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. We have more info about how to use git when contributing to Rust under the git section.

r?

All pull requests are reviewed by another person. We have a bot, @rustbot, that will automatically assign a random person to review your request based on which files you changed.

If you want to request that a specific person reviews your pull request, you can add an r? to the pull request description or in a comment. For example, if you want to ask a review to @awesome-reviewer, add

r? @awesome-reviewer

to the end of the pull request description, and @rustbot will assign them instead of a random person. This is entirely optional.

You can also assign a random reviewer from a specific team by writing r? rust-lang/groupname. So if you were making a diagnostics change, then you could get a reviewer from the diagnostics team by adding:

r? rust-lang/diagnostics

For a full list of possible groupname check the adhoc_groups section at the triagebot.toml config file or the list of teams in the rust-lang teams database.

CI

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.

r+

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.

If you encounter merge conflicts or when a reviewer asks you to perform some changes, 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:

@rustbot label -S-waiting-on-author +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:

In contrast to submodule dependencies (see below for those), the subtree dependencies are just regular files and directories which can be updated in tree. However, enhancements, bug fixes, etc. specific to these tools should be filed against the tools directly in their respective upstream repositories.

Synchronizing a subtree

Periodically the changes made to subtree based dependencies need to be synchronized between this repository and the upstream tool repositories.

Subtree synchronizations are typically handled by the respective tool maintainers. Other users are welcome to submit synchronization PRs, however, in order to do so you you will need to modify your local git installation and follow a very precise set of instructions. These instructions are documented, along with several useful tips and tricks, in the syncing subtree changes section in Clippy's Contributing guide. The instructions are applicable for use with any subtree based tool, just be sure to use the correct corresponding subtree directory and remote repository.

The synchronization process goes in two directions: subtree push and subtree pull.

A subtree push 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.

A subtree pull takes all changes since the last subtree pull from the tool repo and adds these commits to the rustc repo along with 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)

Building Rust will also use external git repositories tracked using git submodules. The complete list may be found in the .gitmodules file. Some of these projects are required (like stdarch for the standard library) and some of them are optional (like [Miri]).

Usage of submodules is discussed more in the Using Git chapter.

Some of the submodules are allowed to be in a "broken" state where they either don't build or their tests don't pass, e.g. the documentation books like The Rust Reference. Maintainers of these projects will be notified when the project is in a broken state, and they should fix them as soon as possible. The current status is tracked on the toolstate website. More information may be found on the Forge Toolstate chapter.

Breakage is not allowed in the beta and stable channels, and must be addressed before the PR is merged. They are also not allowed to be broken on master in the week leading up to the beta cut.

Breaking Tools Built With The Compiler

Rust's build system builds a number of tools that make use of the internals of the compiler and that are hosted in a separate repository, and included in Rust via git submodules (such as [Miri]). 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 (in their own repositories) 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.

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, and go update the tools that you broke. Some tools will require waiting for a nightly release before this can happen, while others use the builds uploaded after each bors merge and thus can be updated immediately (check the tool's documentation for details). 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 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/cargo (substituting the submodule that broke for cargo). 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.

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 A-docs 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. 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.

Contributing to rustc-dev-guide

Contributions to the rustc-dev-guide are always welcome, and 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!

Just a few things to keep in mind:

  • Please limit line length to 100 characters. This is enforced by CI, and you can run the checks locally with ci/check_line_lengths.sh.

  • When contributing text to the guide, please contextualize the information with some time period and/or a reason so that the reader knows how much to trust or mistrust the information. Aim to provide a reasonable amount of context, possibly including but not limited to:

    • A reason for why the data may be out of date other than "change", as change is a constant across the project.

    • The date the comment was added, e.g. instead of writing "Currently, ..." or "As of now, ...", consider adding the date, in one of the following formats:

      • Jan 2021
      • January 2021
      • jan 2021
      • january 2021

      There is a CI action (in ~/.github/workflows/date-check.yml) that generates a monthly issue with any of these that are over 6 months old.

      For the action to pick the date, add a special annotation before specifying the date:

      <!-- date-check --> Jul 2022
      

      Example:

      As of <!-- date-check --> Jul 2022, the foo did the bar.
      

      For cases where the date should not be part of the visible rendered output, use the following instead:

      <!-- date-check: Jul 2022 -->
      
    • A link to a relevant WG, tracking issue, rustc rustdoc page, or similar, that may provide further explanation for the change process or a way to verify that the information is not outdated.

  • If a text grows rather long (more than a few page scrolls) or complicated (more than four subsections) it might benefit from having a Table of Contents at the beginning, which you can auto-generate by including the <!-- toc --> marker.

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 discussion at the next meeting of the team tagged using a T-prefixed label. 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 can be assigned by anyone that understand the issue and is able to prioritize it, 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:

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.