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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:

Running tests

Most of the tests are found in the testsuite integration test. This can be run with a simple cargo test.

Some tests only run on the nightly toolchain, and will be ignored on other channels. It is recommended that you run tests with both nightly and stable to ensure everything is working as expected.

Some tests exercise cross compiling to a different target. This will require you to install the appropriate target. This typically is the 32-bit target of your host platform. For example, if your host is a 64-bit x86_64-unknown-linux-gnu, then you should install the 32-bit target with rustup target add i686-unknown-linux-gnu. If you don't have the alternate target installed, there should be an error message telling you what to do. You may also need to install additional tools for the target. For example, on Ubuntu you should install the gcc-multilib package.

If you can't install an alternate target, you can set the CFG_DISABLE_CROSS_TESTS=1 environment variable to disable these tests. Unfortunately 32-bit support on macOS is going away, so you may not be able to run these tests on macOS. The Windows cross tests only support the MSVC toolchain.

Some of the nightly tests require the rustc-dev component installed. This component includes the compiler as a library. This may already be installed with your nightly toolchain, but it if isn't, run rustup component add rustc-dev --toolchain=nightly.

There are several other packages in the repo for running specialized tests, and you will need to run these tests separately by changing into its directory and running cargo test:

The build-std tests are disabled by default, but you can run them by setting the CARGO_RUN_BUILD_STD_TESTS=1 environment variable and running cargo test --test build-std. This requires the nightly channel, and also requires the rust-src component installed with rustup component add rust-src --toolchain=nightly.

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

See the documentation README for details on building the documentation.

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: