# 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](#contributing-to-clippy)
  - [The Clippy book](#the-clippy-book)
  - [High level approach](#high-level-approach)
  - [Finding something to fix/improve](#finding-something-to-fiximprove)
  - [Getting code-completion for rustc internals to work](#getting-code-completion-for-rustc-internals-to-work)
    - [RustRover](#rustrover)
    - [Rust Analyzer](#rust-analyzer)
  - [How Clippy works](#how-clippy-works)
  - [Issue and PR triage](#issue-and-pr-triage)
  - [Contributions](#contributions)
  - [License](#license)

[Zulip]: https://rust-lang.zulipchat.com/#narrow/stream/clippy
[Rust Code of Conduct]: https://www.rust-lang.org/policies/code-of-conduct

## 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.

[Clippy book]: https://doc.rust-lang.org/nightly/clippy/index.html
[developer guide]: https://doc.rust-lang.org/nightly/clippy/development/index.html

## 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 [Basics docs](book/src/development/basics.md)
   to get set up
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 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](https://github.com/rust-lang/rust-clippy/blob/master/triagebot.toml#L18)

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][deep-nesting].
But we can make it nest-less by using [let chains], [like this][nest-less].

[`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).

[`good first issue`]: https://github.com/rust-lang/rust-clippy/labels/good%20first%20issue
[`S-inactive-closed`]: https://github.com/rust-lang/rust-clippy/pulls?q=is%3Aclosed+label%3AS-inactive-closed
[`T-AST`]: https://github.com/rust-lang/rust-clippy/labels/T-AST
[`T-middle`]: https://github.com/rust-lang/rust-clippy/labels/T-middle
[`E-medium`]: https://github.com/rust-lang/rust-clippy/labels/E-medium
[`ty`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty
[nodes in the AST docs]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/
[deep-nesting]: https://github.com/rust-lang/rust-clippy/blob/5e4f0922911536f80d9591180fa604229ac13939/clippy_lints/src/mem_forget.rs#L31-L45
[let chains]: https://github.com/rust-lang/rust/pull/94927
[nest-less]: https://github.com/rust-lang/rust-clippy/blob/5e4f0922911536f80d9591180fa604229ac13939/clippy_lints/src/bit_mask.rs#L133-L159

## Getting code-completion for rustc internals to work

### RustRover
Since [`RustRover`][RustRover_homepage] 2026.1, no additional setup is required.
Just open the project in RustRover as usual.

[RustRover_homepage]: https://www.jetbrains.com/rust/

### Rust Analyzer
For [`rust-analyzer`][ra_homepage] to work correctly make sure that in the `rust-analyzer` configuration you set

```json
{ "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

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

[ra_homepage]: https://rust-analyzer.github.io/

## How Clippy works

[`clippy_lints/src/lib.rs`][lint_crate_entry] imports all the different lint modules and registers them in the
[`LintStore`]. All early passes are folded into a single `CombinedEarlyLintPass`, and all late passes into a single
`CombinedLateLintPass`, each registered once with the store. A pass is added to one of these by listing it in the
`early_lint_methods!` or `late_lint_methods!` macro invocation. For example, the
[`else_if_without_else`][else_if_without_else] lint is added like this:

```rust
// ./clippy_lints/src/lib.rs

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

rustc_lint::early_lint_methods!(
    crate::combined_early_lint_pass,
    [CombinedEarlyLintPass, (/* ... */), [
        // ...
        ElseIfWithoutElse: else_if_without_else::ElseIfWithoutElse = else_if_without_else::ElseIfWithoutElse,
        // ...
    ]]
);
```

Each entry has the form `Field: Type = constructor`, where the constructor builds the pass (passing `conf` when the pass
needs the user configuration). The combined passes implement [`EarlyLintPass`][early_lint_pass] and
[`LateLintPass`][late_lint_pass] respectively, so each listed pass must also implement the matching trait. 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.

```rust
// ./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 is listed in `early_lint_methods!`. Because the
[actual lint logic][else_if_without_else] does not depend on any type information.

[lint_crate_entry]: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/lib.rs
[else_if_without_else]: https://github.com/rust-lang/rust-clippy/blob/4253aa7137cb7378acc96133c787e49a345c2b3c/clippy_lints/src/else_if_without_else.rs
[`LintStore`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/struct.LintStore.html
[early_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html
[late_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.LateLintPass.html

## Issue and PR triage

Clippy is following the [Rust triage procedure][triage] 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][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][search_ice], 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][sugg_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][sugg_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][false_positive], 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.

[triage]: https://forge.rust-lang.org/release/triage-procedure.html
[search_ice]: https://github.com/rust-lang/rust-clippy/issues?q=sort%3Aupdated-desc+state%3Aopen+label%3A%22I-ICE%22
[sugg_causes_bug]: https://github.com/rust-lang/rust-clippy/issues?q=sort%3Aupdated-desc%20state%3Aopen%20label%3AI-suggestion-causes-bug
[sugg_causes_error]: https://github.com/rust-lang/rust-clippy/issues?q=sort%3Aupdated-desc%20state%3Aopen%20label%3AI-suggestion-causes-error%20
[false_positive]: https://github.com/rust-lang/rust-clippy/issues?q=sort%3Aupdated-desc%20state%3Aopen%20label%3AI-false-positive
[tracking_issue]: https://github.com/rust-lang/rust-clippy/issues/15086

## 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
```

[changelog]: CHANGELOG.md

## License

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

<!-- adapted from https://github.com/servo/servo/blob/master/CONTRIBUTING.md -->

[Apache-2.0]: https://www.apache.org/licenses/LICENSE-2.0
[MIT]: https://opensource.org/licenses/MIT
