Contributing to Clippy

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

First: if you're unsure or afraid of anything, just ask or submit the issue or pull request anyway. You won't be yelled at for giving it your best effort. The worst that can happen is that you'll be politely asked to change something. We appreciate any sort of contributions, and don't want a wall of rules to get in the way of that.

Clippy welcomes contributions from everyone. There are many ways to contribute to Clippy and the following document explains how you can contribute and how to get started. If you have any questions about contributing or need help with anything, feel free to ask questions on issues or visit the #clippy on Zulip.

All contributors are expected to follow the Rust Code of Conduct.

Contributing to Clippy

The Clippy book

If you're new to Clippy and don't know where to start, the Clippy book includes a developer guide and is a good place to start your journey.

High level approach

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

Finding something to fix/improve

All issues on Clippy are mentored, if you want help simply ask someone from the Clippy team directly by mentioning them in the issue or over on Zulip. All currently active team members can be found here

Some issues are easier than others. The good first issue label can be used to find the easy issues. You can use @rustbot claim to assign the issue to yourself.

There are also some abandoned PRs, marked with S-inactive-closed. Pretty often these PRs are nearly completed and just need some extra steps (formatting, addressing review comments, ...) to be merged. If you want to complete such a PR, please leave a comment in the PR and open a new one based on it.

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 unpretty=ast-tree 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. But we can make it nest-less by using let chains, like this.

E-medium issues are generally pretty easy too, though it's recommended you work on an good first issue first. Sometimes they are only somewhat involved code wise, but not difficult per-se. Note that E-medium issues may require some knowledge of Clippy internals or some debugging to find the actual problem behind the issue.

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).

Getting code-completion for rustc internals to work

RustRover

Unfortunately, RustRover does not (yet?) understand how Clippy uses compiler-internals using extern crate and it also needs to be able to read the source files of the rustc-compiler which are not available via a rustup component at the time of writing. To work around this, you need to have a copy of the rustc-repo available which can be obtained via git clone https://github.com/rust-lang/rust/. Then you can run a cargo dev command to automatically make Clippy use the rustc-repo via path-dependencies which RustRover will be able to understand. Run cargo dev setup intellij --repo-path <repo-path> where <repo-path> is a path to the rustc repo you just cloned. The command will add path-dependencies pointing towards rustc-crates inside the rustc repo to Clippy's Cargo.tomls and should allow RustRover to understand most of the types that Clippy uses. Just make sure to remove the dependencies again before finally making a pull request!

Rust Analyzer

For rust-analyzer to work correctly make sure that in the rust-analyzer configuration you set

{ "rust-analyzer.rustc.source": "discover" }

You should be able to see information on things like Expr or EarlyContext now if you hover them, also a lot more type hints.

To have rust-analyzer also work in the clippy_dev and lintcheck crates, add the following configuration

{
    "rust-analyzer.linkedProjects": [
        "./Cargo.toml",
        "clippy_dev/Cargo.toml",
        "lintcheck/Cargo.toml",
    ]
}

How Clippy works

clippy_lints/src/lib.rs imports all the different lint modules and registers in the LintStore. For example, the else_if_without_else lint is registered like this:

// ./clippy_lints/src/lib.rs

// ...
pub mod else_if_without_else;
// ...

pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
    // ...
    store.register_early_pass(|| Box::new(else_if_without_else::ElseIfWithoutElse));
    // ...
}

The rustc_lint::LintStore provides two methods to register lints: register_early_pass and register_late_pass. Both take an object that implements an EarlyLintPass or LateLintPass respectively. This is done in every single lint. It's worth noting that the majority of clippy_lints/src/lib.rs is autogenerated by cargo dev update_lints. When you are writing your own lint, you can use that script to save you some time.

// ./clippy_lints/src/else_if_without_else.rs

use rustc_lint::{EarlyLintPass, EarlyContext};

// ...

pub struct ElseIfWithoutElse;

// ...

impl EarlyLintPass for ElseIfWithoutElse {
    // ... the functions needed, to make the lint work
}

The difference between EarlyLintPass and LateLintPass is that the methods of the EarlyLintPass trait only provide AST information. The methods of the LateLintPass trait are executed after type checking and contain type information via the LateContext parameter.

That's why the else_if_without_else example uses the register_early_pass function. Because the actual lint logic does not depend on any type information.

Issue and PR triage

Clippy is following the Rust triage procedure for issues and pull requests.

However, we are a smaller project with all contributors being volunteers currently. Between writing new lints, fixing issues, reviewing pull requests and responding to issues there may not always be enough time to stay on top of it all.

To find things to fix, go to the tracking issue, find an issue that you like, and claim it with @rustbot claim.

As a general metric and always taking into account your skill and knowledge level, you can use this guide:

🟥 ICEs, these are compiler errors that causes Clippy to panic and crash. Usually involves high-level debugging, sometimes interacting directly with the upstream compiler. Difficult to fix but a great challenge that improves a lot developer workflows!
🟧 Suggestion causes bug, Clippy suggested code that changed logic in some silent way. Unacceptable, as this may have disastrous consequences. Easier to fix than ICEs
🟨 Suggestion causes error, Clippy suggested code snippet that caused a compiler error when applied. We need to make sure that Clippy doesn't suggest using a variable twice at the same time or similar easy-to-happen occurrences.
🟩 False positives, a lint should not have fired, the easiest of them all, as this is "just" identifying the root of a false positive and making an exception for those cases.

Note that false negatives do not have priority unless the case is very clear, as they are a feature-request in a trench coat.

Contributions

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 PRs should include a changelog entry with a short comment explaining the change. The rule of thumb is basically, "what do you believe is important from an outsider's perspective?" Often, PRs are only related to a single property of a lint, and then it's good to mention that one. Otherwise, it's better to include too much detail than too little.

Clippy's changelog is created from these comments. Every release, someone gets all merge commits with a changelog: XYZ entry and combines them into the changelog. This is a manual process.

Examples:

New lint

changelog: new lint: [`missing_trait_methods`]

False positive fix

changelog: Fix [`unused_peekable`] false positive when peeked in a closure or called as `f(&mut peekable)`

Purely internal change
```
changelog: none
```

Note this it is fine for a PR to include multiple changelog entries, e.g.:

changelog: Something 1
changelog: Something 2
changelog: Something 3

License

All code in this repository is under the Apache-2.0 or the MIT license.

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