Contributing to Rust

Thank you for your interest in contributing to Rust! There are many ways to contribute, and we appreciate all of them.

If you have questions, please make a post on internals.rust-lang.org or hop on the Rust Discord server or Rust Zulip server.

As a reminder, all contributors are expected to follow our Code of Conduct.

If this is your first time contributing, the Getting Started and walkthrough chapters can give you a good example of how a typical contribution would go.

Feature Requests

Feature requests need to go through a process to be approved by the relevant teams. Usually this requires a Final Comment Period (FCP) or even a Request for Comments (RFC). See Getting Started for more information about these processes.

Bug Reports

While bugs are unfortunate, they're a reality in software. We can't fix what we don't know about, so please report liberally. If you're not sure if something is a bug or not, feel free to file a bug anyway.

If you believe reporting your bug publicly represents a security risk to Rust users, please follow our instructions for reporting security vulnerabilities.

If you're using the nightly channel, please check if the bug exists in the latest toolchain before filing your bug. It might be fixed already.

If you have the chance, before reporting a bug, please search existing issues, as it's possible that someone else has already reported your error. This doesn't always work, and sometimes it's hard to know what to search for, so consider this extra credit. We won't mind if you accidentally file a duplicate report.

Similarly, to help others who encountered the bug find your issue, consider filing an issue with a descriptive title, which contains information that might be unique to it. This can be the language or compiler feature used, the conditions that trigger the bug, or part of the error message if there is any. An example could be: "impossible case reached" on lifetime inference for impl Trait in return position.

Opening an issue is as easy as following this link and filling out the fields in the appropriate provided template.

Pull Requests

Pull requests (or PRs for short) are the primary mechanism we use to change Rust. GitHub itself has some great documentation on using the Pull Request feature. We use the "fork and pull" model described here, where contributors push changes to their personal fork and create pull requests to bring those changes into the source repository.

All pull requests are reviewed by another person. We have a bot, @rust-highfive, that will automatically assign a random person to review your request.

If you want to request that a specific person reviews your pull request, you can add an r? to the pull request description. For example, Steve usually reviews documentation changes. So if you were to make a documentation change, add

r? @steveklabnik

to the end of the pull request description, and @rust-highfive will assign @steveklabnik instead of a random person. This is entirely optional.

In addition to being reviewed by a human, pull requests are automatically tested thanks to continuous integration (CI). Basically, every time you open and update a pull request, CI builds the compiler and tests it against the compiler test suite, and also performs other tests such as checking that your pull request is in compliance with Rust's style guidelines.

Running continuous integration tests allows PR authors to catch mistakes early without going through a first review cycle, and also helps reviewers stay aware of the status of a particular pull request.

Rust has plenty of CI capacity, and you should never have to worry about wasting computational resources each time you push a change. It is also perfectly fine (and even encouraged!) to use the CI to test your changes if it can help your productivity. In particular, we don't recommend running the full x.py test suite locally, since it takes a very long time to execute.

After someone has reviewed your pull request, they will leave an annotation on the pull request with an r+. It will look something like this:

@bors r+

This tells @bors, our lovable integration bot, that your pull request has been approved. The PR then enters the merge queue, where @bors will run all the tests on every platform we support. If it all works out, @bors will merge your code into master and close the pull request.

Depending on the scale of the change, you may see a slightly different form of r+:

@bors r+ rollup

The additional rollup tells @bors that this change should always be "rolled up". Changes that are rolled up are tested and merged alongside other PRs, to speed the process up. Typically only small changes that are expected not to conflict with one another are marked as "always roll up".

Opening a PR

You are now ready to file a pull request? Great! Here are a few points you should be aware of.

All pull requests should be filed against the master branch, except in very particular scenarios. Unless you know for sure that you should target another branch, master will be the right choice (it's also the default).

Make sure your pull request is in compliance with Rust's style guidelines by running

$ ./x.py test tidy --bless

We recommend to make this check before every pull request (and every new commit in a pull request); you can add git hooks before every push to make sure you never forget to make this check. The CI will also run tidy and will fail if tidy fails.

