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 - [Cheatsheet](#cheatsheet)
30 See the [Basics](basics.md#get-the-code) documentation.
34 There is a bit of boilerplate code that needs to be set up when creating a new
35 lint. Fortunately, you can use the clippy dev tools to handle this for you. We
36 are naming our new lint `foo_functions` (lints are generally written in snake
37 case), and we don't need type information so it will have an early pass type
38 (more on this later on). If you're not sure if the name you chose fits the lint,
39 take a look at our [lint naming guidelines][lint_naming]. To get started on this
40 lint you can run `cargo dev new_lint --name=foo_functions --pass=early
41 --category=pedantic` (category will default to nursery if not provided). This
42 command will create two files: `tests/ui/foo_functions.rs` and
43 `clippy_lints/src/foo_functions.rs`, as well as run `cargo dev update_lints` to
44 register the new lint. For cargo lints, two project hierarchies (fail/pass) will
45 be created by default under `tests/ui-cargo`.
47 Next, we'll open up these files and add our lint!
51 Let's write some tests first that we can execute while we iterate on our lint.
53 Clippy uses UI tests for testing. UI tests check that the output of Clippy is
54 exactly as expected. Each test is just a plain Rust file that contains the code
55 we want to check. The output of Clippy is compared against a `.stderr` file.
56 Note that you don't have to create this file yourself, we'll get to
57 generating the `.stderr` files further down.
59 We start by opening the test file created at `tests/ui/foo_functions.rs`.
61 Update the file with some examples to get started:
64 #![warn(clippy::foo_functions)]
74 // Default trait methods
87 // We also don't want to lint method calls
94 Now we can run the test with `TESTNAME=foo_functions cargo uitest`,
95 currently this test is meaningless though.
97 While we are working on implementing our lint, we can keep running the UI
98 test. That allows us to check if the output is turning into what we want.
100 Once we are satisfied with the output, we need to run
101 `tests/ui/update-all-references.sh` to update the `.stderr` file for our lint.
102 Please note that, we should run `TESTNAME=foo_functions cargo uitest`
103 every time before running `tests/ui/update-all-references.sh`.
104 Running `TESTNAME=foo_functions cargo uitest` should pass then. When we commit
105 our lint, we need to commit the generated `.stderr` files, too. In general, you
106 should only commit files changed by `tests/ui/update-all-references.sh` for the
107 specific lint you are creating/editing.
111 For cargo lints, the process of testing differs in that we are interested in
112 the `Cargo.toml` manifest file. We also need a minimal crate associated
115 If our new lint is named e.g. `foo_categories`, after running `cargo dev new_lint`
116 we will find by default two new crates, each with its manifest file:
118 * `tests/ui-cargo/foo_categories/fail/Cargo.toml`: this file should cause the new lint to raise an error.
119 * `tests/ui-cargo/foo_categories/pass/Cargo.toml`: this file should not trigger the lint.
121 If you need more cases, you can copy one of those crates (under `foo_categories`) and rename it.
123 The process of generating the `.stderr` file is the same, and prepending the `TESTNAME`
124 variable to `cargo uitest` works too, but the script to update the references
125 is in another path: `tests/ui-cargo/update-all-references.sh`.
129 If the lint you are working on is making use of structured suggestions, the
130 test file should include a `// run-rustfix` comment at the top. This will
131 additionally run [rustfix] for that test. Rustfix will apply the suggestions
132 from the lint to the code of the test file and compare that to the contents of
135 Use `tests/ui/update-all-references.sh` to automatically generate the
136 `.fixed` file after running the tests.
138 [rustfix]: https://github.com/rust-lang/rustfix
140 ## Edition 2018 tests
142 Some features require the 2018 edition to work (e.g. `async_await`), but
143 compile-test tests run on the 2015 edition by default. To change this behavior
144 add `// edition:2018` at the top of the test file (note that it's space-sensitive).
148 Manually testing against an example file can be useful if you have added some
149 `println!`s and the test suite output becomes unreadable. To try Clippy with
150 your local modifications, run `env CLIPPY_TESTS=true cargo run --bin
151 clippy-driver -- -L ./target/debug input.rs` from the working copy root.
153 With tests in place, let's have a look at implementing our lint now.
157 Let's start by opening the new file created in the `clippy_lints` crate
158 at `clippy_lints/src/foo_functions.rs`. That's the crate where all the
159 lint code is. This file has already imported some initial things we will need:
162 use rustc_lint::{EarlyLintPass, EarlyContext};
163 use rustc_session::{declare_lint_pass, declare_tool_lint};
164 use rustc_ast::ast::*;
167 The next step is to update the lint declaration. Lints are declared using the
168 [`declare_clippy_lint!`][declare_clippy_lint] macro, and we just need to update
169 the auto-generated lint declaration to have a real description, something like this:
172 declare_clippy_lint! {
173 /// **What it does:**
175 /// **Why is this bad?**
177 /// **Known problems:** None.
