-## Adding a new lint
+# Adding a new lint
You are probably here because you want to add a new lint to Clippy. If this is
the first time you're contributing to Clippy, this document guides you through
- [PR Checklist](#pr-checklist)
- [Cheatsheet](#cheatsheet)
-### Setup
+## Setup
When working on Clippy, you will need the current git master version of rustc,
which can change rapidly. Make sure you're working near rust-clippy's master,
and use the `setup-toolchain.sh` script to configure the appropriate toolchain
for the Clippy directory.
-### Getting Started
+## Getting Started
There is a bit of boilerplate code that needs to be set up when creating a new
lint. Fortunately, you can use the clippy dev tools to handle this for you. We
`cargo dev new_lint --name=foo_functions --pass=early --category=pedantic`
(category will default to nursery if not provided). This command will create
two files: `tests/ui/foo_functions.rs` and `clippy_lints/src/foo_functions.rs`,
-as well as run `cargo dev update_lints` to register the new lint. Next, we'll
-open up these files and add our lint!
+as well as run `cargo dev update_lints` to register the new lint. For cargo lints,
+two project hierarchies (fail/pass) will be created by default under `tests/ui-cargo`.
-### Testing
+Next, we'll open up these files and add our lint!
+
+## Testing
Let's write some tests first that we can execute while we iterate on our lint.
let a = A;
a.foo();
}
-
```
-Now we can run the test with `TESTNAME=foo_functions cargo uitest`.
-Currently this test will fail. If you go through the output you will see that we
-are told that `clippy::foo_functions` is an unknown lint, which is expected.
+Now we can run the test with `TESTNAME=foo_functions cargo uitest`,
+currently this test is meaningless though.
While we are working on implementing our lint, we can keep running the UI
test. That allows us to check if the output is turning into what we want.
Please note that, we should run `TESTNAME=foo_functions cargo uitest`
every time before running `tests/ui/update-all-references.sh`.
Running `TESTNAME=foo_functions cargo uitest` should pass then. When we commit
-our lint, we need to commit the generated `.stderr` files, too.
+our lint, we need to commit the generated `.stderr` files, too. In general, you
+should only commit files changed by `tests/ui/update-all-references.sh` for the
+specific lint you are creating/editing.
+
+### Cargo lints
+
+For cargo lints, the process of testing differs in that we are interested in
+the `Cargo.toml` manifest file. We also need a minimal crate associated
+with that manifest.
+
+If our new lint is named e.g. `foo_categories`, after running `cargo dev new_lint`
+we will find by default two new crates, each with its manifest file:
+
+* `tests/ui-cargo/foo_categories/fail/Cargo.toml`: this file should cause the new lint to raise an error.
+* `tests/ui-cargo/foo_categories/pass/Cargo.toml`: this file should not trigger the lint.
+
+If you need more cases, you can copy one of those crates (under `foo_categories`) and rename it.
-### Rustfix tests
+The process of generating the `.stderr` file is the same, and prepending the `TESTNAME`
+variable to `cargo uitest` works too, but the script to update the references
+is in another path: `tests/ui-cargo/update-all-references.sh`.
+
+## Rustfix tests
If the lint you are working on is making use of structured suggestions, the
test file should include a `// run-rustfix` comment at the top. This will
-additionally run [rustfix](https://github.com/rust-lang-nursery/rustfix) for
-that test. Rustfix will apply the suggestions from the lint to the code of the
-test file and compare that to the contents of a `.fixed` file.
+additionally run [rustfix] for that test. Rustfix will apply the suggestions
+from the lint to the code of the test file and compare that to the contents of
+a `.fixed` file.
Use `tests/ui/update-all-references.sh` to automatically generate the
`.fixed` file after running the tests.
-### Edition 2018 tests
+[rustfix]: https://github.com/rust-lang/rustfix
+
+## Edition 2018 tests
Some features require the 2018 edition to work (e.g. `async_await`), but
compile-test tests run on the 2015 edition by default. To change this behavior
-add `// compile-flags: --edition 2018` at the top of the test file.
+add `// edition:2018` at the top of the test file (note that it's space-sensitive).
-### Testing manually
+## Testing manually
Manually testing against an example file can be useful if you have added some
`println!`s and the test suite output becomes unreadable. To try Clippy with
With tests in place, let's have a look at implementing our lint now.
-### Lint declaration
+## Lint declaration
Let's start by opening the new file created in the `clippy_lints` crate
at `clippy_lints/src/foo_functions.rs`. That's the crate where all the
```rust
use rustc_lint::{EarlyLintPass, EarlyContext};
use rustc_session::{declare_lint_pass, declare_tool_lint};
-use syntax::ast::*;
+use rustc_ast::ast::*;
```
The next step is to update the lint declaration. Lints are declared using the
```
* The section of lines prefixed with `///` constitutes the lint documentation
- section. This is the default documentation style and will be displayed at
- https://rust-lang.github.io/rust-clippy/master/index.html.
