Developing with a local Glean build

FOG uses a release version of Glean, as published on

For local development and try runs you can replace this Glean implementation with a local or remote version.

  1. To tell mach where to find your Glean, patch the top-level Cargo.toml. E.g. like this:

    glean = { git = "", branch = "my-feature-branch" }
    glean-core = { git = "", branch = "my-feature-branch" }

    Both crates are required to ensure they are in sync.

    You can specify the exact code to use by branch, tag or rev (Git commit). See the cargo documentation for details.

    You can also use a path dependency:

    glean = { path = "../glean/glean-core/rlb" }
    glean_core = { path = "../glean/glean-core" }
  2. If the crate version in the patched repository is not semver-compatible with the version required by the fog and fog_control crates, you need to change the version in the following files to match the ones in your glean repo:

  3. Update the Cargo lockfile:

    cargo update -p glean
  4. Vendor the changed crates:

    ./mach vendor rust

    Note: If you’re using a path dependency, mach vendor rust doesn’t actually change files. Instead it pulls the files directly from the location on disk you specify.

  5. Finally build Firefox:

    ./mach build

A remote reference works for try runs as well, but a path dependency will not.

Please ensure to not land a non-release version of Glean.