is
505 installed for the nightly toolchain.
507 [Rustfmt]: https://github.com/rust-lang/rustfmt
511 If you want to debug parts of your lint implementation, you can use the [`dbg!`]
512 macro anywhere in your code. Running the tests should then include the debug
513 output in the `stdout` part.
515 [`dbg!`]: https://doc.rust-lang.org/std/macro.dbg.html
519 Before submitting your PR make sure you followed all of the basic requirements:
521 <!-- Sync this with `.github/PULL_REQUEST_TEMPLATE` -->
523 - \[ ] Followed [lint naming conventions][lint_naming]
524 - \[ ] Added passing UI tests (including committed `.stderr` file)
525 - \[ ] `cargo test` passes locally
526 - \[ ] Executed `cargo dev update_lints`
527 - \[ ] Added lint documentation
528 - \[ ] Run `cargo dev fmt`
530 ## Adding configuration to a lint
532 Clippy supports the configuration of lints values using a `clippy.toml` file in the workspace
533 directory. Adding a configuration to a lint can be useful for thresholds or to constrain some
534 behavior that can be seen as a false positive for some users. Adding a configuration is done
535 in the following steps:
537 1. Adding a new configuration entry to [clippy_lints::utils::conf](/clippy_lints/src/utils/conf.rs)
540 /// Lint: LINT_NAME. <The configuration field doc comment>
541 (configuration_ident, "configuration_value": Type, DefaultValue),
543 The configuration value and identifier should usually be the same. The doc comment will be
544 automatically added to the lint documentation.
545 2. Adding the configuration value to the lint impl struct:
546 1. This first requires the definition of a lint impl struct. Lint impl structs are usually
547 generated with the `declare_lint_pass!` macro. This struct needs to be defined manually
548 to add some kind of metadata to it:
550 // Generated struct definition
551 declare_lint_pass!(StructName => [
555 // New manual definition struct
556 #[derive(Copy, Clone)]
557 pub struct StructName {}
559 impl_lint_pass!(StructName => [
564 2. Next add the configuration value and a corresponding creation method like this:
566 #[derive(Copy, Clone)]
567 pub struct StructName {
568 configuration_ident: Type,
574 pub fn new(configuration_ident: Type) -> Self {
581 3. Passing the configuration value to the lint impl struct:
583 First find the struct construction in the [clippy_lints lib file](/clippy_lints/src/lib.rs).
584 The configuration value is now cloned or copied into a local value that is then passed to the
585 impl struct like this:
587 // Default generated registration:
588 store.register_*_pass(|| box module::StructName);
590 // New registration with configuration value
591 let configuration_ident = conf.configuration_ident.clone();
592 store.register_*_pass(move || box module::StructName::new(configuration_ident));
595 Congratulations the work is almost done. The configuration value can now be accessed
596 in the linting code via `self.configuration_ident`.
599 1. The default configured value can be tested like any normal lint in [`tests/ui`](/tests/ui).
600 2. The configuration itself will be tested separately in [`tests/ui-toml`](/tests/ui-toml).
601 Simply add a new subfolder with a fitting name. This folder contains a `clippy.toml` file
602 with the configuration value and a rust file that should be linted by Clippy. The test can
603 otherwise be written as usual.
607 Here are some pointers to things you are likely going to need for every lint:
609 * [Clippy utils][utils] - Various helper functions. Maybe the function you need
610 is already in here (`implements_trait`, `match_path`, `snippet`, etc)
611 * [Clippy diagnostics][diagnostics]
612 * [The `if_chain` macro][if_chain]
613 * [`from_expansion`][from_expansion] and [`in_external_macro`][in_external_macro]
615 * [`Applicability`][applicability]
616 * [Common tools for writing lints](common_tools_writing_lints.md) helps with common operations
617 * [The rustc-dev-guide][rustc-dev-guide] explains a lot of internal compiler concepts
618 * [The nightly rustc docs][nightly_docs] which has been linked to throughout
621 For `EarlyLintPass` lints:
623 * [`EarlyLintPass`][early_lint_pass]
624 * [`rustc_ast::ast`][ast]
626 For `LateLintPass` lints:
628 * [`LateLintPass`][late_lint_pass]
631 While most of Clippy's lint utils are documented, most of rustc's internals lack
632 documentation currently. This is unfortunate, but in most cases you can probably
633 get away with copying things from existing similar lints. If you are stuck,
634 don't hesitate to ask on [Zulip] or in the issue/PR.
636 [utils]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/mod.rs
637 [if_chain]: https://docs.rs/if_chain/*/if_chain/
638 [from_expansion]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html#method.from_expansion
639 [in_external_macro]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/lint/fn.in_external_macro.html
640 [span]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html
641 [applicability]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/enum.Applicability.html
642 [rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/
643 [nightly_docs]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/
644 [ast]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/index.html
645 [ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/sty/index.html
646 [Zulip]: https://rust-lang.zulipchat.com/#narrow/stream/clippy