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.

Commit guidelines

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 don’t use Conventional Commits and instead start the commit message with <topic>: rather than like chore: , feat: and fix: . 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.

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.

The project unfortunately has a shortage of reviewers (a common problem in open-source projects). To maximize the chances of getting a timely review, do what you can to provide reviewers (and other future readers) with the necessary context. For example, if you’re adding a new feature, explain why that feature is useful, how it works from the user’s point of view, how it’s designed, and what the limitations of your design are. If you have considered other designs, it may be useful to mention them. Another way of speeding up reviews is to review others’ PRs so the maintainers don’t need to spend as much time on them.

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 invite you to the jj-vcs/contributors team to 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.

If your employer pays anyone (not necessarily you) to contribute to Jujutsu, please make sure your GitHub username is recorded. 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++.

Style guide

See here.

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 1.89 # also specified in Cargo.toml cargo install —locked bacon cargo install —locked cargo-insta cargo install —locked cargo-nextest

During development (adapt according to your preference):

bacon clippy-all cargo +nightly fmt # Occasionally cargo nextest run —workspace # Occasionally cargo insta test —workspace —test-runner nextest # Occasionally

Caution

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 1.89.0.

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 bacon, to automatically build, check, and / or run tests.

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.

Configuring jj fix to run rustfmt

Run this in the jj repo:

jj config set --repo fix.tools.rustfmt '{ command = ["rustfmt", "+nightly"], patterns = ["glob:**/*.rs"] }'

Note: users of Nix and direnv should drop the "+nightly" argument above since the devShell is already configured to pull in a nightly version of rustfmt:

jj config set --repo fix.tools.rustfmt '{ command = ["rustfmt"], patterns = ["glob:**/*.rs"] }'

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.

Set up a RAM disk for faster tests on macOS

jj tests can be sped up significantly on macOS by using a RAM disk instead of the usual /tmp directory (on one Mac, the tests sped up ~3 times from 180 seconds to 55 seconds). You can set this up as follows:

sudo mkdir -p /Volumes/RAMDisk
sudo chmod a+wx /Volumes/RAMDisk
sudo mount_tmpfs /Volumes/RAMDisk  # Add `-e` to make it case-sensitive
                      # Add `-o nobrowse` to hide the tmpfs from Finder

You will need to re-run this after a reboot. Consider making it a script you can run with sudo. (The first two commands don’t need sudo if you change the dir to somewhere in your HOME, the last one always does.)

Then, you can rust tests as, for example,

TMPDIR=/Volumes/RAMDisk cargo insta test --workspace --test-runner nextest

You can double-check whether or not the RAM disk is mounted with

mount | grep tmpfs

Some details and speculation

Hard drive speed is not the issue here. Experimentally, when /tmp is physical, Ilya experienced the tests doing writes at 60MB/s (as reported by the Activity Monitor) on an SSD that should be many times as fast.

Instead, it seems to have something to do with file locking and fdatasync. A likely curlprit is the presence of global kernel locks in APFS. There are some benchmarks in this PR.

It might be possible to add an entry to /etc/fstab to mount tmpfs automatically. If you succeed with this, let us (and Ilya in particular) know!

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"] }
      }
    }
  }
}

Helix

.helix/languages.toml

[language-server.rust-analyzer.config.rustfmt]
extraArgs = ["+nightly"]

Alternative development setup with Nix

If you have Nix installed, you can use the provided flake.nix to get a development environment with all necessary tools pre-configured:

nix develop

Once inside the shell, you can run cargo build, cargo test etc. as usual. Other tools like uv, cargo-insta, cargo-nextest and more are installed, and so you should be able to follow any other recommendations in this guide as needed.

!!! tip “Editor integration”

Launch your editor from within the nix develop shell so that rust-analyzer and other LSP tools can find the correct toolchain. For example:

nix develop
code .  # or your editor of choice

!!! note “direnv users”

If you use direnv with nix-direnv, you can echo "source_env .envrc.recommended" >> .envrc to automatically activate the development environment when entering the directory.