186 "function named `foo`, which is not a descriptive name"
190 * The section of lines prefixed with `///` constitutes the lint documentation
191 section. This is the default documentation style and will be displayed
192 [like this][example_lint_page].
193 * `FOO_FUNCTIONS` is the name of our lint. Be sure to follow the
194 [lint naming guidelines][lint_naming] here when naming your lint.
195 In short, the name should state the thing that is being checked for and
196 read well when used with `allow`/`warn`/`deny`.
197 * `pedantic` sets the lint level to `Allow`.
198 The exact mapping can be found [here][category_level_mapping]
199 * The last part should be a text that explains what exactly is wrong with the
202 The rest of this file contains an empty implementation for our lint pass,
203 which in this case is `EarlyLintPass` and should look like this:
206 // clippy_lints/src/foo_functions.rs
208 // .. imports and lint declaration ..
210 declare_lint_pass!(FooFunctions => [FOO_FUNCTIONS]);
212 impl EarlyLintPass for FooFunctions {}
215 Normally after declaring the lint, we have to run `cargo dev update_lints`,
216 which updates some files, so Clippy knows about the new lint. Since we used
217 `cargo dev new_lint ...` to generate the lint declaration, this was done
218 automatically. While `update_lints` automates most of the things, it doesn't
219 automate everything. We will have to register our lint pass manually in the
220 `register_plugins` function in `clippy_lints/src/lib.rs`:
223 store.register_early_pass(|| box foo_functions::FooFunctions);
226 [declare_clippy_lint]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L60
227 [example_lint_page]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure
228 [lint_naming]: https://rust-lang.github.io/rfcs/0344-conventions-galore.html#lints
229 [category_level_mapping]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L110
233 Writing a lint that only checks for the name of a function means that we only
234 have to deal with the AST and don't have to deal with the type system at all.
235 This is good, because it makes writing this particular lint less complicated.
237 We have to make this decision with every new Clippy lint. It boils down to using
238 either [`EarlyLintPass`][early_lint_pass] or [`LateLintPass`][late_lint_pass].
240 In short, the `LateLintPass` has access to type information while the
241 `EarlyLintPass` doesn't. If you don't need access to type information, use the
242 `EarlyLintPass`. The `EarlyLintPass` is also faster. However linting speed
243 hasn't really been a concern with Clippy so far.
245 Since we don't need type information for checking the function name, we used
246 `--pass=early` when running the new lint automation and all the imports were
249 [early_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html
250 [late_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.LateLintPass.html
254 With UI tests and the lint declaration in place, we can start working on the
255 implementation of the lint logic.
257 Let's start by implementing the `EarlyLintPass` for our `FooFunctions`:
260 impl EarlyLintPass for FooFunctions {
261 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
262 // TODO: Emit lint here
267 We implement the [`check_fn`][check_fn] method from the
268 [`EarlyLintPass`][early_lint_pass] trait. This gives us access to various
269 information about the function that is currently being checked. More on that in
270 the next section. Let's worry about the details later and emit our lint for
271 *every* function definition first.
273 Depending on how complex we want our lint message to be, we can choose from a
274 variety of lint emission functions. They can all be found in
275 [`clippy_lints/src/utils/diagnostics.rs`][diagnostics].
277 `span_lint_and_help` seems most appropriate in this case. It allows us to
278 provide an extra help message and we can't really suggest a better name
279 automatically. This is how it looks:
282 impl EarlyLintPass for FooFunctions {
283 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
288 "function named `foo`",
290 "consider using a more meaningful name"
296 Running our UI test should now produce output that contains the lint message.
298 According to [the rustc-dev-guide], the text should be matter of fact and avoid
299 capitalization and periods, unless multiple sentences are needed.
300 When code or an identifier must appear in a message or label, it should be
301 surrounded with single acute accents \`.
303 [check_fn]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html#method.check_fn
304 [diagnostics]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/diagnostics.rs
305 [the rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/diagnostics.html
307 ## Adding the lint logic
309 Writing the logic for your lint will most likely be different from our example,
310 so this section is kept rather short.
312 Using the [`check_fn`][check_fn] method gives us access to [`FnKind`][fn_kind]
313 that has the [`FnKind::Fn`] variant. It provides access to the name of the
314 function/method via an [`Ident`][ident].
316 With that we can expand our `check_fn` method to:
319 impl EarlyLintPass for FooFunctions {
320 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
321 if is_foo_fn(fn_kind) {
326 "function named `foo`",
328 "consider using a more meaningful name"
335 We separate the lint conditional from the lint emissions because it makes the
336 code a bit easier to read. In some cases this separation would also allow to
337 write some unit tests (as opposed to only UI tests) for the separate function.
339 In our example, `is_foo_fn` looks like:
342 // use statements, impl EarlyLintPass, check_fn, ..
