Summary

This is the second PR out of three that adds support for enabling/disabling lint rules in Red Knot. You may want to take a look at the first PR in this stack to familiarize yourself with the used terminology.

This PR adds a new syntax to define a lint:

declare_lint! {
    /// ## What it does
    /// Checks for references to names that are not defined.
    ///
    /// ## Why is this bad?
    /// Using an undefined variable will raise a `NameError` at runtime.
    ///
    /// ## Example
    ///
    /// ```python
    /// print(x)  # NameError: name 'x' is not defined
    /// ```
    pub(crate) static UNRESOLVED_REFERENCE = {
        summary: "detects references to names that are not defined",
        status: LintStatus::preview("1.0.0"),
        default_level: Level::Warn,
    }
}

A lint has a name and metadata about its status (preview, stable, removed, deprecated), the default diagnostic level (unless the configuration changes), and documentation. I use a macro here to derive the kebab-case name and extract the documentation automatically.

This PR doesn't yet add any mechanism to discover all known lints. This will be added in the next and last PR in this stack.

Documentation

I documented some rules but then decided that it's probably not my best use of time if I document all of them now (it also means that I play catch-up with all of you forever). That's why I left some rules undocumented (marked with TODO)

Where is the best place to define all lints?

I'm not sure. I think what I have in this PR is fine but I also don't love it because most lints are in a single place but not all of them. If you have ideas, let me know.

Why is the message not part of the lint, unlike Ruff's Violation

I understand that the main motivation for defining message on Violation in Ruff is to remove the need to repeat the same message over and over again. I'm not sure if this is an actual problem. Most rules only emit a diagnostic in a single place and they commonly use different messages if they emit diagnostics in different code paths, requiring extra fields on the Violation struct.

That's why I'm not convinced that there's an actual need for it and there are alternatives that can reduce the repetition when creating a diagnostic:

• Create a helper function. We already do this in red knot with the add_xy methods
• Create a custom Diagnostic implementation that tailors the entire diagnostic and pre-codes e.g. the message

Avoiding an extra field on the Violation also removes the need to allocate intermediate strings as it is commonly the place in Ruff. Instead, Red Knot can use a borrowed string with format_args

Test Plan

cargo test