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 - [Specifying the lint's minimum supported Rust version (MSRV)](#specifying-the-lints-minimum-supported-rust-version-msrv)
22 - [Author lint](#author-lint)
23 - [Documentation](#documentation)
24 - [Running rustfmt](#running-rustfmt)
25 - [Debugging](#debugging)
26 - [PR Checklist](#pr-checklist)
27 - [Adding configuration to a lint](#adding-configuration-to-a-lint)
28 - [Cheatsheet](#cheatsheet)
32 See the [Basics](basics.md#get-the-code) documentation.
36 There is a bit of boilerplate code that needs to be set up when creating a new
37 lint. Fortunately, you can use the clippy dev tools to handle this for you. We
38 are naming our new lint `foo_functions` (lints are generally written in snake
39 case), and we don't need type information so it will have an early pass type
40 (more on this later on). If you're not sure if the name you chose fits the lint,
41 take a look at our [lint naming guidelines][lint_naming]. To get started on this
42 lint you can run `cargo dev new_lint --name=foo_functions --pass=early
43 --category=pedantic` (category will default to nursery if not provided). This
44 command will create two files: `tests/ui/foo_functions.rs` and
45 `clippy_lints/src/foo_functions.rs`, as well as run `cargo dev update_lints` to
46 register the new lint. For cargo lints, two project hierarchies (fail/pass) will
47 be created by default under `tests/ui-cargo`.
49 Next, we'll open up these files and add our lint!
53 Let's write some tests first that we can execute while we iterate on our lint.
55 Clippy uses UI tests for testing. UI tests check that the output of Clippy is
56 exactly as expected. Each test is just a plain Rust file that contains the code
57 we want to check. The output of Clippy is compared against a `.stderr` file.
58 Note that you don't have to create this file yourself, we'll get to
59 generating the `.stderr` files further down.
61 We start by opening the test file created at `tests/ui/foo_functions.rs`.
63 Update the file with some examples to get started:
66 #![warn(clippy::foo_functions)]
76 // Default trait methods
89 // We also don't want to lint method calls
96 Now we can run the test with `TESTNAME=foo_functions cargo uitest`,
97 currently this test is meaningless though.
99 While we are working on implementing our lint, we can keep running the UI
100 test. That allows us to check if the output is turning into what we want.
102 Once we are satisfied with the output, we need to run
103 `cargo dev bless` to update the `.stderr` file for our lint.
104 Please note that, we should run `TESTNAME=foo_functions cargo uitest`
105 every time before running `cargo dev bless`.
106 Running `TESTNAME=foo_functions cargo uitest` should pass then. When we commit
107 our lint, we need to commit the generated `.stderr` files, too. In general, you
108 should only commit files changed by `cargo dev bless` for the
109 specific lint you are creating/editing. Note that if the generated files are
110 empty, they should be removed.
112 Note that you can run multiple test files by specifying a comma separated list:
113 `TESTNAME=foo_functions,test2,test3`.
117 For cargo lints, the process of testing differs in that we are interested in
118 the `Cargo.toml` manifest file. We also need a minimal crate associated
121 If our new lint is named e.g. `foo_categories`, after running `cargo dev new_lint`
122 we will find by default two new crates, each with its manifest file:
124 * `tests/ui-cargo/foo_categories/fail/Cargo.toml`: this file should cause the new lint to raise an error.
125 * `tests/ui-cargo/foo_categories/pass/Cargo.toml`: this file should not trigger the lint.
127 If you need more cases, you can copy one of those crates (under `foo_categories`) and rename it.
129 The process of generating the `.stderr` file is the same, and prepending the `TESTNAME`
130 variable to `cargo uitest` works too.
134 If the lint you are working on is making use of structured suggestions, the
135 test file should include a `// run-rustfix` comment at the top. This will
136 additionally run [rustfix] for that test. Rustfix will apply the suggestions
137 from the lint to the code of the test file and compare that to the contents of
140 Use `cargo dev bless` to automatically generate the
141 `.fixed` file after running the tests.