Rust follows a no merge-commit policy, meaning, when you encounter merge conflicts you are expected to always rebase instead of merging. E.g. always use rebase when bringing the latest changes from the master branch to your feature branch. Also, please make sure that fixup commits are squashed into other related commits with meaningful commit messages.

If you encounter merge conflicts, your PR will get marked as S-waiting-on-author. When you resolve them, you should use @rustbot to mark it as S-waiting-on-review. See this chapter for more details.

GitHub allows closing issues using keywords. This feature should be used to keep the issue tracker tidy. However, it is generally preferred to put the "closes #123" text in the PR description rather than the issue commit; particularly during rebasing, citing the issue number in the commit can "spam" the issue in question.

External Dependencies (subtree)

As a developer to this repository, you don't have to treat the following external projects differently from other crates that are directly in this repo:

  • Clippy

They are just regular files and directories. This is in contrast to submodule dependencies (see below for those). Only tool authors will actually use any operations here.

Synchronizing a subtree

There are two synchronization directions: subtree push and subtree pull.

git subtree push -P src/tools/clippy git@github.com:your-github-name/rust-clippy sync-from-rust

takes all the changes that happened to the copy in this repo and creates commits on the remote repo that match the local changes. Every local commit that touched the subtree causes a commit on the remote repo, but is modified to move the files from the specified directory to the tool repo root.

Make sure to not pick the master branch on the tool repo, so you can open a normal PR to the tool to merge that subrepo push.

git subtree pull -P src/tools/clippy https://github.com/rust-lang/rust-clippy master

takes all changes since the last subtree pull from the tool repo repo and adds these commits to the rustc repo + a merge commit that moves the tool changes into the specified directory in the rust repository.

It is recommended that you always do a push first and get that merged to the tool master branch. Then, when you do a pull, the merge works without conflicts. While it's definitely possible to resolve conflicts during a pull, you may have to redo the conflict resolution if your PR doesn't get merged fast enough and there are new conflicts. Do not try to rebase the result of a git subtree pull, rebasing merge commits is a bad idea in general.

You always need to specify the -P prefix to the subtree directory and the corresponding remote repository. If you specify the wrong directory or repository you'll get very fun merges that try to push the wrong directory to the wrong remote repository. Luckily you can just abort this without any consequences by throwing away either the pulled commits in rustc or the pushed branch on the remote and try again. It is usually fairly obvious that this is happening because you suddenly get thousands of commits that want to be synchronized.

Creating a new subtree dependency

If you want to create a new subtree dependency from an existing repository, call (from this repository's root directory!)

git subtree add -P src/tools/clippy https://github.com/rust-lang/rust-clippy.git master

This will create a new commit, which you may not rebase under any circumstances! Delete the commit and redo the operation if you need to rebase.

Now you're done, the src/tools/clippy directory behaves as if Clippy were part of the rustc monorepo, so no one but you (or others that synchronize subtrees) actually needs to use git subtree.

External Dependencies (submodules)

Currently building Rust will also build the following external projects:

We allow breakage of these tools in the nightly channel. Maintainers of these projects will be notified of the breakages and should fix them as soon as possible.

After the external is fixed, one could add the changes with

git add path/to/submodule

outside the submodule.

In order to prepare your tool-fixing PR, you can run the build locally by doing ./x.py build src/tools/TOOL. If you will be editing the sources there, you may wish to set submodules = false in the config.toml to prevent x.py from resetting to the original branch.

Breakage is not allowed in the beta and stable channels, and must be addressed before the PR is merged.

Breaking Tools Built With The Compiler

Rust's build system builds a number of tools that make use of the internals of the compiler. This includes RLS and rustfmt. If these tools break because of your changes, you may run into a sort of "chicken and egg" problem. These tools rely on the latest compiler to be built so you can't update them to reflect your changes to the compiler until those changes are merged into the compiler. At the same time, you can't get your changes merged into the compiler because the rust-lang/rust build won't pass until those tools build and pass their tests.

