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. Note that if the generated files are
108 empty, they should be removed.
112 For cargo lints, the process of testing differs in that we are interested in
113 the `Cargo.toml` manifest file. We also need a minimal crate associated
116 If our new lint is named e.g. `foo_categories`, after running `cargo dev new_lint`
117 we will find by default two new crates, each with its manifest file:
119 * `tests/ui-cargo/foo_categories/fail/Cargo.toml`: this file should cause the new lint to raise an error.
120 * `tests/ui-cargo/foo_categories/pass/Cargo.toml`: this file should not trigger the lint.
122 If you need more cases, you can copy one of those crates (under `foo_categories`) and rename it.
124 The process of generating the `.stderr` file is the same, and prepending the `TESTNAME`
125 variable to `cargo uitest` works too, but the script to update the references
126 is in another path: `tests/ui-cargo/update-all-references.sh`.
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 `tests/ui/update-all-references.sh` 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 `env CLIPPY_TESTS=true cargo run --bin
152 clippy-driver -- -L ./target/debug input.rs` from the working copy root.
154 With tests in place, let's have a look at implementing our lint now.
158 Let's start by opening the new file created in the `clippy_lints` crate
159 at `clippy_lints/src/foo_functions.rs`. That's the crate where all the
160 lint code is. This file has already imported some initial things we will need:
163 use rustc_lint::{EarlyLintPass, EarlyContext};
164 use rustc_session::{declare_lint_pass, declare_tool_lint};
165 use rustc_ast::ast::*;
168 The next step is to update the lint declaration. Lints are declared using the
169 [`declare_clippy_lint!`][declare_clippy_lint] macro, and we just need to update
170 the auto-generated lint declaration to have a real description, something like this:
173 declare_clippy_lint! {
174 /// **What it does:**
176 /// **Why is this bad?**
178 /// **Known problems:** None.
187 "function named `foo`, which is not a descriptive name"
191 * The section of lines prefixed with `///` constitutes the lint documentation
192 section. This is the default documentation style and will be displayed
193 [like this][example_lint_page]. To render and open this documentation locally
194 in a browser, run `cargo dev serve`.
195 * `FOO_FUNCTIONS` is the name of our lint. Be sure to follow the
196 [lint naming guidelines][lint_naming] here when naming your lint.
197 In short, the name should state the thing that is being checked for and
198 read well when used with `allow`/`warn`/`deny`.
199 * `pedantic` sets the lint level to `Allow`.
200 The exact mapping can be found [here][category_level_mapping]
201 * The last part should be a text that explains what exactly is wrong with the
204 The rest of this file contains an empty implementation for our lint pass,
205 which in this case is `EarlyLintPass` and should look like this:
208 // clippy_lints/src/foo_functions.rs
210 // .. imports and lint declaration ..
212 declare_lint_pass!(FooFunctions => [FOO_FUNCTIONS]);
214 impl EarlyLintPass for FooFunctions {}
217 Normally after declaring the lint, we have to run `cargo dev update_lints`,
218 which updates some files, so Clippy knows about the new lint. Since we used
219 `cargo dev new_lint ...` to generate the lint declaration, this was done
220 automatically. While `update_lints` automates most of the things, it doesn't
221 automate everything. We will have to register our lint pass manually in the
222 `register_plugins` function in `clippy_lints/src/lib.rs`:
225 store.register_early_pass(|| box foo_functions::FooFunctions);
228 As one may expect, there is a corresponding `register_late_pass` method
229 available as well. Without a call to one of `register_early_pass` or
230 `register_late_pass`, the lint pass in question will not be run.
232 One reason that `cargo dev` does not automate this step is that multiple lints
233 can use the same lint pass, so registering the lint pass may already be done
234 when adding a new lint. Another reason that this step is not automated is that
235 the order that the passes are registered determines the order the passes
236 actually run, which in turn affects the order that any emitted lints are output
239 [declare_clippy_lint]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L60
240 [example_lint_page]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure
241 [lint_naming]: https://rust-lang.github.io/rfcs/0344-conventions-galore.html#lints
242 [category_level_mapping]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L110
246 Writing a lint that only checks for the name of a function means that we only
247 have to deal with the AST and don't have to deal with the type system at all.