143 [rustfix]: https://github.com/rust-lang/rustfix
145 ## Edition 2018 tests
147 Some features require the 2018 edition to work (e.g. `async_await`), but
148 compile-test tests run on the 2015 edition by default. To change this behavior
149 add `// edition:2018` at the top of the test file (note that it's space-sensitive).
153 Manually testing against an example file can be useful if you have added some
154 `println!`s and the test suite output becomes unreadable. To try Clippy with
155 your local modifications, run
158 env __CLIPPY_INTERNAL_TESTS=true cargo run --bin clippy-driver -- -L ./target/debug input.rs
161 from the working copy root. With tests in place, let's have a look at
162 implementing our lint now.
166 Let's start by opening the new file created in the `clippy_lints` crate
167 at `clippy_lints/src/foo_functions.rs`. That's the crate where all the
168 lint code is. This file has already imported some initial things we will need:
171 use rustc_lint::{EarlyLintPass, EarlyContext};
172 use rustc_session::{declare_lint_pass, declare_tool_lint};
173 use rustc_ast::ast::*;
176 The next step is to update the lint declaration. Lints are declared using the
177 [`declare_clippy_lint!`][declare_clippy_lint] macro, and we just need to update
178 the auto-generated lint declaration to have a real description, something like this:
181 declare_clippy_lint! {
182 /// **What it does:**
184 /// **Why is this bad?**
186 /// **Known problems:** None.
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 * `FOO_FUNCTIONS` is the name of our lint. Be sure to follow the
204 [lint naming guidelines][lint_naming] here when naming your lint.
205 In short, the name should state the thing that is being checked for and
206 read well when used with `allow`/`warn`/`deny`.
207 * `pedantic` sets the lint level to `Allow`.
208 The exact mapping can be found [here][category_level_mapping]
209 * The last part should be a text that explains what exactly is wrong with the
212 The rest of this file contains an empty implementation for our lint pass,
213 which in this case is `EarlyLintPass` and should look like this:
216 // clippy_lints/src/foo_functions.rs
218 // .. imports and lint declaration ..
220 declare_lint_pass!(FooFunctions => [FOO_FUNCTIONS]);
222 impl EarlyLintPass for FooFunctions {}
225 Normally after declaring the lint, we have to run `cargo dev update_lints`,
226 which updates some files, so Clippy knows about the new lint. Since we used
227 `cargo dev new_lint ...` to generate the lint declaration, this was done
228 automatically. While `update_lints` automates most of the things, it doesn't
229 automate everything. We will have to register our lint pass manually in the
230 `register_plugins` function in `clippy_lints/src/lib.rs`:
233 store.register_early_pass(|| box foo_functions::FooFunctions);
236 As one may expect, there is a corresponding `register_late_pass` method
237 available as well. Without a call to one of `register_early_pass` or
238 `register_late_pass`, the lint pass in question will not be run.
240 One reason that `cargo dev` does not automate this step is that multiple lints
241 can use the same lint pass, so registering the lint pass may already be done
242 when adding a new lint. Another reason that this step is not automated is that
243 the order that the passes are registered determines the order the passes
244 actually run, which in turn affects the order that any emitted lints are output
247 [declare_clippy_lint]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L60
248 [example_lint_page]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure
249 [lint_naming]: https://rust-lang.github.io/rfcs/0344-conventions-galore.html#lints
250 [category_level_mapping]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L110
254 Writing a lint that only checks for the name of a function means that we only
255 have to deal with the AST and don't have to deal with the type system at all.
256 This is good, because it makes writing this particular lint less complicated.
258 We have to make this decision with every new Clippy lint. It boils down to using
259 either [`EarlyLintPass`][early_lint_pass] or [`LateLintPass`][late_lint_pass].
261 In short, the `LateLintPass` has access to type information while the
262 `EarlyLintPass` doesn't. If you don't need access to type information, use the
263 `EarlyLintPass`. The `EarlyLintPass` is also faster. However linting speed
264 hasn't really been a concern with Clippy so far.
266 Since we don't need type information for checking the function name, we used
267 `--pass=early` when running the new lint automation and all the imports were
270 [early_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html
271 [late_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.LateLintPass.html
275 With UI tests and the lint declaration in place, we can start working on the
276 implementation of the lint logic.
278 Let's start by implementing the `EarlyLintPass` for our `FooFunctions`:
281 impl EarlyLintPass for FooFunctions {
282 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
283 // TODO: Emit lint here
288 We implement the [`check_fn`][check_fn] method from the
289 [`EarlyLintPass`][early_lint_pass] trait. This gives us access to various
290 information about the function that is currently being checked. More on that in
291 the next section. Let's worry about the details later and emit our lint for
292 *every* function definition first.
294 Depending on how complex we want our lint message to be, we can choose from a
295 variety of lint emission functions. They can all be found in
296 [`clippy_utils/src/diagnostics.rs`][diagnostics].
298 `span_lint_and_help` seems most appropriate in this case. It allows us to
299 provide an extra help message and we can't really suggest a better name
300 automatically. This is how it looks:
303 impl EarlyLintPass for FooFunctions {
304 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
309 "function named `foo`",
311 "consider using a more meaningful name"
317 Running our UI test should now produce output that contains the lint message.
319 According to [the rustc-dev-guide], the text should be matter of fact and avoid
320 capitalization and periods, unless multiple sentences are needed.
321 When code or an identifier must appear in a message or label, it should be
322 surrounded with single grave accents \`.
324 [check_fn]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html#method.check_fn
325 [diagnostics]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_utils/src/diagnostics.rs
326 [the rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/diagnostics.html
328 ## Adding the lint logic
330 Writing the logic for your lint will most likely be different from our example,
331 so this section is kept rather short.
333 Using the [`check_fn`][check_fn] method gives us access to [`FnKind`][fn_kind]
334 that has the [`FnKind::Fn`] variant. It provides access to the name of the
335 function/method via an [`Ident`][ident].
337 With that we can expand our `check_fn` method to:
340 impl EarlyLintPass for FooFunctions {
341 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
342 if is_foo_fn(fn_kind) {
347 "function named `foo`",
349 "consider using a more meaningful name"
356 We separate the lint conditional from the lint emissions because it makes the
357 code a bit easier to read. In some cases this separation would also allow to
358 write some unit tests (as opposed to only UI tests) for the separate function.
360 In our example, `is_foo_fn` looks like:
363 // use statements, impl EarlyLintPass, check_fn, ..
365 fn is_foo_fn(fn_kind: FnKind<'_>) -> bool {
367 FnKind::Fn(_, ident, ..) => {
368 // check if `fn` name is `foo`
369 ident.name.as_str() == "foo"
372 FnKind::Closure(..) => false
377 Now we should also run the full test suite with `cargo test`. At this point
378 running `cargo test` should produce the expected output. Remember to run
379 `cargo dev bless` to update the `.stderr` file.
381 `cargo test` (as opposed to `cargo uitest`) will also ensure that our lint
382 implementation is not violating any Clippy lints itself.
384 That should be it for the lint implementation. Running `cargo test` should now
387 [fn_kind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html
388 [`FnKind::Fn`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html#variant.Fn
389 [ident]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Ident.html
391 ## Specifying the lint's minimum supported Rust version (MSRV)
393 Sometimes a lint makes suggestions that require a certain version of Rust. For example, the `manual_strip` lint suggests
394 using `str::strip_prefix` and `str::strip_suffix` which is only available after Rust 1.45. In such cases, you need to
395 ensure that the MSRV configured for the project is >= the MSRV of the required Rust feature. If multiple features are
396 required, just use the one with a lower MSRV.
398 First, add an MSRV alias for the required feature in [`clippy_utils::msrvs`](/clippy_utils/src/msrvs.rs). This can be
399 accessed later as `msrvs::STR_STRIP_PREFIX`, for example.
404 1,45,0 { STR_STRIP_PREFIX }
408 In order to access the project-configured MSRV, you need to have an `msrv` field in the LintPass struct, and a
409 constructor to initialize the field. The `msrv` value is passed to the constructor in `clippy_lints/lib.rs`.
