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-merge the PR (so please manually squash any fixup commits before sending for review).

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 they 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. We start the commit message with <topic>: and don't use conventional commits. This means if you modified a command in the CLI, use its name as the topic, e.g. next/prev: <your-modification> or conflicts: <your-modification>. We don't currently have a specific guidelines on what to write in the topic field, but the reviewers will help you provide a topic if you have difficulties choosing it. How to Write a Git Commit Message is a good guide if you're new to writing good commit messages. We are not particularly strict about the style, but please do explain the reason for the change unless it's obvious.

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 creating a new commit on top of the initial commit (jj new <commit>) and then squash in the changes when you're done (jj squash). jj git push will automatically force-push the bookmark.

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.

To avoid conflicts of interest, please don't merge a PR that has only been approved by someone from the same organization. Similarly, as a reviewer, there is no need to approve your coworkers' PRs, since the author should await an approval from someone else anyway. It is of course still appreciated if you review and comment on their PRs. Also, if the PR seems completely unrelated to your company's interests, do feel free to approve it.

Community Guidelines

This project follows Google's Open Source Community Guidelines.

Contributing large patches

Before sending a PR for a large change which designs/redesigns or reworks an existing component, we require an architecture review from multiple stakeholders, which we do with Design Docs, see the process here.

Contributing to the documentation

We appreciate bug reports about any problems, however small, lurking in our documentation website or in the jj help <command> docs. If a part of the bug report template does not apply, you can just delete it.

Before reporting a problem with the documentation website, we'd appreciate it if you could check that the problem still exists in the "prerelease" version of the documentation (as opposed to the docs for one of the released versions of jj). You can use the version switcher in the top-left of the website to do so.

If you are willing to make a PR fixing a documentation problem, even better!

The documentation website sources are Markdown files located in the docs/ directory. You do not need to know Rust to work with them. See below for instructions on how to preview the HTML docs as you edit the Markdown files. Doing so is optional, but recommended.

The jj help docs are sourced from the "docstring" comments inside the Rust sources, currently from the cli/src/commands directory. Working on them requires setting up a Rust development environment, as described below, and may occasionally require adjusting a test.

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

One-time setup:

rustup toolchain add nightly  # wanted for 'rustfmt'
rustup toolchain add 1.76     # also specified in Cargo.toml
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 +1.76 check --workspace --all-targets'
cargo +nightly fmt # Occasionally
cargo nextest run --workspace # Occasionally
cargo insta test --workspace --test-runner nextest # 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." As of this writing, that version is 1.76.0.

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.

   On Linux, you may be able to speed up nextest even further by using the mold linker, as explained below.

Using mold for faster tests on Linux

On a machine with a multi-core CPU, one way to speed up cargo nextest on Linux is to use the multi-threaded mold linker. This linker may help if, currently, your CPU is underused while Rust is linking test binaries. Before proceeding with mold, you can check whether this is an issue worth solving using a system monitoring tool such as htop.

mold is packaged for many distributions. On Debian, for example, sudo apt install mold should just work.

A simple way to use mold is via the -run option, e.g.:

mold -run cargo insta test --workspace --test-runner nextest

There will be no indication that a different linker is used, except for higher CPU usage while linking and, hopefully, faster completion. You can verify that mold was indeed used by running readelf -p .comment target/debug/jj.

There are also ways of having Rust use mold by default, see the "How to use" instructions.

On recent versions of MacOS, the default linker Rust uses is already multi-threaded. It should use all the CPU cores without any configuration.

Editor setup

Visual Studio Code

We recommend at least these settings:

{
    "files.insertFinalNewline": true,
    "files.trimTrailingWhitespace": true,
    "[rust]": {
        "files.trimTrailingWhitespace": false
    }
}

Zed

// .zed/settings.json
{
  "ensure_final_newline_on_save": true,
  "remove_trailing_whitespace_on_save": true,

  "languages": {
    // We don't use a formatter for Markdown files, so format_on_save would just
    // mess with others' docs
    "Markdown": { "format_on_save": "off" }
    "Rust": {
      "format_on_save": "on",
      // Avoid removing trailing spaces within multi-line string literals
      "remove_trailing_whitespace_on_save": false
    }
  },

  "lsp": {
    "rust-analyzer": {
      "initialization_options": {
        // If you are working on docs and don't need `cargo check`, uncomment
        // this option:
        //
        //   "checkOnSave": false,

        // Use nightly `rustfmt`, equivalent to `cargo +nightly fmt`
        "rustfmt": { "extraArgs": ["+nightly"] }
      }
    }
  }
}

Previewing the HTML documentation

The documentation for jj is automatically published online at https://martinvonz.github.io/jj/.

When editing documentation, you should check your changes locally — especially if you are adding a new page, or doing a major rewrite.

