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Contributing to Cargo

Thank you for your interest in contributing to Cargo! Good places to start are this document,, which describes the high-level structure of Cargo and E-easy bugs on the issue tracker.

If you have a general question about Cargo or it's internals, feel free to ask on Discord.

Code of Conduct

All contributors are expected to follow our Code of Conduct.

Bug reports

We can't fix what we don't know about, so please report problems liberally. This includes problems with understanding the documentation, unhelpful error messages and unexpected behavior.

If you think that you have identified an issue with Cargo that might compromise its users' security, please do not open a public issue on GitHub. Instead, we ask you to refer to Rust's security policy.

Opening an issue is as easy as following this link and filling out the fields. Here's a template that you can use to file an issue, though it's not necessary to use it exactly:

<short summary of the problem>

I tried this: <minimal example that causes the problem>

I expected to see this happen: <explanation>

Instead, this happened: <explanation>

I'm using <output of `cargo --version`>

All three components are important: what you did, what you expected, what happened instead. Please use if your examples run long.

Feature requests

Cargo follows the general Rust model of evolution. All major features go through an RFC process. Therefore, before opening a feature request issue create a Pre-RFC thread on the internals forum to get preliminary feedback. Implementing a feature as a custom subcommand is encouraged as it helps demonstrate the demand for the functionality and is a great way to deliver a working solution faster as it can iterate outside of cargo's release cadence.

Working on issues

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

Feel free to ask for guidelines on how to tackle a problem on Discord or open a new issue. This is especially important if you want to add new features to Cargo or make large changes to the already existing code-base. Cargo's core developers will do their best to provide help.

If you start working on an already-filed issue, post a comment on this issue to let people know that somebody is working it. Feel free to ask for comments if you are unsure about the solution you would like to submit.

While Cargo does make use of some Rust-features available only through the nightly toolchain, it must compile on stable Rust. Code added to Cargo is encouraged to make use of the latest stable features of the language and stdlib.

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. This process is partly automated: Pull requests are made against Cargo's master-branch, tested and reviewed. Once a change is approved to be merged, a friendly bot merges the changes into an internal branch, runs the full test-suite on that branch and only then merges into master. This ensures that Cargo's master branch passes the test-suite at all times.

Your basic steps to get going:

  • Fork Cargo and create a branch from master for the issue you are working on.
  • Please adhere to the code style that you see around the location you are working on.
  • Commit as you go.
  • Include tests that cover all non-trivial code. The existing tests in test/ provide templates on how to test Cargo's behavior in a sandbox-environment. The internal module testsuite/support provides a vast amount of helpers to minimize boilerplate. See testsuite/support/ for an introduction to writing tests.
  • Make sure cargo test passes. If you do not have the cross-compilers installed locally, install them using the instructions returned by cargo test cross_compile::cross_tests (twice, with --toolchain nightly added to get the nightly cross target too); alternatively just ignore the cross-compile test failures or disable them by using CFG_DISABLE_CROSS_TESTS=1 cargo test. Note that some tests are enabled only on nightly toolchain. If you can, test both toolchains.
  • All code changes are expected to comply with the formatting suggested by rustfmt. You can use rustup component add --toolchain nightly rustfmt to install rustfmt and use rustfmt +nightly --unstable-features --skip-children on the changed files to automatically format your code.
  • Push your commits to GitHub and create a pull request against Cargo's master branch.

Pull requests

After the pull request is made, a friendly bot will automatically assign a reviewer; the review-process will make sure that the proposed changes are sound. Please give the assigned reviewer sufficient time, especially during weekends. If you don't get a reply, you may poke the core developers on Discord.

A merge of Cargo's master-branch and your changes is immediately queued to be tested after the pull request is made. In case unforeseen problems are discovered during this step (e.g., a failure on a platform you originally did not develop on), you may ask for guidance. Push additional commits to your branch to tackle these problems.

The reviewer might point out changes deemed necessary. Please add them as extra commits; this ensures that the reviewer can see what has changed since the code was previously reviewed. Large or tricky changes may require several passes of review and changes.

Once the reviewer approves your pull request, a friendly bot picks it up and merges it into Cargo's master branch.

Contributing to the documentation

To contribute to the documentation, all you need to do is change the markdown files in the src/doc directory. To view the rendered version of changes you have made locally, make sure you have mdbook installed and run:

cd src/doc
mdbook build
open book/index.html

To install mdbook run cargo install mdbook.

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.

Contributors with sufficient permissions on the Rust-repository can help by adding labels to triage issues:

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

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

  • Light purple, C-prefixed labels represent the category of an issue. In particular, C-feature-request marks proposals for new features. If an issue is C-feature-request, but is not Feature accepted or I-nominated, then it was not thoroughly discussed, and might need some additional design or perhaps should be implemented as an external subcommand first. Ping @rust-lang/cargo if you want to send a PR for such issue.

  • Dark purple, Command-prefixed labels mean the issue has to do with a specific cargo command.

  • Green, E-prefixed labels explain the level of experience or effort necessary to fix the issue. E-mentor issues also have some instructions on how to get started.

  • 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.

  • 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-nominated label.

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

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