rust/CONTRIBUTING.md

# 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](#getting-started)
  * [Finding something to fix/improve](#finding-something-to-fiximprove)
* [Writing code](#writing-code)
* [How Clippy works](#how-clippy-works)
* [Fixing nightly build failures](#fixing-build-failures-caused-by-rust)
* [Issue and PR Triage](#issue-and-pr-triage)
* [Bors and Homu](#bors-and-homu)
* [Contributions](#contributions)

[Discord]: https://discord.gg/rust-lang
[Rust Code of Conduct]: https://www.rust-lang.org/policies/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](doc/adding_lints.md) 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][deep-nesting].
But we can make it nest-less by using [if_chain] macro, [like this][nest-less].

[`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.

[`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/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/mem_forget.rs#L29-L43
[if_chain]: https://docs.rs/if_chain/*/if_chain
[nest-less]: https://github.com/rust-lang/rust-clippy/blob/557f6848bd5b7183f55c1e1522a326e9e1df6030/clippy_lints/src/bit_mask.rs#L124-L150

## Writing code

Have a look at the [docs for writing lints][adding_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][rfc_stability] and
[lint categories][rfc_lint_cats] sections of the [Clippy 1.0 RFC][clippy_rfc] a
quick read.

[adding_lints]: https://github.com/rust-lang/rust-clippy/blob/master/doc/adding_lints.md
[clippy_rfc]: https://github.com/rust-lang/rfcs/blob/master/text/2476-clippy-uno.md
[rfc_stability]: https://github.com/rust-lang/rfcs/blob/master/text/2476-clippy-uno.md#stability-guarantees
[rfc_lint_cats]: https://github.com/rust-lang/rfcs/blob/master/text/2476-clippy-uno.md#lint-audit-and-categories

## How Clippy works

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

```rust
// ./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`][`LintStore`] provides two methods to register lints:
[register_early_pass][reg_early_pass] and [register_late_pass][reg_late_pass]. Both take an object
that implements an [`EarlyLintPass`][early_lint_pass] or [`LateLintPass`][late_lint_pass] 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.

```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 uses the `register_early_pass` function. 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
[reg_early_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/struct.LintStore.html#method.register_early_pass
[reg_late_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/struct.LintStore.html#method.register_late_pass
[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

## Fixing build failures caused by Rust

Clippy currently gets build 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 `subtree`s in the Rust repository see [Rust's
`CONTRIBUTING.md`][subtree].

Here is a TL;DR version of the sync process:

1. Clone the [`rust-lang/rust`] repository (all of the following commands have
   to be run inside the `rust` directory)
2. Sync the changes to the copy of Clippy to your fork of the Clippy repository:
    ```bash
    # 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
    ```
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 copy of Clippy:
    ```bash
    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`]

[subtree]: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#external-dependencies-subtree
[`rust-lang/rust`]: https://github.com/rust-lang/rust

## 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.

Our highest priority is fixing [crashes][l-crash] and [bugs][l-bug]. 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][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][homu_queue].

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

[triage]: https://forge.rust-lang.org/release/triage-procedure.html
[l-crash]: https://github.com/rust-lang/rust-clippy/labels/L-crash%20%3Aboom%3A
[l-bug]: https://github.com/rust-lang/rust-clippy/labels/L-bug%20%3Abeetle%3A
[homu]: https://github.com/rust-lang/homu
[homu_instructions]: https://buildbot2.rust-lang.org/homu/
[homu_queue]: https://buildbot2.rust-lang.org/homu/queue/clippy

## 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.

<!-- 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