Third-party Rust Crates


OSRB approval is required for third-party crates. To get approval, you will need to follow the instructions under the “Process for 3rd Party Hosted Code” section in this document.

Third-party crates depended on by rustc_binary and rustc_library targets are stored in //third-party/rust_crates/vendor. This set of crates is based on the dependencies listed in //third_party/rust_crates/Cargo.toml, and is updated by running fx update-rustc-third-party, which will update the precise versions of the crates used in the Cargo.lock file and download any necessary crates into the vendor dir.

Adding a new vendored dependency

If a crate is not available in the vendor directory, it can to be added with the following steps:

  1. Reference the crates you need in third_party/rust_crates/Cargo.toml.
  2. Run scripts/fx update-rustc-third-party. This will download all crates listed in rust_crates/Cargo.toml as well as their dependencies and place them in the vendor directory.
  3. git add the Cargo.toml, Cargo.lock and vendor directory.
  4. Do not upload this change to gerrit yet.
  5. Get OSRB approval. Make sure to include the requested information for all new crates pulled in by your new dependency. If there are any files in the source repository that are not included when vendored, make a note of that for the OSRB reviewer. For example, font files that are only used for testing but are excluded when the crate is vendored. If you are not a Google employee, you will need to ask a Google employee to do this part for you.
  6. After getting the OSRB approval, upload the change to Gerrit. Get code-review+2, merge the change into third_party/rust_crates.

Adding a new mirror

  1. Request the addition of a mirror on;
  2. Add the mirror to the Jiri manifest for the Rust runtime;
  3. Add a patch section for the crate to the workspace;
  4. Run the update script.

Unicode crates

If the project requires importing a new third-party crate to handle functionality related to Unicode and internationalization, prefer crates from the UNIC project when available.

Grandfathered non-UNIC crates

The following non-UNIC crates are already vendored and are grandfathered, but we will aim to migrate to UNIC equivalents when possible.

  • unicode-bidi
  • unicode-normalization
  • unicode-segmentation
  • unicode-width
  • unicode-xid

We should encourage upstream dependencies to migrate to UNIC as well.

Rationale for standardization

UNIC crates have distinct advantages over other crates:

  • UNIC crates are developed in a single repo, with shared common code and a single version scheme.

    • Independently developed crates do not share a common release schedule, versioning scheme, or adherence to any particular version of the Unicode standard.
  • UNIC crates are generated from a consistent set of Unicode data files.

    • Each of the independent crates uses an arbitrary version and subset of the data. For example, different crates might have different assumptions about whether a particular code point is assigned, what its properties are, etc.
  • The UNIC project is aiming for comprehensive feature coverage, to be like ICU for Rust. If the project succeeds, our dependencies on unrelated Unicode crates should be reduced over time.