That means that, in the default state, you can't update the compiler without first fixing rustfmt, rls and the other tools that the compiler builds.

Luckily, a feature was added to Rust's build to make all of this easy to handle. The idea is that we allow these tools to be "broken", so that the rust-lang/rust build passes without trying to build them, then land the change in the compiler, wait for a nightly, and go update the tools that you broke. Once you're done and the tools are working again, you go back in the compiler and update the tools so they can be distributed again.

This should avoid a bunch of synchronization dances and is also much easier on contributors as there's no need to block on rls/rustfmt/other tools changes going upstream.

Here are those same steps in detail:

  1. (optional) First, if it doesn't exist already, create a config.toml by copying config.toml.example in the root directory of the Rust repository. Set submodules = false in the [build] section. This will prevent x.py from resetting to the original branch after you make your changes. If you need to update any submodules to their latest versions, see the section of this file about that for more information.
  2. (optional) Run ./x.py test src/tools/rustfmt (substituting the submodule that broke for rustfmt). Fix any errors in the submodule (and possibly others).
  3. (optional) Make commits for your changes and send them to upstream repositories as a PR.
  4. (optional) Maintainers of these submodules will not merge the PR. The PR can't be merged because CI will be broken. You'll want to write a message on the PR referencing your change, and how the PR should be merged once your change makes it into a nightly.
  5. Wait for your PR to merge.
  6. Wait for a nightly
  7. (optional) Help land your PR on the upstream repository now that your changes are in nightly.
  8. (optional) Send a PR to rust-lang/rust updating the submodule.

Updating submodules

These instructions are specific to updating rustfmt, however they may apply to the other submodules as well. Please help by improving these instructions if you find any discrepancies or special cases that need to be addressed.

To update the rustfmt submodule, start by running the appropriate git submodule command. For example, to update to the latest commit on the remote master branch, you may want to run:

git submodule update --remote src/tools/rustfmt

If you run ./x.py build now, and you are lucky, it may just work. If you see an error message about patches that did not resolve to any crates, you will need to complete a few more steps which are outlined with their rationale below.

(This error may change in the future to include more information.)

error: failed to resolve patches for `https://github.com/rust-lang/rustfmt`

Caused by:
  patch for `rustfmt-nightly` in `https://github.com/rust-lang/rustfmt` did not resolve to any crates
failed to run: ~/rust/build/x86_64-unknown-linux-gnu/stage0/bin/cargo build --manifest-path ~/rust/src/bootstrap/Cargo.toml

The [patch] section of Cargo.toml can be very useful for testing. In addition to that, you should read the Overriding dependencies section of the documentation.

Specifically, the following section in Overriding dependencies reveals what the problem is:

Next up we need to ensure that our lock file is updated to use this new version of uuid so our project uses the locally checked out copy instead of one from crates.io. The way [patch] works is that it'll load the dependency at ../path/to/uuid and then whenever crates.io is queried for versions of uuid it'll also return the local version.

This means that the version number of the local checkout is significant and will affect whether the patch is used. Our manifest declared uuid = "1.0" which means we'll only resolve to >= 1.0.0, < 2.0.0, and Cargo's greedy resolution algorithm also means that we'll resolve to the maximum version within that range. Typically this doesn't matter as the version of the git repository will already be greater or match the maximum version published on crates.io, but it's important to keep this in mind!

This says that when we updated the submodule, the version number in our src/tools/rustfmt/Cargo.toml changed. The new version is different from the version in Cargo.lock, so the build can no longer continue.

To resolve this, we need to update Cargo.lock. Luckily, cargo provides a command to do this easily.

$ cargo update -p rustfmt-nightly

This should change the version listed in Cargo.lock to the new version you updated the submodule to. Running ./x.py build should work now.

Writing Documentation

Documentation improvements are very welcome. The source of doc.rust-lang.org is located in src/doc in the tree, and standard API documentation is generated from the source code itself (e.g. lib.rs). Documentation pull requests function in the same way as other pull requests.

To find documentation-related issues, sort by the T-doc label.

You can find documentation style guidelines in RFC 1574.