-* `FOO_FUNCTIONS` is the name of our lint. Be sure to follow the [lint naming
- guidelines][lint_naming] here when naming your lint. In short, the name should
- state the thing that is being checked for and read well when used with
- `allow`/`warn`/`deny`.
+ section. This is the default documentation style and will be displayed
+ [like this][example_lint_page].
+* `FOO_FUNCTIONS` is the name of our lint. Be sure to follow the
+ [lint naming guidelines][lint_naming] here when naming your lint.
+ In short, the name should state the thing that is being checked for and
+ read well when used with `allow`/`warn`/`deny`.
* `pedantic` sets the lint level to `Allow`.
The exact mapping can be found [here][category_level_mapping]
* The last part should be a text that explains what exactly is wrong with the
`register_plugins` function in `clippy_lints/src/lib.rs`:
```rust
-store.register_early_pass(box foo_functions::FooFunctions);
+store.register_early_pass(|| box foo_functions::FooFunctions);
```
-This should fix the `unknown clippy lint: clippy::foo_functions` error that we
-saw when we executed our tests the first time. The next decision we have to make
-is which lint pass our lint is going to need.
+[declare_clippy_lint]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L60
+[example_lint_page]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure
+[lint_naming]: https://rust-lang.github.io/rfcs/0344-conventions-galore.html#lints
+[category_level_mapping]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/lib.rs#L110
-### Lint passes
+## Lint passes
Writing a lint that only checks for the name of a function means that we only
have to deal with the AST and don't have to deal with the type system at all.
`--pass=early` when running the new lint automation and all the imports were
added accordingly.
-### Emitting a lint
+[early_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html
+[late_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.LateLintPass.html
+
+## Emitting a lint
With UI tests and the lint declaration in place, we can start working on the
implementation of the lint logic.
```rust
impl EarlyLintPass for FooFunctions {
- fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, _: &FnDecl, span: Span, _: NodeId) {
+ fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
// TODO: Emit lint here
}
}
```rust
impl EarlyLintPass for FooFunctions {
- fn check_fn(&mut self, cx: &EarlyContext<'_>, _: FnKind<'_>, _: &FnDecl, span: Span, _: NodeId) {
+ fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
span_lint_and_help(
cx,
FOO_FUNCTIONS,
span,
"function named `foo`",
+ None,
"consider using a more meaningful name"
);
}
Running our UI test should now produce output that contains the lint message.
-### Adding the lint logic
+[check_fn]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html#method.check_fn
+[diagnostics]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/diagnostics.rs
+
+## Adding the lint logic
Writing the logic for your lint will most likely be different from our example,
so this section is kept rather short.
Using the [`check_fn`][check_fn] method gives us access to [`FnKind`][fn_kind]
-that has two relevant variants for us `FnKind::ItemFn` and `FnKind::Method`.
-Both provide access to the name of the function/method via an [`Ident`][ident].
+that has the [`FnKind::Fn`] variant. It provides access to the name of the
+function/method via an [`Ident`][ident].
With that we can expand our `check_fn` method to:
```rust
impl EarlyLintPass for FooFunctions {
- fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, _: &FnDecl, span: Span, _: NodeId) {
+ fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
if is_foo_fn(fn_kind) {
span_lint_and_help(
cx,
FOO_FUNCTIONS,
span,
"function named `foo`",
+ None,
"consider using a more meaningful name"
);
}
fn is_foo_fn(fn_kind: FnKind<'_>) -> bool {
match fn_kind {
- FnKind::ItemFn(ident, ..) | FnKind::Method(ident, ..) => {
- ident.name == "foo"
- },
+ FnKind::Fn(_, ident, ..) => {
+ // check if `fn` name is `foo`
+ ident.name.as_str() == "foo"
+ }
+ // ignore closures
FnKind::Closure(..) => false
}
}
That should be it for the lint implementation. Running `cargo test` should now
pass.
-### Author lint
+[fn_kind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html
+[`FnKind::Fn`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/enum.FnKind.html#variant.Fn
+[ident]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Ident.html
+
+## Author lint
If you have trouble implementing your lint, there is also the internal `author`
lint to generate Clippy code that detects the offending pattern. It does not
work for all of the Rust syntax, but can give a good starting point.
-The quickest way to use it, is the [Rust playground: play.rust-lang.org][Play].
+The quickest way to use it, is the
+[Rust playground: play.rust-lang.org][author_example].
