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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 Discord.

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

Getting started

High level approach:

  1. Find something to fix/improve
  2. Change code (likely some file in clippy_lints/src/)
  3. Follow the instructions in the docs for writing lints such as running the setup-toolchain.sh script
  4. Run cargo test in the root directory and wiggle code until it passes
  5. 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 with a bug just ask @Manishearth, @flip1995, @phansch or @yaahc.

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!

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 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. But we can make it nest-less by using if_chain macro, like this.

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

Have a look at the docs for writing lints for more details.

If you want to add a new lint or change existing ones apart from bugfixing, it's also a good idea to give the stability guarantees and lint categories sections of the Clippy 1.0 RFC a quick read.

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_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf: &Conf) {
    // ...
    store.register_early_pass(|| box else_if_without_else::ElseIfWithoutElse);
    // ...

    store.register_group(true, "clippy::restriction", Some("clippy_restriction"), vec![
        // ...
        LintId::of(&else_if_without_else::ELSE_IF_WITHOUT_ELSE),
        // ...
    ]);
}

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.

Fixing build failures caused by Rust

Clippy currently gets built with rustc of the rust-lang/rust master branch. Most of the times we have to adapt to the changes and only very rarely there's an actual bug in Rust.

If you decide to make Clippy work again with a Rust commit that breaks it, you have to sync the rust-lang/rust-clippy repository with the subtree copy of Clippy in the rust-lang/rust repository.

For general information about subtrees in the Rust repository see Rust's CONTRIBUTING.md.

Here is a TL;DR version of the sync process (all of the following commands have to be run inside the rust directory):

  1. Clone the rust-lang/rust repository

  2. Sync the changes to the rust-copy of Clippy to your Clippy fork:

    # Make sure to change `your-github-name` to your github name in the following command
git subtree push -P src/tools/clippy git@github.com:your-github-name/rust-clippy sync-from-rust

    Note: This will directly push to the remote repository. You can also push to your local copy by replacing the remote address with /path/to/rust-clippy directory.

    Note: Most of the time you have to create a merge commit in the rust-clippy repo (this has to be done in the Clippy repo, not in the rust-copy of Clippy):

    git fetch origin && git fetch upstream
git checkout sync-from-rust
git merge upstream/master

  3. Open a PR to rust-lang/rust-clippy and wait for it to get merged (to accelerate the process ping the @rust-lang/clippy team in your PR and/or annoy ask them in the Discord channel.)

  4. Sync the rust-lang/rust-clippy master to the rust-copy of Clippy:

    git checkout -b sync-from-clippy
git subtree pull -P src/tools/clippy https://github.com/rust-lang/rust-clippy master

  5. Open a PR to rust-lang/rust

Also, you may want to define remotes, so you don't have to type out the remote addresses on every sync. You can do this with the following commands (these commands still have to be run inside the rust directory):

# Set clippy-upstream remote for pulls
$ git remote add clippy-upstream https://github.com/rust-lang/rust-clippy
# Make sure to not push to the upstream repo
$ git remote set-url --push clippy-upstream DISABLED
# Set clippy-origin remote to your fork for pushes
$ git remote add clippy-origin git@github.com:your-github-name/rust-clippy
# Set a local remote
$ git remote add clippy-local /path/to/rust-clippy

You can then sync with the remote names from above, e.g.:

$ git subtree push -P src/tools/clippy clippy-local sync-from-rust

Note: The first time running git subtree push a cache has to be built. This involves going through the complete Clippy history once. For this you have to increase the stack limit though, which you can do with ulimit -s 60000. For this to work, you will need the fix of git subtree available here.

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.

Our highest priority is fixing crashes and bugs. We don't want Clippy to crash on your code and we want it to be as reliable as the suggestions from Rust compiler errors.

Bors and Homu

We use a bot powered by Homu to help automate testing and landing of pull requests in Clippy. The bot's username is @bors.

You can find the Clippy bors queue here.

If you have @bors permissions, you can find an overview of the available commands here.

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