Unstable features

Most new features should go through the unstable process. This means that the feature will only be usable on the nightly channel, and requires a specific opt-in by the user. Small changes can skip this process, but please consult with the Cargo team first.

Unstable feature opt-in

For features that require behavior changes or new syntax in Cargo.toml, then it will need a cargo-features value placed at the top of Cargo.toml to enable it. The process for doing adding a new feature is described in the features module. Code that implements the feature will need to manually check that the feature is enabled for the current manifest.

For features that add new command-line flags, config options, or environment variables, then the -Z flags will be needed to enable them. The features module also describes how to add these. New flags should use the fail_if_stable_opt method to check if the -Z unstable-options flag has been passed.

Unstable documentation

Every unstable feature should have a section added to the unstable chapter describing how to use the feature.

-Z CLI flags should be documented in the built-in help in the cli module.

Tracking issues

Each unstable feature should get a tracking issue. These issues are typically created when a PR is close to being merged, or soon after it is merged. Use the tracking issue template when creating a tracking issue.

Larger features should also get a new label in the issue tracker so that when issues are filed, they can be easily tied together.


After some period of time, typically measured in months, the feature can be considered to be stabilized. The feature should not have any significant known bugs or issues, and any design concerns should be resolved.

The stabilization process depends on the kind of feature. For smaller features, you can leave a comment on the tracking issue expressing interest in stabilizing it. It can usually help to indicate that the feature has received some real-world testing, and has exhibited some demand for broad use.

For larger features that have not gone through the RFC process, then an RFC to call for stabilization might be warranted. This gives the community a final chance to provide feedback about the proposed design.

For a small feature, or one that has already gone through the RFC process, a Cargo Team member may decide to call for a "final comment period" using rfcbot. This is a public signal that a major change is being made, and gives the Cargo Team members an opportunity to confirm or block the change. This process can take a few days or weeks, or longer if a concern is raised.

Once the stabilization has been approved, the person who called for stabilization should prepare a PR to stabilize the feature. This PR should:

  • Flip the feature to stable in the features module.
  • Remove any unstable checks that aren't automatically handled by the feature system.
  • Move the documentation from the unstable chapter into the appropriate places in the Cargo book and man pages.
  • Remove the -Z flags and help message if applicable.
  • Update all tests to remove nightly checks.
  • Tag the PR with relnotes label if it seems important enough to highlight in the Rust release notes.