248 This is good, because it makes writing this particular lint less complicated.
250 We have to make this decision with every new Clippy lint. It boils down to using
251 either [`EarlyLintPass`][early_lint_pass] or [`LateLintPass`][late_lint_pass].
253 In short, the `LateLintPass` has access to type information while the
254 `EarlyLintPass` doesn't. If you don't need access to type information, use the
255 `EarlyLintPass`. The `EarlyLintPass` is also faster. However linting speed
256 hasn't really been a concern with Clippy so far.
258 Since we don't need type information for checking the function name, we used
259 `--pass=early` when running the new lint automation and all the imports were
262 [early_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html
263 [late_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.LateLintPass.html
267 With UI tests and the lint declaration in place, we can start working on the
268 implementation of the lint logic.
270 Let's start by implementing the `EarlyLintPass` for our `FooFunctions`:
273 impl EarlyLintPass for FooFunctions {
274 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
275 // TODO: Emit lint here
280 We implement the [`check_fn`][check_fn] method from the
281 [`EarlyLintPass`][early_lint_pass] trait. This gives us access to various
282 information about the function that is currently being checked. More on that in
283 the next section. Let's worry about the details later and emit our lint for
284 *every* function definition first.
286 Depending on how complex we want our lint message to be, we can choose from a
287 variety of lint emission functions. They can all be found in
288 [`clippy_lints/src/utils/diagnostics.rs`][diagnostics].
290 `span_lint_and_help` seems most appropriate in this case. It allows us to
291 provide an extra help message and we can't really suggest a better name
292 automatically. This is how it looks:
295 impl EarlyLintPass for FooFunctions {
296 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
301 "function named `foo`",
303 "consider using a more meaningful name"
309 Running our UI test should now produce output that contains the lint message.
311 According to [the rustc-dev-guide], the text should be matter of fact and avoid
312 capitalization and periods, unless multiple sentences are needed.
313 When code or an identifier must appear in a message or label, it should be
314 surrounded with single grave accents \`.
316 [check_fn]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html#method.check_fn
317 [diagnostics]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/diagnostics.rs
318 [the rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/diagnostics.html
320 ## Adding the lint logic
322 Writing the logic for your lint will most likely be different from our example,
323 so this section is kept rather short.
325 Using the [`check_fn`][check_fn] method gives us access to [`FnKind`][fn_kind]
326 that has the [`FnKind::Fn`] variant. It provides access to the name of the
327 function/method via an [`Ident`][ident].
329 With that we can expand our `check_fn` method to:
332 impl EarlyLintPass for FooFunctions {
333 fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
334 if is_foo_fn(fn_kind) {
339 "function named `foo`",
341 "consider using a more meaningful name"
348 We separate the lint conditional from the lint emissions because it makes the
349 code a bit easier to read. In some cases this separation would also allow to
350 write some unit tests (as opposed to only UI tests) for the separate function.
352 In our example, `is_foo_fn` looks like:
355 // use statements, impl EarlyLintPass, check_fn, ..
357 fn is_foo_fn(fn_kind: FnKind<'_>) -> bool {
359 FnKind::Fn(_, ident, ..) => {
360 // check if `fn` name is `foo`
361 ident.name.as_str() == "foo"
364 FnKind::Closure(..) => false
369 Now we should also run the full test suite with `cargo test`. At this point
370 running `cargo test` should produce the expected output. Remember to run
371 `tests/ui/update-all-references.sh` to update the `.stderr` file.
373 `cargo test` (as opposed to `cargo uitest`) will also ensure that our lint
374 implementation is not violating any Clippy lints itself.
376 That should be it for the lint implementation. Running `cargo test` should now
379 [fn_kind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html
380 [`FnKind::Fn`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html#variant.Fn
381 [ident]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Ident.html
385 If you have trouble implementing your lint, there is also the internal `author`
386 lint to generate Clippy code that detects the offending pattern. It does not
387 work for all of the Rust syntax, but can give a good starting point.
389 The quickest way to use it, is the
390 [Rust playground: play.rust-lang.org][author_example].