Put the code you want to lint into the editor and add the `#[clippy::author]`
attribute above the item. Then run Clippy via `Tools -> Clippy` and you should
see the generated code in the output below.
If the command was executed successfully, you can copy the code over to where
you are implementing your lint.
-### Documentation
+[author_example]: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=9a12cb60e5c6ad4e3003ac6d5e63cf55
+
+## Documentation
The final thing before submitting our PR is to add some documentation to our
lint declaration.
Once your lint is merged, this documentation will show up in the [lint
list][lint_list].
-### Running rustfmt
+[lint_list]: https://rust-lang.github.io/rust-clippy/master/index.html
+
+## Running rustfmt
-[Rustfmt](https://github.com/rust-lang/rustfmt) is a tool for formatting Rust
-code according to style guidelines. Your code has to be formatted by `rustfmt`
-before a PR can be merged. Clippy uses nightly `rustfmt` in the CI.
+[Rustfmt] is a tool for formatting Rust code according to style guidelines.
+Your code has to be formatted by `rustfmt` before a PR can be merged.
+Clippy uses nightly `rustfmt` in the CI.
It can be installed via `rustup`:
Use `cargo dev fmt` to format the whole codebase. Make sure that `rustfmt` is
installed for the nightly toolchain.
-### Debugging
+[Rustfmt]: https://github.com/rust-lang/rustfmt
-If you want to debug parts of your lint implementation, you can use the `dbg!`
+## Debugging
+
+If you want to debug parts of your lint implementation, you can use the [`dbg!`]
macro anywhere in your code. Running the tests should then include the debug
output in the `stdout` part.
-### PR Checklist
+[`dbg!`]: https://doc.rust-lang.org/std/macro.dbg.html
+
+## PR Checklist
Before submitting your PR make sure you followed all of the basic requirements:
- [ ] Added lint documentation
- [ ] Run `cargo dev fmt`
-### Cheatsheet
+## Cheatsheet
Here are some pointers to things you are likely going to need for every lint:
* [`from_expansion`][from_expansion] and [`in_external_macro`][in_external_macro]
* [`Span`][span]
* [`Applicability`][applicability]
+* [Common tools for writing lints](common_tools_writing_lints.md) helps with common operations
* [The rustc-dev-guide][rustc-dev-guide] explains a lot of internal compiler concepts
* [The nightly rustc docs][nightly_docs] which has been linked to throughout
this guide
For `EarlyLintPass` lints:
* [`EarlyLintPass`][early_lint_pass]
-* [`syntax::ast`][ast]
+* [`rustc_ast::ast`][ast]
For `LateLintPass` lints:
* [`LateLintPass`][late_lint_pass]
* [`Ty::TyKind`][ty]
-
While most of Clippy's lint utils are documented, most of rustc's internals lack
documentation currently. This is unfortunate, but in most cases you can probably
get away with copying things from existing similar lints. If you are stuck,
-don't hesitate to ask on Discord, IRC or in the issue/PR.
+don't hesitate to ask on [Discord] or in the issue/PR.
-[lint_list]: https://rust-lang.github.io/rust-clippy/master/index.html
-[lint_naming]: https://rust-lang.github.io/rfcs/0344-conventions-galore.html#lints
-[category_level_mapping]: https://github.com/rust-lang/rust-clippy/blob/bd23cb89ec0ea63403a17d3fc5e50c88e38dd54f/clippy_lints/src/lib.rs#L43
-[declare_clippy_lint]: https://github.com/rust-lang/rust-clippy/blob/a71acac1da7eaf667ab90a1d65d10e5cc4b80191/clippy_lints/src/lib.rs#L39
-[check_fn]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/trait.EarlyLintPass.html#method.check_fn
-[early_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/trait.EarlyLintPass.html
-[late_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/trait.LateLintPass.html
-[fn_kind]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/visit/enum.FnKind.html
-[diagnostics]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/diagnostics.rs
[utils]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/mod.rs
-[ident]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/source_map/symbol/struct.Ident.html
-[span]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html
-[applicability]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/enum.Applicability.html
[if_chain]: https://docs.rs/if_chain/*/if_chain/
-[ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/ty/sty/index.html
-[ast]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/ast/index.html
[from_expansion]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html#method.from_expansion
-[in_external_macro]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/fn.in_external_macro.html
-[play]: https://play.rust-lang.org
-[author_example]: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=f093b986e80ad62f3b67a1f24f5e66e2
+[in_external_macro]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/lint/fn.in_external_macro.html
+[span]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html
+[applicability]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/enum.Applicability.html
[rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/
-[nightly_docs]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/
+[nightly_docs]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/
+[ast]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/index.html
+[ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/sty/index.html
+[Discord]: https://discord.gg/rust-lang
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