Alternative development setup with mise

An experimental development setup is available using mise. If you try it, file bugs, PRs, or tell us on Discord/IRC/discussions if you experience problems or if this config is too inflexible. If we can make it work for most platforms and most people’s needs, including people previously unfamiliar with mise, we may make mise the recommended way to set up a development environment.

This tool manages the necessary dependencies for you, eliminating the need for a separate setup process. mise automatically installs the required tools when they are needed.

Here are some of the commands you may find yourself using frequently during development:

• mise test: Runs all tests.
• mise test <string>: Runs tests that contain a specific string. For example, mise test squash would run tests with “squash” in their name.
• mise build: Compiles jj.
• mise build:release: Compiles jj in release mode.
• mise build:docs: Builds the documentation for jj.

For a complete list of all available tasks, you can run mise tasks or review the configuration file at .config/mise.toml.

You can customize this configuration using a mise.local.toml file.

Previewing the HTML documentation

The documentation for jj is automatically published online at https://docs.jj-vcs.dev/.

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/Linux

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

Windows

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

Homebrew

brew install uv

Cargo

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

Other options

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

Build the docs

To build the docs, run while inside 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.

Offline distribution

To build the rendered docs for offline distribution or to view them from your file system, run while inside the jj repository:

MKDOCS_OFFLINE=true uv run mkdocs build

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://docs.jj-vcs.dev redirects to https://docs.jj-vcs.dev/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 (jj git clone), 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 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 v1.33.1 latest --push

7. (Optional) When you are done, you may want to reset the gh-pages bookmark to the same spot as it is 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 a few environment variables and invokes uv run mike deploy with some default arguments and whatever arguments were passed to docs-build-deploy. 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://docs.jj-vcs.dev/ (instead of the usual main bookmark).

Previously, the version switcher would not work unless the value of the site_url config in mkdocs.yml matched the actual URL the site is being served from. This bug should now be fixed, but if you are not serving the site from https://docs.jj-vcs.dev/ and something fails weirdly, you might want to adjust the site_url to something like https://jjfan.github.io/jj.

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 run JJ_LOG=debug jj to get debug level logs enabled for all targets. You can also use the --debug global option, which turns on debug log level for jj-lib and jj-cli only.

Profiling

Introduction

jj provides two main approaches to profiling:

1. Tracing-based profiling - Uses the Rust tracing crate to record spans and events. Good for understanding control flow and finding where time is spent in instrumented code.
2. Sampling-based profiling - Uses system profilers (perf, dtrace) to sample the call stack. Good for finding hot spots including in uninstrumented code and external libraries.

Tracing-based profiling

jj uses the tracing crate with Chrome Trace Format output.

Set the JJ_TRACE environment variable to generate a trace file:

# Generate trace with auto-generated filename (jj-trace-{timestamp}.json)
JJ_TRACE= jj st

# Generate trace with specific filename
JJ_TRACE=/tmp/trace.json jj st

To include function argument values in the trace (useful for detailed debugging):

JJ_TRACE_INCLUDE_ARGS=1 JJ_TRACE=/tmp/trace.json jj log -r 'ancestors(@, 10)'

The produced profiles can be imported into chrome://tracing (on Google Chrome) or https://ui.perfetto.dev/ (all browsers).

Only functions annotated with #[tracing::instrument] or explicit trace_span! macros appear in traces. If you need to profile a specific area that isn’t instrumented, you can add #[instrument] attributes to the relevant functions.

Sampling-based profiling

Sampling profilers periodically capture the call stack to identify hot spots. This approach:

• Has lower overhead than tracing
• Captures all code, including uninstrumented functions and external libraries (like gitoxide)
• May miss short-lived function calls since it samples rather than traces every call

Using samply

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

cargo install samply
samply record jj diff

Then just open the link it prints.

Using cargo-flamegraph

cargo-flamegraph generates flamegraph visualizations using system profilers:

cargo install flamegraph
cargo flamegraph --bin jj -- diff

You may need to use sudo or install perf on your system, see flamegraph’s documentation for more.