To get started, we will create a lint that detects functions called `foo`,
because that's clearly a non-descriptive name.
+* [Setup](#Setup)
* [Testing](#Testing)
+* [Rustfix tests](#Rustfix-tests)
* [Lint declaration](#Lint-declaration)
* [Lint passes](#Lint-passes)
* [Emitting a lint](#Emitting-a-lint)
* [Adding the lint logic](#Adding-the-lint-logic)
+* [Author lint](#Author-lint)
* [Documentation](#Documentation)
+* [Running rustfmt](#Running-rustfmt)
* [Debugging](#Debugging)
* [PR Checklist](#PR-Checklist)
* [Cheatsheet](#Cheatsheet)
+### 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.
+
### Testing
Let's write some tests first that we can execute while we iterate on our lint.
Clippy uses UI tests for testing. UI tests check that the output of Clippy is
exactly as expected. Each test is just a plain Rust file that contains the code
we want to check. The output of Clippy is compared against a `.stderr` file.
+Note that you don't have to create this file yourself, we'll get to
+generating the `.stderr` files further down.
-Let's create the test file at `tests/ui/foo_functions.rs`. It doesn't really
-matter what the file is called, but it's a good convention to name it after the
-lint it is testing, so `foo_functions.rs` it is.
+We start by creating the test file at `tests/ui/foo_functions.rs`. It doesn't
+really matter what the file is called, but it's a good convention to name it
+after the lint it is testing, so `foo_functions.rs` it is.
-Inside we put some examples to get started:
+Inside the file we put some examples to get started:
```rust
#![warn(clippy::foo_functions)]
Now we can run the test with `TESTNAME=ui/foo_functions cargo uitest`.
Currently this test will fail. If you go through the output you will see that we
-have to add some missing imports to our lint file.
+are told that `clippy::foo_functions` is an unknown lint, which is expected.
+
+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.
+
+Once we are satisfied with the output, we need to run
+`tests/ui/update-all-references.sh` to update the `.stderr` file for our lint.
+Running `TESTNAME=ui/foo_functions cargo uitest` should pass then. When we
+commit our lint, we need to commit the generated `.stderr` files, too.
+
+### Rustfix tests
-While you are working on implementing your lint, you can keep running the UI
-test. That allows you to check if the output is turning into what you want.
+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.
-Once you are satisfied with the output, you need to run
-`tests/ui/update-all-references.sh` to update the `stderr` file for your lint.
-Running `TESTNAME=ui/foo_functions cargo uitest` should pass then. When you
-commit your lint, be sure to commit the `*.stderr` files, too.
+Use `tests/ui/update-all-references.sh` to automatically generate the
+`.fixed` file after running the tests.
-Let's have a look at implementing our lint now.
+With tests in place, let's have a look at implementing our lint now.
+
+### 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
+your local modifications, run `env CLIPPY_TESTS=true cargo run --bin
+clippy-driver -- -L ./target/debug input.rs` from the working copy root.
### Lint declaration
`clippy_lints/src/foo_functions.rs` and import some initial things we need:
```rust
-use rustc::lint::{LintArray, LintPass};
-use rustc::{declare_tool_lint, lint_array};
+use rustc::lint::{LintArray, LintPass, EarlyLintPass};
+use rustc::{declare_lint_pass, declare_tool_lint};
```
The next step is to provide a lint declaration. Lints are declared using the
* The last part should be a text that explains what exactly is wrong with the
code
-With our lint declaration done, we will now make sure that our lint declaration
-is assigned to a lint pass:
+With our lint declaration done, we will now make sure that it is assigned to a
+lint pass:
```rust
// clippy_lints/src/foo_functions.rs
// .. imports and lint declaration ..
-#[derive(Copy, Clone)]
-pub struct FooFunctionsPass;
+declare_lint_pass!(FooFunctions => [FOO_FUNCTIONS]);
-impl LintPass for FooFunctionsPass {
- fn get_lints(&self) -> LintArray {
- lint_array!(
- FOO_FUNCTIONS,
- )
- }
-
- fn name(&self) -> &'static str {
- "FooFunctions"
- }
-}
+impl EarlyLintPass for FooFunctions {}
```
Don't worry about the `name` method here. As long as it includes the name of the
lint pass it should be fine.
-Next you should run `util/dev update_lints` to register the lint in various
+Next we need to run `util/dev update_lints` to register the lint in various
places, mainly in `clippy_lints/src/lib.rs`.
While `update_lints` automates some things, it doesn't automate everything. We
in `clippy_lints/src/lib.rs`:
```rust
-reg.register_early_lint_pass(box foo_functions::FooFunctionsPass);
+reg.register_early_lint_pass(box foo_functions::FooFunctions);
```
-Without that, running the UI tests would produce an error like `unknown clippy
-lint: clippy::foo_functions`. The next decision we have to make is which lint
-pass our lint is going to need.
+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.
### Lint passes
-Writing a lint that just checks for the name of a function means that we just
+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.
This is good, because it makes writing this particular lint less complicated.
We have to make this decision with every new Clippy lint. It boils down to using
either [`EarlyLintPass`][early_lint_pass] or [`LateLintPass`][late_lint_pass].
-This is a result of Rust's compilation process. You can read more about it [in
-the rustc guide][compilation_stages].
In short, the `LateLintPass` has access to type information while the
`EarlyLintPass` doesn't. If you don't need access to type information, use the
### Emitting a lint
-With UI tests in place, we can start working on the implementation of the lint logic. We can keep executing the tests until we make them pass.
+With UI tests and the lint declaration in place, we can start working on the
+implementation of the lint logic.
