Fuchsia integration tests

Fuchsia is an open-source operating system with about 2 million lines of Rust code.1 It has caught a large number of regressions in the past and was subsequently included in CI.

Building Fuchsia in CI

Fuchsia builds as part of the suite of bors tests that run before a pull request is merged.

If you are worried that a pull request might break the Fuchsia builder and want to test it out before submitting it to the bors queue, simply add this line to your PR description:

try-job: x86_64-fuchsia

Then when you @bors try it will pick the job that builds Fuchsia.

Building Fuchsia locally

Because Fuchsia uses languages other than Rust, it does not use Cargo as a build system. It also requires the toolchain build to be configured in a certain way.

The recommended way to build Fuchsia is to use the Docker scripts that check out and run a Fuchsia build for you. If you've run Docker tests before, you can simply run this command from your Rust checkout to download and build Fuchsia using your local Rust toolchain.

src/ci/docker/run.sh x86_64-fuchsia

See the Testing with Docker chapter for more details on how to run and debug jobs with Docker.

Note that a Fuchsia checkout is large – as of this writing, a checkout and build takes 46G of space – and as you might imagine, it takes awhile to complete.

Modifying the Fuchsia checkout

The main reason you would want to build Fuchsia locally is because you need to investigate a regression. After running a Docker build, you'll find the Fuchsia checkout inside the obj/fuchsia directory of your Rust checkout. If you modify the KEEP_CHECKOUT line in the build-fuchsia.sh script to KEEP_CHECKOUT=1, you can change the checkout as needed and rerun the build command above. This will reuse all the build results from before.

You can find more options to customize the Fuchsia checkout in the build-fuchsia.sh script.

Customizing the Fuchsia build

You can find more info about the options used to build Fuchsia in Rust CI in the build_fuchsia_from_rust_ci.sh script invoked by build-fuchsia.sh.

The Fuchsia build system uses GN, a metabuild system that generates Ninja files and then hands off the work of running the build to Ninja.

Fuchsia developers use fx to run builds and perform other development tasks. This tool is located in .jiri_root/bin of the Fuchsia checkout; you may need to add this to your $PATH for some workflows.

There are a few fx subcommands that are relevant, including:

  • fx set accepts build arguments, writes them to out/default/args.gn, and runs GN.
  • fx build builds the Fuchsia project using Ninja. It will automatically pick up changes to build arguments and rerun GN. By default it builds everything, but it also accepts target paths to build specific targets (see below).
  • fx clippy runs Clippy on specific Rust targets (or all of them). We use this in the Rust CI build to avoid running codegen on most Rust targets. Underneath it invokes Ninja, just like fx build. The clippy results are saved in json files inside the build output directory before being printed.

Target paths

GN uses paths like the following to identify build targets:


The initial // means the root of the checkout, and the remaining slashes are directory names. The string after : is the target name of a target defined in the BUILD.gn file of that directory.

The target name can be omitted if it is the same as the directory name. In other words, //src/starnix/kernel is the same as //src/starnix/kernel:kernel.

These target paths are used inside BUILD.gn files to reference dependencies, and can also be used in fx build.

Modifying compiler flags

You can put custom compiler flags inside a GN config that is added to a target. As a simple example:

config("everybody_loops") {
    rustflags = [ "-Zeverybody-loops" ]

rustc_binary("example") {
    crate_root = "src/bin.rs"
    # ...existing keys here...
    configs += [ ":everybody_loops" ]

This will add the flag -Zeverybody-loops to rustc when building the example target. Note that you can also use public_configs for a config to be added to every target that depends on that target.

If you want to add a flag to every Rust target in the build, you can add rustflags to the //build/config:compiler config or to the OS-specific configs referenced in that file. Note that cflags and ldflags are ignored on Rust targets.

Running ninja and rustc commands directly

Going down one layer, fx build invokes ninja, which in turn eventually invokes rustc. All build actions are run inside the out directory, which is usually out/default inside the Fuchsia checkout.

You can get ninja to print the actual command it invokes by forcing that command to fail, e.g. by adding a syntax error to one of the source files of the target. Once you have the command, you can run it from inside the output directory.

After changing the toolchain itself, the build setting rustc_version_string in out/default/args.gn needs to be changed so that fx build or ninja will rebuild all the Rust targets. This can be done in a text editor and the contents of the string do not matter, as long as it changes from one build to the next. build_fuchsia_from_rust_ci.sh does this for you by hashing the toolchain directory.

The Fuchsia website has more detailed documentation of the build system.

Other tips and tricks

When using build_fuchsia_from_rust_ci.sh you can comment out the fx set command after the initial run so it won't rerun GN each time. If you do this you can also comment out the version_string line to save a couple seconds.

export NINJA_PERSISTENT_MODE=1 to get faster ninja startup times after the initial build.

Fuchsia target support

To learn more about Fuchsia target support, see the Fuchsia chapter in the rustc book.


As of June 2024, Fuchsia had about 2 million lines of first-party Rust code and a roughly equal amount of third-party code, as counted by tokei (excluding comments and blanks).