344 fn is_foo_fn(fn_kind: FnKind<'_>) -> bool {
346 FnKind::Fn(_, ident, ..) => {
347 // check if `fn` name is `foo`
348 ident.name.as_str() == "foo"
351 FnKind::Closure(..) => false
356 Now we should also run the full test suite with `cargo test`. At this point
357 running `cargo test` should produce the expected output. Remember to run
358 `tests/ui/update-all-references.sh` to update the `.stderr` file.
360 `cargo test` (as opposed to `cargo uitest`) will also ensure that our lint
361 implementation is not violating any Clippy lints itself.
363 That should be it for the lint implementation. Running `cargo test` should now
366 [fn_kind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html
367 [`FnKind::Fn`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html#variant.Fn
368 [ident]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Ident.html
372 If you have trouble implementing your lint, there is also the internal `author`
373 lint to generate Clippy code that detects the offending pattern. It does not
374 work for all of the Rust syntax, but can give a good starting point.
376 The quickest way to use it, is the
377 [Rust playground: play.rust-lang.org][author_example].
378 Put the code you want to lint into the editor and add the `#[clippy::author]`
379 attribute above the item. Then run Clippy via `Tools -> Clippy` and you should
380 see the generated code in the output below.
382 [Here][author_example] is an example on the playground.
384 If the command was executed successfully, you can copy the code over to where
385 you are implementing your lint.
387 [author_example]: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=9a12cb60e5c6ad4e3003ac6d5e63cf55
391 The final thing before submitting our PR is to add some documentation to our
394 Please document your lint with a doc comment akin to the following:
397 declare_clippy_lint! {
398 /// **What it does:** Checks for ... (describe what the lint matches).
400 /// **Why is this bad?** Supply the reason for linting the code.
402 /// **Known problems:** None. (Or describe where it could go wrong.)
408 /// Insert a short example of code that triggers the lint
411 /// Insert a short example of improved code that doesn't trigger the lint
415 "function named `foo`, which is not a descriptive name"
419 Once your lint is merged, this documentation will show up in the [lint
422 [lint_list]: https://rust-lang.github.io/rust-clippy/master/index.html
426 [Rustfmt] is a tool for formatting Rust code according to style guidelines.
427 Your code has to be formatted by `rustfmt` before a PR can be merged.
428 Clippy uses nightly `rustfmt` in the CI.
430 It can be installed via `rustup`:
433 rustup component add rustfmt --toolchain=nightly
436 Use `cargo dev fmt` to format the whole codebase. Make sure that `rustfmt` is
437 installed for the nightly toolchain.
439 [Rustfmt]: https://github.com/rust-lang/rustfmt
443 If you want to debug parts of your lint implementation, you can use the [`dbg!`]
444 macro anywhere in your code. Running the tests should then include the debug
445 output in the `stdout` part.
447 [`dbg!`]: https://doc.rust-lang.org/std/macro.dbg.html
451 Before submitting your PR make sure you followed all of the basic requirements:
453 <!-- Sync this with `.github/PULL_REQUEST_TEMPLATE` -->
455 - [ ] Followed [lint naming conventions][lint_naming]
456 - [ ] Added passing UI tests (including committed `.stderr` file)
457 - [ ] `cargo test` passes locally
458 - [ ] Executed `cargo dev update_lints`
459 - [ ] Added lint documentation
460 - [ ] Run `cargo dev fmt`
464 Here are some pointers to things you are likely going to need for every lint:
466 * [Clippy utils][utils] - Various helper functions. Maybe the function you need
467 is already in here (`implements_trait`, `match_path`, `snippet`, etc)
468 * [Clippy diagnostics][diagnostics]
469 * [The `if_chain` macro][if_chain]
470 * [`from_expansion`][from_expansion] and [`in_external_macro`][in_external_macro]
472 * [`Applicability`][applicability]
473 * [Common tools for writing lints](common_tools_writing_lints.md) helps with common operations
474 * [The rustc-dev-guide][rustc-dev-guide] explains a lot of internal compiler concepts
475 * [The nightly rustc docs][nightly_docs] which has been linked to throughout
478 For `EarlyLintPass` lints:
480 * [`EarlyLintPass`][early_lint_pass]
481 * [`rustc_ast::ast`][ast]
483 For `LateLintPass` lints:
485 * [`LateLintPass`][late_lint_pass]
488 While most of Clippy's lint utils are documented, most of rustc's internals lack
489 documentation currently. This is unfortunate, but in most cases you can probably
490 get away with copying things from existing similar lints. If you are stuck,
491 don't hesitate to ask on [Zulip] or in the issue/PR.
493 [utils]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/mod.rs
494 [if_chain]: https://docs.rs/if_chain/*/if_chain/
495 [from_expansion]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html#method.from_expansion
496 [in_external_macro]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/lint/fn.in_external_macro.html
497 [span]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html
498 [applicability]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/enum.Applicability.html
499 [rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/
500 [nightly_docs]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/
501 [ast]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/index.html
502 [ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/sty/index.html
503 [Zulip]: https://rust-lang.zulipchat.com/#narrow/stream/clippy