Basics for hacking on Clippy

This document explains the basics for hacking on Clippy. Besides others, this includes how to build and test Clippy. For a more in depth description on the codebase take a look at Adding Lints or Common Tools.

Get the Code

First, make sure you have checked out the latest version of Clippy. If this is your first time working on Clippy, create a fork of the repository and clone it afterwards with the following command:

git clone<your-username>/rust-clippy

If you've already cloned Clippy in the past, update it to the latest version:

# If the upstream remote has not been added yet
git remote add upstream
# upstream has to be the remote of the rust-lang/rust-clippy repo
git fetch upstream
# make sure that you are on the master branch
git checkout master
# rebase your master branch on the upstream master
git rebase upstream/master
# push to the master branch of your fork
git push

Building and Testing

You can build and test Clippy like every other Rust project:

cargo build  # builds Clippy
cargo test   # tests Clippy

Since Clippy's test suite is pretty big, there are some commands that only run a subset of Clippy's tests:

# only run UI tests
cargo uitest
# only run UI tests starting with `test_`
TESTNAME="test_" cargo uitest
# only run dogfood tests
cargo dev dogfood

If the output of a UI test differs from the expected output, you can update the reference file with:

cargo dev bless

For example, this is necessary, if you fix a typo in an error message of a lint or if you modify a test file to add a test case.

Note: This command may update more files than you intended. In that case only commit the files you wanted to update.

cargo dev

Clippy has some dev tools to make working on Clippy more convenient. These tools can be accessed through the cargo dev command. Available tools are listed below. To get more information about these commands, just call them with --help.

# formats the whole Clippy codebase and all tests
cargo dev fmt
# register or update lint names/groups/...
cargo dev update_lints
# create a new lint and register it
cargo dev new_lint
# deprecate a lint and attempt to remove code relating to it
cargo dev deprecate
# automatically formatting all code before each commit
cargo dev setup git-hook
# (experimental) Setup Clippy to work with IntelliJ-Rust
cargo dev setup intellij
# runs the `dogfood` tests
cargo dev dogfood

More about intellij command usage and reasons here


cargo lintcheck will build and run clippy on a fixed set of crates and generate a log of the results. You can git diff the updated log against its previous version and see what impact your lint made on a small set of crates. If you add a new lint, please audit the resulting warnings and make sure there are no false positives and that the suggestions are valid.

Refer to the tools README for more details.


We follow a rustc no merge-commit policy. See

Common Abbreviations

UBUndefined Behavior
FPFalse Positive
FNFalse Negative
ICEInternal Compiler Error
ASTAbstract Syntax Tree
MIRMid-Level Intermediate Representation
HIRHigh-Level Intermediate Representation
TCXType context

This is a concise list of abbreviations that can come up during Clippy development. An extensive general list can be found in the rustc-dev-guide glossary. Always feel free to ask if an abbreviation or meaning is unclear to you.

Install from source

If you are hacking on Clippy and want to install it from source, do the following:

First, take note of the toolchain override in /rust-toolchain. We will use this override to install Clippy into the right toolchain.

Tip: You can view the active toolchain for the current directory with rustup show active-toolchain.

From the Clippy project root, run the following command to build the Clippy binaries and copy them into the toolchain directory. This will override the currently installed Clippy component.

cargo build --release --bin cargo-clippy --bin clippy-driver -Zunstable-options --out-dir "$(rustc --print=sysroot)/bin"

Now you may run cargo clippy in any project, using the toolchain where you just installed Clippy.

cd my-project
cargo +nightly-2021-07-01 clippy

...or clippy-driver

clippy-driver +nightly-2021-07-01 <filename>

If you need to restore the default Clippy installation, run the following (from the Clippy project root).

rustup component remove clippy
rustup component add clippy

DO NOT install using cargo install --path . --force since this will overwrite rustup proxies. That is, ~/.cargo/bin/cargo-clippy and ~/.cargo/bin/clippy-driver should be hard or soft links to ~/.cargo/bin/rustup. You can repair these by running rustup update.