In many cases, you don't need a full ./x.py doc --stage 2, which will build the entire stage 2 compiler and compile the various books published on doc.rust-lang.org. When updating documentation for the standard library, first try ./x.py doc library/std. If that fails, or if you need to see the output from the latest version of rustdoc, add --stage 1. Results should appear in build/$TARGET/doc.

You can also use rustdoc directly to check small fixes. For example, rustdoc src/doc/reference.md will render reference to doc/reference.html. The CSS might be messed up, but you can verify that the HTML is right.

Additionally, contributions to the rustc-dev-guide are always welcome. Contributions can be made directly at the rust-lang/rustc-dev-guide repo. The issue tracker in that repo is also a great way to find things that need doing. There are issues for beginners and advanced compiler devs alike!

Issue Triage

Sometimes, an issue will stay open, even though the bug has been fixed. And sometimes, the original bug may go stale because something has changed in the meantime.

It can be helpful to go through older bug reports and make sure that they are still valid. Load up an older issue, double check that it's still true, and leave a comment letting us know if it is or is not. The least recently updated sort is good for finding issues like this.

Thanks to @rustbot, anyone can help triage issues by adding appropriate labels to issues that haven't been triaged yet:

  • Yellow, A-prefixed labels state which area of the project an issue relates to.

  • Magenta, B-prefixed labels identify bugs which are blockers.

  • Dark blue, beta- labels track changes which need to be backported into the beta branches.

  • Light purple, C-prefixed labels represent the category of an issue.

  • Green, E-prefixed labels explain the level of experience necessary to fix the issue.

  • The dark blue final-comment-period label marks bugs that are using the RFC signoff functionality of rfcbot and are currently in the final comment period.

  • Red, I-prefixed labels indicate the importance of the issue. The I-nominated label indicates that an issue has been nominated for prioritizing at the next triage meeting. Similarly, the I-prioritize indicates that an issue has been requested to be prioritized by the appropriate team.

  • The purple metabug label marks lists of bugs collected by other categories.

  • Purple gray, O-prefixed labels are the operating system or platform that this issue is specific to.

  • Orange, P-prefixed labels indicate a bug's priority. These labels are only assigned during triage meetings, and replace the I-prioritize label.

  • The gray proposed-final-comment-period label marks bugs that are using the RFC signoff functionality of rfcbot and are currently awaiting signoff of all team members in order to enter the final comment period.

  • Pink, regression-prefixed labels track regressions from stable to the release channels.

  • The light orange relnotes label marks issues that should be documented in the release notes of the next release.

  • Gray, S-prefixed labels are used for tracking the status of pull requests.

  • Blue, T-prefixed bugs denote which team the issue belongs to.

If you're looking for somewhere to start, check out the E-easy tag.

Out-of-tree Contributions

There are a number of other ways to contribute to Rust that don't deal with rust-lang/rust:

Helpful Links and Information

For people new to Rust, and just starting to contribute, or even for more seasoned developers, some useful places to look for information are:

  • This guide contains information about how various parts of the compiler work and how to contribute to the compiler
  • Rust Forge contains additional documentation, including write-ups of how to achieve common tasks
  • The Rust Internals forum, a place to ask questions and discuss Rust's internals
  • The generated documentation for Rust's compiler
  • The Rust reference, even though it doesn't specifically talk about Rust's internals, is a great resource nonetheless
  • Although out of date, Tom Lee's great blog article is very helpful
  • rustaceans.org is helpful, but mostly dedicated to IRC
  • The Rust Compiler Testing Docs
  • For @bors, this cheat sheet is helpful
  • Google is always helpful when programming. You can search all Rust documentation (the standard library, the compiler, the books, the references, and the guides) to quickly find information about the language and compiler.
  • You can also use Rustdoc's built-in search feature to find documentation on types and functions within the crates you're looking at. You can also search by type signature! For example, searching for * -> vec should find all functions that return a Vec<T>. Hint: Find more tips and keyboard shortcuts by typing ? on any Rustdoc page!
  • Don't be afraid to ask! The Rust community is friendly and helpful.