412 pub struct ManualStrip {
413 msrv: Option<RustcVersion>,
418 pub fn new(msrv: Option<RustcVersion>) -> Self {
424 The project's MSRV can then be matched against the feature MSRV in the LintPass
425 using the `meets_msrv` utility function.
428 if !meets_msrv(self.msrv.as_ref(), &msrvs::STR_STRIP_PREFIX) {
433 The project's MSRV can also be specified as an inner attribute, which overrides
434 the value from `clippy.toml`. This can be accounted for using the
435 `extract_msrv_attr!(LintContext)` macro and passing
436 `LateContext`/`EarlyContext`.
439 impl<'tcx> LateLintPass<'tcx> for ManualStrip {
440 fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
443 extract_msrv_attr!(LateContext);
447 Once the `msrv` is added to the lint, a relevant test case should be added to
448 `tests/ui/min_rust_version_attr.rs` which verifies that the lint isn't emitted
449 if the project's MSRV is lower.
451 As a last step, the lint should be added to the lint documentation. This is done
452 in `clippy_lints/src/utils/conf.rs`:
456 /// Lint: LIST, OF, LINTS, <THE_NEWLY_ADDED_LINT>. The minimum rust version that the project supports
457 (msrv: Option<String> = None),
464 If you have trouble implementing your lint, there is also the internal `author`
465 lint to generate Clippy code that detects the offending pattern. It does not
466 work for all of the Rust syntax, but can give a good starting point.
468 The quickest way to use it, is the
469 [Rust playground: play.rust-lang.org][author_example].
470 Put the code you want to lint into the editor and add the `#[clippy::author]`
471 attribute above the item. Then run Clippy via `Tools -> Clippy` and you should
472 see the generated code in the output below.
474 [Here][author_example] is an example on the playground.
476 If the command was executed successfully, you can copy the code over to where
477 you are implementing your lint.
479 [author_example]: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=9a12cb60e5c6ad4e3003ac6d5e63cf55
483 The final thing before submitting our PR is to add some documentation to our
486 Please document your lint with a doc comment akin to the following:
489 declare_clippy_lint! {
490 /// **What it does:** Checks for ... (describe what the lint matches).
492 /// **Why is this bad?** Supply the reason for linting the code.
494 /// **Known problems:** None. (Or describe where it could go wrong.)
500 /// Insert a short example of code that triggers the lint
503 /// Insert a short example of improved code that doesn't trigger the lint
507 "function named `foo`, which is not a descriptive name"
511 Once your lint is merged, this documentation will show up in the [lint
514 [lint_list]: https://rust-lang.github.io/rust-clippy/master/index.html
518 [Rustfmt] is a tool for formatting Rust code according to style guidelines.
519 Your code has to be formatted by `rustfmt` before a PR can be merged.
520 Clippy uses nightly `rustfmt` in the CI.
522 It can be installed via `rustup`:
525 rustup component add rustfmt --toolchain=nightly
528 Use `cargo dev fmt` to format the whole codebase. Make sure that `rustfmt` is
529 installed for the nightly toolchain.
531 [Rustfmt]: https://github.com/rust-lang/rustfmt
535 If you want to debug parts of your lint implementation, you can use the [`dbg!`]
536 macro anywhere in your code. Running the tests should then include the debug
537 output in the `stdout` part.
539 [`dbg!`]: https://doc.rust-lang.org/std/macro.dbg.html
543 Before submitting your PR make sure you followed all of the basic requirements:
545 <!-- Sync this with `.github/PULL_REQUEST_TEMPLATE` -->
547 - \[ ] Followed [lint naming conventions][lint_naming]
548 - \[ ] Added passing UI tests (including committed `.stderr` file)
549 - \[ ] `cargo test` passes locally
550 - \[ ] Executed `cargo dev update_lints`
551 - \[ ] Added lint documentation
552 - \[ ] Run `cargo dev fmt`
554 ## Adding configuration to a lint
556 Clippy supports the configuration of lints values using a `clippy.toml` file in the workspace
557 directory. Adding a configuration to a lint can be useful for thresholds or to constrain some
558 behavior that can be seen as a false positive for some users. Adding a configuration is done
559 in the following steps:
561 1. Adding a new configuration entry to [clippy_utils::conf](/clippy_utils/src/conf.rs)
564 /// Lint: LINT_NAME. <The configuration field doc comment>
565 (configuration_ident: Type = DefaultValue),
567 The configuration value and identifier should usually be the same. The doc comment will be
568 automatically added to the lint documentation.
