Contributing to rust-clippy

Hello fellow Rustacean! Great to see your interest in compiler internals and lints!

Getting started

High level approach:

Find something to fix/improve
Change code (likely some file in clippy_lints/src/)
Run cargo test in the root directory and wiggle code until it passes
Open a PR (also can be done between 2. and 3. if you run into problems)

Finding something to fix/improve

All issues on Clippy are mentored, if you want help with a bug just ask @Manishearth, @llogiq, @mcarton or @oli-obk.

Some issues are easier than others. The good first issue label can be used to find the easy issues. If you want to work on an issue, please leave a comment so that we can assign it to you!

Issues marked T-AST involve simple matching of the syntax tree structure, and are generally easier than T-middle issues, which involve types and resolved paths.

T-AST issues will generally need you to match against a predefined syntax structure. To figure out how this syntax structure is encoded in the AST, it is recommended to run rustc -Z ast-json on an example of the structure and compare with the nodes in the AST docs. Usually the lint will end up to be a nested series of matches and ifs, like so.

E-medium issues are generally pretty easy too, though it's recommended you work on an E-easy issue first. They are mostly classified as E-medium, since they might be somewhat involved code wise, but not difficult per-se.

T-middle issues can be more involved and require verifying types. The ty module contains a lot of methods that are useful, though one of the most useful would be expr_ty (gives the type of an AST expression). match_def_path() in Clippy's utils module can also be useful.

Writing code

Compiling clippy from scratch can take almost a minute or more depending on your machine. However, since Rust 1.24.0 incremental compilation is enabled by default and compile times for small changes should be quick.

Llogiq's blog post on lints is a nice primer to lint-writing, though it does get into advanced stuff. Most lints consist of an implementation of LintPass with one or more of its default methods overridden. See the existing lints for examples of this.

Author 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.

Create a new UI test with the pattern you want to match:

// ./tests/ui/my_lint.rs

// The custom_attribute needs to be enabled for the author lint to work
#![feature(plugin, custom_attribute)]

fn main() {
    #[clippy(author)]
    let arr: [i32; 1] = [7]; // Replace line with the code you want to match
}

Now you run TESTNAME=ui/my_lint cargo test --test compile-test to produce the file with the generated code:

// ./tests/ui/my_lint.stdout

if_chain! {
    if let Expr_::ExprArray(ref elements) = stmt.node;
    if elements.len() == 1;
    if let Expr_::ExprLit(ref lit) = elements[0].node;
    if let LitKind::Int(7, _) = lit.node;
    then {
        // report your lint here
    }
}

Documentation

Please document your lint with a doc comment akin to the following:

/// **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
/// // 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
/// ```

Once your lint is merged it will show up in the lint list

Running test suite

Clippy uses UI tests. UI tests check that the output of the compiler is exactly as expected. Of course there's little sense in writing the output yourself or copying it around. Therefore you can simply run tests/ui/update-all-references.sh (after running cargo test) and check whether the output looks as you expect with git diff. Commit all *.stderr files, too.

If you don't want to wait for all tests to finish, you can also execute a single test file by using TESTNAME to specify the test to run:

TESTNAME=ui/empty_line_after_outer_attr cargo test --test compile-test

Testing manually

Manually testing against an example file is useful if you have added some println!s and test suite output becomes unreadable. To try clippy with your local modifications, run cargo run --bin clippy-driver -- -L ./target/debug input.rs from the working copy root. Your test file, here input.rs, needs to have clippy enabled as a plugin:

#![feature(plugin)]
#![plugin(clippy)]

Contributions

Clippy welcomes contributions from everyone.

Contributions to Clippy should be made in the form of GitHub pull requests. Each pull request will be reviewed by a core contributor (someone with permission to land patches) and either landed in the main tree or given feedback for changes that would be required.

All code in this repository is under the Mozilla Public License, 2.0

Conduct

We follow the Rust Code of Conduct.

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