jj/docs/contributing.md

How to Contribute

Policies

We'd love to accept your patches and contributions to this project. There are just a few small guidelines you need to follow.

Contributor License Agreement

Contributions to this project must be accompanied by a Contributor License Agreement. You (or your employer) retain the copyright to your contribution; this simply gives us permission to use and redistribute your contributions as part of the project. Head over to https://cla.developers.google.com/ to see your current agreements on file or to sign a new one.

You generally only need to submit a CLA once, so if you've already submitted one (even if it was for a different project), you probably don't need to do it again.

Code reviews

All submissions, including submissions by project members, require review. We use GitHub pull requests for this purpose. Consult GitHub Help for more information on using pull requests.

Unlike many GitHub projects (but like many VCS projects), we care more about the contents of commits than about the contents of PRs. We review each commit separately, and we don't squash them when the PR is ready.

Each commit should ideally do one thing. For example, if you need to refactor a function in order to add a new feature cleanly, put the refactoring in one commit and the new feature in a different commit. If the refactoring itself consists of many parts, try to separate out those into separate commits. You can use jj split to do it if you didn't realize ahead of time how it should be split up. Include tests and documentation in the same commit as the code the test and document. The commit message should describe the changes in the commit; the PR description can even be empty, but feel free to include a personal message.

When you address comments on a PR, don't make the changes in a commit on top (as is typical on GitHub). Instead, please make the changes in the appropriate commit. You can do that by checking out the commit (jj checkout/new <commit>) and then squash in the changes when you're done (jj squash). jj git push will automatically force-push the branch.

When your first PR has been approved, we typically give you contributor access, so you can address any remaining minor comments and then merge the PR yourself when you're ready. If you realize that some comments require non-trivial changes, please ask your reviewer to take another look.

Community Guidelines

This project follows Google's Open Source Community Guidelines.

Learning Rust

In addition to the Rust Book and the other excellent resources at https://www.rust-lang.org/learn, we recommend the "Comprehensive Rust" mini-course for an overview, especially if you are familiar with C++.

Setting up a development environment

To develop jj, the mandatory steps are simply to install Rust (the default installer options are fine), clone the repository, and use cargo build , cargo fmt, cargo clippy --workspace --all-targets, and
cargo test --workspace. If you are preparing a PR, there are some additional recommended steps.

Summary

If you already have rustup installed, then moving into the source code directory will install the MSRV for you.

One-time setup:

rustup toolchain add nightly  # wanted for 'rustfmt'
cargo install cargo-insta
cargo install cargo-watch
cargo install cargo-nextest

During development (adapt according to your preference):

cargo watch --ignore '.jj/**' -s \
  'cargo clippy --workspace --all-targets \
   && cargo check --workspace --all-targets'
cargo +nightly fmt # Occasionally
cargo nextest run --workspace # Occasionally
cargo insta test --workspace # Occasionally

WARNING: Build artifacts from debug builds and especially from repeated invocations of cargo test can quickly take up 10s of GB of disk space. Cargo will happily use up your entire hard drive. If this happens, run cargo clean.

Explanation

These are listed roughly in order of decreasing importance.

1. Nearly any change to jj's CLI will require writing or updating snapshot tests that use the insta crate. To make this convenient, install the cargo-insta binary. Use cargo insta test --workspace to run tests, and cargo insta review --workspace to update the snapshot tests. The --workspace flag is needed to run the tests on all crates; by default, only the crate in the current directory is tested.

2. Github CI checks require that the code is formatted with the nightly version of rustfmt. To do this on your computer, install the nightly toolchain and use cargo +nightly fmt.

3. Your code will be rejected if it cannot be compiled with the minimal supported version of Rust ("MSRV"). Currently, jj follows a rather casual MSRV policy: "The current rustc stable version, minus one."

4. Your code needs to pass cargo clippy. You can also use cargo +nightly clippy if you wish to see more warnings.

5. You may also want to install and use cargo-watch. In this case, you should exclude .jj. directory from the filesystem watcher, as it gets updated on every jj log.

6. To run tests more quickly, use cargo nextest run --workspace. To use nextest with insta, use cargo insta test --workspace --test-runner nextest.

Modifying protobuffers (this is not common)

Occasionally, you may need to change the .proto files that define jj's data storage format. In this case, you will need to add a few steps to the above workflow.

• Install the protoc compiler. This usually means either apt-get install protobuf-compiler or downloading an official release. The prost library docs have additional advice.
• Run cargo run -p gen-protos regularly (or after every edit to a .proto file). This is the same as running cargo run from lib/gen-protos. The gen-protos binary will use the prost-build library to compile the .proto files into .rs files.
• If you are adding a new .proto file, you will need to edit the list of these files in lib/gen-protos/src/main.rs.

The .rs files generated from .proto files are included in the repository, and there is a Github CI check that will complain if they do not match.