569 2. Adding the configuration value to the lint impl struct:
570 1. This first requires the definition of a lint impl struct. Lint impl structs are usually
571 generated with the `declare_lint_pass!` macro. This struct needs to be defined manually
572 to add some kind of metadata to it:
574 // Generated struct definition
575 declare_lint_pass!(StructName => [
579 // New manual definition struct
580 #[derive(Copy, Clone)]
581 pub struct StructName {}
583 impl_lint_pass!(StructName => [
588 2. Next add the configuration value and a corresponding creation method like this:
590 #[derive(Copy, Clone)]
591 pub struct StructName {
592 configuration_ident: Type,
598 pub fn new(configuration_ident: Type) -> Self {
605 3. Passing the configuration value to the lint impl struct:
607 First find the struct construction in the [clippy_lints lib file](/clippy_lints/src/lib.rs).
608 The configuration value is now cloned or copied into a local value that is then passed to the
609 impl struct like this:
611 // Default generated registration:
612 store.register_*_pass(|| box module::StructName);
614 // New registration with configuration value
615 let configuration_ident = conf.configuration_ident.clone();
616 store.register_*_pass(move || box module::StructName::new(configuration_ident));
619 Congratulations the work is almost done. The configuration value can now be accessed
620 in the linting code via `self.configuration_ident`.
623 1. The default configured value can be tested like any normal lint in [`tests/ui`](/tests/ui).
624 2. The configuration itself will be tested separately in [`tests/ui-toml`](/tests/ui-toml).
625 Simply add a new subfolder with a fitting name. This folder contains a `clippy.toml` file
626 with the configuration value and a rust file that should be linted by Clippy. The test can
627 otherwise be written as usual.
631 Here are some pointers to things you are likely going to need for every lint:
633 * [Clippy utils][utils] - Various helper functions. Maybe the function you need
634 is already in here (`implements_trait`, `match_def_path`, `snippet`, etc)
635 * [Clippy diagnostics][diagnostics]
636 * [The `if_chain` macro][if_chain]
637 * [`from_expansion`][from_expansion] and [`in_external_macro`][in_external_macro]
639 * [`Applicability`][applicability]
640 * [Common tools for writing lints](common_tools_writing_lints.md) helps with common operations
641 * [The rustc-dev-guide][rustc-dev-guide] explains a lot of internal compiler concepts
642 * [The nightly rustc docs][nightly_docs] which has been linked to throughout
645 For `EarlyLintPass` lints:
647 * [`EarlyLintPass`][early_lint_pass]
648 * [`rustc_ast::ast`][ast]
650 For `LateLintPass` lints:
652 * [`LateLintPass`][late_lint_pass]
655 While most of Clippy's lint utils are documented, most of rustc's internals lack
656 documentation currently. This is unfortunate, but in most cases you can probably
657 get away with copying things from existing similar lints. If you are stuck,
658 don't hesitate to ask on [Zulip] or in the issue/PR.
660 [utils]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_utils/src/lib.rs
661 [if_chain]: https://docs.rs/if_chain/*/if_chain/
662 [from_expansion]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html#method.from_expansion
663 [in_external_macro]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/lint/fn.in_external_macro.html
664 [span]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html
665 [applicability]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/enum.Applicability.html
666 [rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/
667 [nightly_docs]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/
668 [ast]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/index.html
669 [ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/sty/index.html
670 [Zulip]: https://rust-lang.zulipchat.com/#narrow/stream/clippy