# 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](https://help.github.com/articles/about-pull-requests/) 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](https://opensource.google/conduct/).


## Setting up a development environment

To develop `jj`, the mandatory steps are simply to [install
Rust](https://www.rust-lang.org/tools/install) (the default installer options
are fine), clone the repository, and use `cargo build`, `cargo fmt`,
`cargo test`, etc.  If you are preparing a PR, there are some additional
recommended steps.

### Summary

One-time setup:

    rustup toolchain add nightly  # If this is not your default
    rustup toolchain add 1.60
    cargo install cargo-insta
    cargo install cargo-watch

During development (adapt according to your preference):

    cargo watch --ignore '.jj/**' -s \
      'cargo +nightly fmt && cargo clippy --all-targets && cargo +1.60 check --all-targets'
    cargo insta test  # 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 interface will require writing or updating
golden tests that use the [`insta`](https://insta.rs/) crate.  To make
this convenient, install the `cargo-insta` binary. Use `cargo insta tests`
to run tests, and `cargo insta review` to update the golden tests.

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. This version is listed as
`rust-version` in [`Cargo.toml`](../Cargo.toml); it is 1.60 as
of this writing.

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`.