Install uv

The only thing you need is uv (version 0.5.1 or newer).

uv is a Python project manager written in Rust. It will fetch the right Python version and the dependencies needed to build the docs. Install it like so:

macOS/LinuxWindowsHomebrewCargoOther options

curl -LsSf https://astral.sh/uv/install.sh | sh

Note

If you don't have ~/.local/bin in your PATH, the installer will modify your shell profile. To avoid it:

curl -LsSf https://astral.sh/uv/install.sh | env INSTALLER_NO_MODIFY_PATH=1 sh

powershell -ExecutionPolicy ByPass -c "irm https://astral.sh/uv/install.ps1 | iex"

brew install uv

# This might take a while
cargo install --git https://github.com/astral-sh/uv uv

• Directly download the binaries from GitHub: uv releases.
• Even more options: Installing uv.

Build the docs

To build the docs, run from the root of the jj repository:

uv run mkdocs serve

Open http://127.0.0.1:8000 in your browser to see the docs.

As you edit the .md files in docs/, the website should be rebuilt and reloaded in your browser automatically.

If the docs are not updating

Check the terminal from which you ran uv run mkdocs serve for any build errors or warnings. Warnings about "GET /versions.json HTTP/1.1" code 404 are expected and harmless.

Building the entire website

Tip

Building the entire website is not usually necessary. If you are editing documentation, the previous section is enough.

These instructions are relevant if you are working on the versioning of the documentation that we currently do with mike.

The full jj website includes the documentation for several jj versions (prerelease, latest release, and the older releases). The top-level URL https://martinvonz.github.io/jj redirects to https://martinvonz.github.io/jj/latest, which in turn redirects to the docs for the last stable version.

The different versions of documentation are managed and deployed with mike, which can be run with uv run mike.

On a POSIX system or WSL, one way to build the entire website is as follows (on Windows, you'll need to understand and adapt the shell script):

1. Check out jj as a co-located jj + git repository (jj clone --colocate), cloned from your fork of jj (e.g. github.com/jjfan/jj). You can also use a pure Git repo if you prefer.

2. Make sure github.com/jjfan/jj includes the gh-pages bookmark of the jj repo and run git fetch origin gh-pages.

3. Go to the GitHub repository settings, enable GitHub Pages, and configure them to use the gh-pages bookmark (this is usually the default).

4. Install uv as explained in Previewing the HTML documentation, and run the same sh script that is used in GitHub CI (details below):

   .github/scripts/docs-build-deploy 'https://jjfan.github.io/jj/'\
       prerelease main --push
   

   This should build the version of the docs from the current commit, deploy it as a new commit to the gh-pages bookmark, and push the gh-pages bookmark to the origin.

5. Now, you should be able to see the full website, including your latest changes to the prerelease version, at https://jjfan.github.io/jj/prerelease/.

6. (Optional) The previous steps actually only rebuild https://jjfan.github.io/jj/prerelease/ and its alias https://jjfan.github.io/jj/main/. If you'd like to test out version switching back and forth, you can also rebuild the docs for the latest release as follows.

   jj new v1.33.1  # Let's say `jj 1.33.1` is the currently the latest release
   .github/scripts/docs-build-deploy 'https://jjfan.github.io/jj/'\
       v1.33.1 latest --push
   

7. (Optional) When you are done, you may want to reset the gh-bookmarks to the same spot as it is in the upstream. If you configured the upstream remote, this can be done with:

   # This will LOSE any changes you made to `gh-pages`
   jj git fetch --remote upstream
   jj bookmark set gh-pages -r gh-pages@upstream
   jj git push --remote origin --bookmark gh-pages
   

   If you want to preserve some of the changes you made, you can do jj bookmark set my-changes -r gh-pages BEFORE running the above commands.

Explanation of the docs-build-deploy script

The script sets up the site_url mkdocs config to 'https://jjfan.github.io/jj/'. If this config does not match the URL where you loaded the website, some minor website features (like the version switching widget) will have reduced functionality.

Then, the script passes the rest of its arguments to uv run mike deploy, which does the rest of the job. Run uv run mike help deploy to find out what the arguments do.

If you need to do something more complicated, you can use uv run mike ... commands. You can also edit the gh-pages bookmark directly, but take care to avoid files that will be overwritten by future invocations of mike. Then, you can submit a PR based on the gh-pages bookmark of https://martinvonz.github.com/jj (instead of the usual main bookmark).

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.

Logging

You can print internal jj logs using JJ_LOG. It acts like the RUST_LOG environment variable, frequent in Rust codebases, and accepts one or more directives. You can also use the --debug global option that sets debug log level for all targets by default. JJ_LOG is still respected when using --debug.

Profiling

One easy-to-use sampling profiler is samply. For example:

cargo install samply
samply record jj diff

Then just open the link it prints.

Another option is to use the instrumentation we've added manually (using tracing::instrument) in various places. For example:

JJ_TRACE=/tmp/trace.json jj diff

Then go to https://ui.perfetto.dev/ in Chrome and load /tmp/trace.json from there.