391 Put the code you want to lint into the editor and add the `#[clippy::author]`
392 attribute above the item. Then run Clippy via `Tools -> Clippy` and you should
393 see the generated code in the output below.
395 [Here][author_example] is an example on the playground.
397 If the command was executed successfully, you can copy the code over to where
398 you are implementing your lint.
400 [author_example]: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=9a12cb60e5c6ad4e3003ac6d5e63cf55
404 The final thing before submitting our PR is to add some documentation to our
407 Please document your lint with a doc comment akin to the following:
410 declare_clippy_lint! {
411 /// **What it does:** Checks for ... (describe what the lint matches).
413 /// **Why is this bad?** Supply the reason for linting the code.
415 /// **Known problems:** None. (Or describe where it could go wrong.)
421 /// Insert a short example of code that triggers the lint
424 /// Insert a short example of improved code that doesn't trigger the lint
428 "function named `foo`, which is not a descriptive name"
432 Once your lint is merged, this documentation will show up in the [lint
435 [lint_list]: https://rust-lang.github.io/rust-clippy/master/index.html
439 [Rustfmt] is a tool for formatting Rust code according to style guidelines.
440 Your code has to be formatted by `rustfmt` before a PR can be merged.
441 Clippy uses nightly `rustfmt` in the CI.
443 It can be installed via `rustup`:
446 rustup component add rustfmt --toolchain=nightly
449 Use `cargo dev fmt` to format the whole codebase. Make sure that `rustfmt` is
450 installed for the nightly toolchain.
452 [Rustfmt]: https://github.com/rust-lang/rustfmt
456 If you want to debug parts of your lint implementation, you can use the [`dbg!`]
457 macro anywhere in your code. Running the tests should then include the debug
458 output in the `stdout` part.
460 [`dbg!`]: https://doc.rust-lang.org/std/macro.dbg.html
464 Before submitting your PR make sure you followed all of the basic requirements:
466 <!-- Sync this with `.github/PULL_REQUEST_TEMPLATE` -->
468 - \[ ] Followed [lint naming conventions][lint_naming]
469 - \[ ] Added passing UI tests (including committed `.stderr` file)
470 - \[ ] `cargo test` passes locally
471 - \[ ] Executed `cargo dev update_lints`
472 - \[ ] Added lint documentation
473 - \[ ] Run `cargo dev fmt`
477 Here are some pointers to things you are likely going to need for every lint:
479 * [Clippy utils][utils] - Various helper functions. Maybe the function you need
480 is already in here (`implements_trait`, `match_path`, `snippet`, etc)
481 * [Clippy diagnostics][diagnostics]
482 * [The `if_chain` macro][if_chain]
483 * [`from_expansion`][from_expansion] and [`in_external_macro`][in_external_macro]
485 * [`Applicability`][applicability]
486 * [Common tools for writing lints](common_tools_writing_lints.md) helps with common operations
487 * [The rustc-dev-guide][rustc-dev-guide] explains a lot of internal compiler concepts
488 * [The nightly rustc docs][nightly_docs] which has been linked to throughout
491 For `EarlyLintPass` lints:
493 * [`EarlyLintPass`][early_lint_pass]
494 * [`rustc_ast::ast`][ast]
496 For `LateLintPass` lints:
498 * [`LateLintPass`][late_lint_pass]
501 While most of Clippy's lint utils are documented, most of rustc's internals lack
502 documentation currently. This is unfortunate, but in most cases you can probably
503 get away with copying things from existing similar lints. If you are stuck,
504 don't hesitate to ask on [Zulip] or in the issue/PR.
506 [utils]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/mod.rs
507 [if_chain]: https://docs.rs/if_chain/*/if_chain/
508 [from_expansion]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html#method.from_expansion
509 [in_external_macro]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/lint/fn.in_external_macro.html
510 [span]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html
511 [applicability]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/enum.Applicability.html
512 [rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/
513 [nightly_docs]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/
514 [ast]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/index.html
515 [ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/sty/index.html
516 [Zulip]: https://rust-lang.zulipchat.com/#narrow/stream/clippy