Third-party Rust crates


Fuchsia uses third-party Rust crates. They are placed in //third-party/rust_crates/vendor. This set of crates is based on the dependencies listed in //third_party/rust_crates/Cargo.toml. If you don't find a crate that you want to use, you may bring that into Fuchsia.

To add a third-party crate, the steps are:

  • Calculate dependencies.
  • Get Open Source Review Board (OSRB) approval.
  • Upload the change for code review.

Pay attention to transitive dependencies: A third-party crate may depend on other third-party crates. List all the new crates that end up with being brought in, in the OSRB review. For OSRB, follow the instructions under the “Process for 3rd Party Hosted Code” section in this document.

Note: You need to get OSRB approval first before uploading a change for review.

Steps to add a third-party crate

  1. Change directory to Fuchsia repo base directory (For example, cd ~/fuchsia).

  2. Add an entry in third_party/rust_crates/Cargo.toml for the crate you want to add.

  3. Run the following command to calculate the dependencies and download the crates:

    fx update-rustc-third-party

    This command downloads all crates listed in rust_crates/Cargo.toml as well as their dependencies, places them in the vendor directory, and updates Cargo.toml and Cargo.lock.

    You may need to provide additional configuration in a [gn.package.<crate>] section inside the Cargo.toml file. For crates that use a script this configuration replaces the script, which is (intentionally) unsupported by our build system. This configuration is used by cargo-gnaw, which generates the GN rules from the Cargo.toml file. See cargo-gnaw's README for more details.

    Note: on Linux, pkg-config needs to be installed.

  4. Do a build test. For example:

    fx set core.x64 && fx build
  5. Do not submit the change for code review. Get OSRB approval first.

    • Create an tracking issue at Open Source Review Board (OSRB) template
    • Do not add a specific ‘owner’ to the request. The OSRB team meets regularly to review the issues. Please allow about a week for a response.
    • Identify all the crates to be brought in (see the diff in //third_party/rust_crates/vendor/).
    • 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.

    Note: As part of OSRB review, you may be asked to import only a subset of the files in a third-party crate. See Importing a subset of files in a crate for how to do this.

  6. After the OSRB approval, upload the change for review to Gerrit. Include the OSRB bug number in the change (e.g. Bug: “1234”).

  7. Get code-review+2 and merge the change into third_party/rust_crates.

Steps to update a third-party crate

Updating is very similar to adding a crate.

To update a third-party crate, do the following:

  1. Start by bumping the version number of the crate in third_party/rust_crates/Cargo.toml and rerunning fx update-rustc-third-party as above.

    You may need to update or provide additional configuration in [gn.package.<crate>] sections inside the Cargo.toml file. For crates that use a script this configuration replaces the script, which is (intentionally) unsupported by our build system. This configuration is used by cargo-gnaw, which generates the GN rules from the Cargo.toml file. See cargo-gnaw's README for more details.

  2. Identify all new library dependencies brought in (see the diff in //third_party/rust_crates/vendor/). Again, do not submit the change for code review until you've received OSRB approval for any new dependencies added.

  3. After OSRB approval, upload the change for review to Gerrit and merge as above.

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.

Importing a subset of files in a crate

In some cases, you may want to import only a subset of files in a crate. For example, there may be an optional license in the third-party repo that‘s incompatible with Fuchsia’s license requirements. Here's an example OSRB review in which this happened.

To do this, you‘ll need to add the crate’s files to /third_party/rust_crates/tiny_mirrors.

  1. Follow the instructions for adding a third-party crate.
  2. After running fx update-rustc-third-party, move the downloaded copy of your crate from /third_party/rust_crates/vendor/<my_crate> to /third_party/rust_crates/tiny_mirrors.
  3. Make the changes you need to make to the imported files.
  4. Add a line to the [patch.crates-io] section of /third_party/rust_crates/Cargo.toml to point to your new crate:
    my_crate = { path = "tiny_mirrors/my_crate" }
  5. Re-run fx update-rustc-third-party and fx build.

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.


Broken Config

After running fx update-rustc-third-party, if you encounter an error like this:

Generating GN file from /$HOME/fuchsia/third_party/rust_crates/Cargo.toml
Error: GNaw config exists for crates that were not found in the Cargo build graph:

library crate, package handlebars version 2.0.0-beta.2
library crate, package core-foundation-sys version 0.7.0
library crate, package pulldown-cmark version 0.6.0
library crate, package nix version 0.18.0

You can fix this issue commenting out your fuchsia target in .cargo/config:

target = "x86_64-fuchsia"

After commenting, it becomes:

# target = "x86_64-fuchsia"

This issue is being tracked upstream.