Hello fellow Rustacean! Great to see your interest in compiler internals and lints!
+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` IRC channel on `irc.mozilla.org`.
+
+All contributors are expected to follow the [Rust Code of Conduct](http://www.rust-lang.org/conduct.html).
+
## Getting started
-All issues on Clippy are mentored, if you want help with a bug just ask @Manishearth or @llogiq.
+High level approach:
+
+1. Find something to fix/improve
+2. Change code (likely some file in `clippy_lints/src/`)
+3. Run `cargo test` in the root directory and wiggle code until it passes
+4. Open a PR (also can be done between 2. and 3. if you run into problems)
-Some issues are easier than others. The [E-easy](https://github.com/Manishearth/rust-clippy/labels/E-easy)
+### Finding something to fix/improve
+
+All issues on Clippy are mentored, if you want help with a bug just ask @Manishearth, @llogiq, @mcarton or @oli-obk.
+
+Some issues are easier than others. The [`good first issue`](https://github.com/rust-lang-nursery/rust-clippy/labels/good%20first%20issue)
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!
-Issues marked [T-AST](https://github.com/Manishearth/rust-clippy/labels/T-AST) involve simple
+Issues marked [`T-AST`](https://github.com/rust-lang-nursery/rust-clippy/labels/T-AST) involve simple
matching of the syntax tree structure, and are generally easier than
-[T-middle](https://github.com/Manishearth/rust-clippy/labels/T-middle) issues, which involve types
+[`T-middle`](https://github.com/rust-lang-nursery/rust-clippy/labels/T-middle) issues, which involve types
and resolved paths.
-Issues marked [E-medium](https://github.com/Manishearth/rust-clippy/labels/E-medium) are generally
-pretty easy too, though it's recommended you work on an E-easy issue first.
+[`T-AST`](https://github.com/rust-lang-nursery/rust-clippy/labels/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](https://doc.rust-lang.org/nightly/nightly-rustc/syntax/ast). Usually
+the lint will end up to be a nested series of matches and ifs,
+[like so](https://github.com/rust-lang-nursery/rust-clippy/blob/de5ccdfab68a5e37689f3c950ed1532ba9d652a0/src/misc.rs#L34).
+
+[`E-medium`](https://github.com/rust-lang-nursery/rust-clippy/labels/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`](https://github.com/rust-lang-nursery/rust-clippy/labels/T-middle) issues can
+be more involved and require verifying types. The
+[`ty`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc/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
+
+Compiling clippy from scratch can take almost a minute or more depending on your machine.
+However, since Rust 1.24.0 incremental compilation is enabled by default and compile times for small changes should be quick.
[Llogiq's blog post on lints](https://llogiq.github.io/2015/06/04/workflows.html) is a nice primer
to lint-writing, though it does get into advanced stuff. Most lints consist of an implementation of
`LintPass` with one or more of its default methods overridden. See the existing lints for examples
of this.
-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](http://manishearth.github.io/rust-internals-docs/syntax/ast/). Usually
-the lint will end up to be a nested series of matches and ifs,
-[like so](https://github.com/Manishearth/rust-clippy/blob/de5ccdfab68a5e37689f3c950ed1532ba9d652a0/src/misc.rs#L34).
-T-middle issues can be more involved and require verifying types. The
-[`middle::ty`](http://manishearth.github.io/rust-internals-docs/rustc/middle/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.
+#### Author lint
+
+There is also the internal `author` lint to generate clippy code that detects the offending pattern. It does not work for all of the Rust syntax, but can give a good starting point.
+
+First, create a new UI test file in the `tests/ui/` directory with the pattern you want to match:
+
+```rust
+// ./tests/ui/my_lint.rs
-Should you add a lint, try it on clippy itself using `util/dogfood.sh`. You may find that clippy
-contains some questionable code itself! Also before making a pull request, please run
-`util/update_lints.py`, which will update `lib.rs` and `README.md` with the lint declarations. Our
-travis build actually checks for this.
+// The custom_attribute needs to be enabled for the author lint to work
+#![feature(plugin, custom_attribute)]
-Also please document your lint with a doc comment akin to the following:
+fn main() {
+ #[clippy(author)]
+ let arr: [i32; 1] = [7]; // Replace line with the code you want to match
+}
```
-/// **What it does:** Describe what the lint matches.
+
+Now you run `TESTNAME=ui/my_lint cargo test --test compile-test` to produce
+a `.stdout` file with the generated code:
+
+```rust
+// ./tests/ui/my_lint.stdout
+
+if_chain! {
+ if let Expr_::ExprArray(ref elements) = stmt.node;
+ if elements.len() == 1;
+ if let Expr_::ExprLit(ref lit) = elements[0].node;
+ if let LitKind::Int(7, _) = lit.node;
+ then {
+ // report your lint here
+ }
+}
+```
+
+If the command was executed successfully, you can copy the code over to where you are implementing your lint.
+
+#### Documentation
+
+Please document your lint with a doc comment akin to the following:
+
+```rust
+/// **What it does:** Checks for ... (describe what the lint matches).
///
-/// **Why is this bad?** Write the reason for linting the code.
+/// **Why is this bad?** Supply the reason for linting the code.
///
-/// **Known problems:** Hopefully none.
+/// **Known problems:** None. (Or describe where it could go wrong.)
///
-/// **Example:** Insert a short example if you have one
+/// **Example:**
+///
+/// ```rust
+/// // Bad
+/// Insert a short example of code that triggers the lint
+///
+/// // Good
+/// Insert a short example of improved code that doesn't trigger the lint
+/// ```
```
-Our `util/update_wiki.py` script can then add your ilnt docs to the wiki.
+Once your lint is merged it will show up in the [lint list](https://rust-lang-nursery.github.io/rust-clippy/master/index.html)
-## Contributions
+### Running test suite
-Clippy welcomes contributions from everyone.
+Clippy uses UI tests. UI tests check that the output of the compiler is exactly as expected.
+Of course there's little sense in writing the output yourself or copying it around.
+Therefore you can simply run `tests/ui/update-all-references.sh` (after running
+`cargo test`) and check whether the output looks as you expect with `git diff`. Commit all
+`*.stderr` files, too.
+
+If you don't want to wait for all tests to finish, you can also execute a single test file by using `TESTNAME` to specify the test to run:
+
+```bash
+TESTNAME=ui/empty_line_after_outer_attr cargo test --test compile-test
+```
+
+### Testing manually
+
+Manually testing against an example file is useful if you have added some
+`println!`s and test suite output becomes unreadable. To try clippy with your
+local modifications, run `cargo run --bin clippy-driver -- -L ./target/debug input.rs` from the
+working copy root. Your test file, here `input.rs`, needs to have clippy
+enabled as a plugin:
+
+```rust
+#![feature(plugin)]
+#![plugin(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
All code in this repository is under the [Mozilla Public License, 2.0](https://www.mozilla.org/MPL/2.0/)
-## Conduct
-
-We follow the [Rust Code of Conduct](http://www.rust-lang.org/conduct.html).
-
-
<!-- adapted from https://github.com/servo/servo/blob/master/CONTRIBUTING.md -->
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