-Let's start by implementing the `EarlyLintPass` for our `FooFunctionsPass`:
+Let's start by implementing the `EarlyLintPass` for our `FooFunctions`:
```rust
-impl EarlyLintPass for FooFunctionsPass {
+impl EarlyLintPass for FooFunctions {
fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, _: &FnDecl, span: Span, _: NodeId) {
// TODO: Emit lint here
}
*every* function definition first.
Depending on how complex we want our lint message to be, we can choose from a
-variety of lint emission functions. They can all be found in
+variety of lint emission functions. They can all be found in
[`clippy_lints/src/utils/diagnostics.rs`][diagnostics].
+`span_help_and_lint` seems most appropriate in this case. It allows us to
+provide an extra help message and we can't really suggest a better name
+automatically. This is how it looks:
```rust
-impl EarlyLintPass for Pass {
+impl EarlyLintPass for FooFunctions {
fn check_fn(&mut self, cx: &EarlyContext<'_>, _: FnKind<'_>, _: &FnDecl, span: Span, _: NodeId) {
span_help_and_lint(
cx,
}
```
+Running our UI test should now produce output that contains the lint message.
+
### Adding the lint logic
-Writing the logic for your lint will most likely be different from this example,
+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]
With that we can expand our `check_fn` method to:
```rust
-impl EarlyLintPass for Pass {
+impl EarlyLintPass for FooFunctions {
fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, _: &FnDecl, span: Span, _: NodeId) {
if is_foo_fn(fn_kind) {
span_help_and_lint(
We separate the lint conditional from the lint emissions because it makes the
code a bit easier to read. In some cases this separation would also allow to
-write some unit tests (as opposed to UI tests) for the separate function.
+write some unit tests (as opposed to only UI tests) for the separate function.
In our example, `is_foo_fn` looks like:
}
```
-Now you'll want to also run the full test suite with `cargo test`. Apart from
-running all other UI tests, this ensures that our lint implementation is not
-violating any Clippy lints itself.
+Now we should also run the full test suite with `cargo test`. At this point
+running `cargo test` should produce the expected output. Remember to run
+`tests/ui/update-all-references.sh` to update the `.stderr` file.
-If you are still following the example, you'll see that the `FooFunctionsPass`
-violates a Clippy lint. So we are going to rename that struct to just `Pass`:
+`cargo test` (as opposed to `cargo uitest`) will also ensure that our lint
+implementation is not violating any Clippy lints itself.
-```rust
-#[derive(Copy, Clone)]
-pub struct Pass;
+That should be it for the lint implementation. Running `cargo test` should now
+pass.
-impl LintPass for Pass { /* .. */ }
-```
+### Author lint
-That should be it for the lint implementation. Running `cargo test` should now
-pass and we can finish up our work by adding some documentation.
+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).
+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.
+
+[Here][author_example] is an example on the playground.
+
+If the command was executed successfully, you can copy the code over to where
+you are implementing your lint.
### Documentation
Please document your lint with a doc comment akin to the following:
```rust
-/// **What it does:** Checks for ... (describe what the lint matches).
-///
-/// **Why is this bad?** Supply the reason for linting the code.
-///
-/// **Known problems:** None. (Or describe where it could go wrong.)
-///
-/// **Example:**
-///
-/// ```rust,ignore
-/// // Bad
-/// Insert a short example of code that triggers the lint
-///
-/// // Good
-/// Insert a short example of improved code that doesn't trigger the lint
-/// ```
-declare_clippy_lint! { /* ... */ }
+declare_clippy_lint! {
+ /// **What it does:** Checks for ... (describe what the lint matches).
+ ///
+ /// **Why is this bad?** Supply the reason for linting the code.
+ ///
+ /// **Known problems:** None. (Or describe where it could go wrong.)
+ ///
+ /// **Example:**
+ ///
+ /// ```rust,ignore
+ /// // Bad
+ /// Insert a short example of code that triggers the lint
+ ///
+ /// // Good
+ /// Insert a short example of improved code that doesn't trigger the lint
+ /// ```
+ pub FOO_FUNCTIONS,
+ pedantic,
+ "function named `foo`, which is not a descriptive name"
+}
```
Once your lint is merged, this documentation will show up in the [lint
list][lint_list].
+### 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.
+
+It can be installed via `rustup`:
+
+```bash
+rustup component add rustfmt
+```
+
+Use `cargo fmt --all` to format the whole codebase.
+
### Debugging
If you want to debug parts of your lint implementation, you can use the `dbg!`
### PR Checklist
-TODO: Prose
+Before submitting your PR make sure you followed all of the basic requirements:
- [ ] Followed [lint naming conventions][lint_naming]
- [ ] Added passing UI tests (including committed `.stderr` file)
- [ ] `cargo test` passes locally
+- [ ] Executed `util/dev update_lints`
- [ ] Added lint documentation
+- [ ] Run `cargo fmt`
### Cheatsheet
Here are some pointers to things you are likely going to need for every lint:
+* [Clippy utils][utils] - Various helper functions. Maybe the function you need
+ is already in here (`implements_trait`, `match_path`, `snippet`, etc)
+* [Clippy diagnostics][diagnostics]
* [The `if_chain` macro][if_chain]
* [`in_macro`][in_macro] and [`in_external_macro`][in_external_macro]
* [`Span`][span]
-* [Clippy diagnostics][diagnostics]
* [`Applicability`][applicability]
+* [The rustc guide][rustc_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:
[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/syntax_pos/struct.Span.html
[applicability]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/enum.Applicability.html
[ast]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/ast/index.html
[in_macro]: https://github.com/rust-lang/rust-clippy/blob/d0717d1f9531a03d154aaeb0cad94c243915a146/clippy_lints/src/utils/mod.rs#L94
[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
+[rustc_guide]: https://rust-lang.github.io/rustc-guide/
+[nightly_docs]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/
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