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# 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 whether you have already signed the CLA or to sign it.

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. In other words, the Google CLA is not scoped to a specific project. If you sign the Google CLA, it applies to other Google open-source projects as well (including future projects).

### 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. If the PR closes an open issue or feature request for the project, include the `Closes #<issue-number>` or `Fixes #<issue-number>` in the commit description.

### 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

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 1.89.0.

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.

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

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 Windows Homebrew Cargo Other 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 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.