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Contributing to rust-clippy

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

Getting started

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

Some issues are easier than others. The E-easy 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.

Issues marked E-medium are generally pretty easy too, though it's recommended you work on an E-easy issue first.

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.

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.

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.

Should you add a lint, try it on clippy itself using util/ You may find that clippy contains some questionable code itself! Also before making a pull request, please run util/, which will update and with the lint declarations. Our travis build actually checks for this.

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/ and check whether the output looks as you expect with git diff. Commit all *.stderr files, too.

Also 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
/// Insert a short example if you have one.
/// ```

Our util/ script can then add your lint docs to the wiki.


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


We follow the Rust Code of Conduct.