Build a custom Rust toolchain for Fuchsia

This guide explains how to build a Rust compiler for use with the Fuchsia. This is useful if you need to build Fuchsia with a patched compiler, or a compiler built with custom options. Building a custom Rust toolchain is not always necessary for building Fuchsia with a different version of Rust; see Build Fuchsia with a custom Rust toolchain for details.


Prior to building a custom Rust toolchain for Fuchsia, you need to do the following:

  1. If you haven't already, clone the Rust source. The Guide to Rustc Development is a good resource to reference whenever you're working on the compiler.

    DEV_ROOT=DEV_ROOT  # parent of your Rust directory
    git clone --recurse-submodules $DEV_ROOT/rust
  2. Run the following command to install cmake and jq/tomlq:

    sudo apt-get install cmake ninja-build jq yq
  3. Run the following command to obtain the infra sources:

    DEV_ROOT=DEV_ROOT  # parent of your Rust directory
    mkdir -p $DEV_ROOT/infra && \
    ( \
      builtin cd $DEV_ROOT/infra && \
      jiri init && \
      jiri import -overwrite -name=fuchsia/manifest infra \
 && \
      jiri update \
  4. Run the following command to use cipd to get a Fuchsia core IDK, a Linux sysroot, a recent version of clang, and the correct beta compiler for building Fuchsia's Rust toolchain:

    cat << "EOF" > cipd.ensure
    @Subdir sdk
    fuchsia/sdk/core/${platform} latest
    @Subdir sysroot/linux
    fuchsia/third_party/sysroot/linux git_revision:db18eec0b4f14b6b16174aa2b91e016663157376
    @Subdir sysroot/focal
    fuchsia/third_party/sysroot/focal latest
    @Subdir clang
    fuchsia/third_party/clang/${platform} integration
    STAGE0_DATE=`jq ${DEV_ROOT}/rust/src/stage0.json --raw-output`
    STAGE0_VERSION=`jq .compiler.version ${DEV_ROOT}/rust/src/stage0.json --raw-output`
    STAGE0_COMMIT_HASH=`curl -s "${STAGE0_DATE}/channel-rust-${STAGE0_VERSION}.toml" | tomlq .pkg.rust.git_commit_hash --raw-output`
    echo "@Subdir stage0" >> cipd.ensure
    echo "fuchsia/third_party/rust/host/\${platform} git_revision:${STAGE0_COMMIT_HASH}" >> cipd.ensure
    echo "fuchsia/third_party/rust/target/${HOST_TRIPLE} git_revision:${STAGE0_COMMIT_HASH}" >> cipd.ensure
    $DEV_ROOT/infra/fuchsia/prebuilt/tools/cipd ensure --root $DEV_ROOT --ensure-file cipd.ensure

    Downloading the Fuchsia-built stage0 compiler is optional, but useful for recreating builds in CI. If the stage0 is not available you may instruct the Rust build to download and use the upstream stage0 compiler by omitting those lines from your cipd.ensure file and removing the --stage0 arguments to below.

Configure Rust for Fuchsia

  1. Change into your Rust directory.
  2. Run the following command to generate a configuration for the Rust toolchain:

    $DEV_ROOT/infra/fuchsia/prebuilt/tools/vpython3 \
      $DEV_ROOT/infra/fuchsia/recipes/recipes/rust_toolchain.resources/ \
        config_toml \
        --clang-prefix=$DEV_ROOT/clang \
        --host-sysroot=$DEV_ROOT/sysroot/linux \
        --stage0=$DEV_ROOT/stage0 \
        --prefix=$(pwd)/install/fuchsia-rust \
       | tee fuchsia-config.toml
    $DEV_ROOT/infra/fuchsia/prebuilt/tools/vpython3 \
        $DEV_ROOT/infra/fuchsia/recipes/recipes/rust_toolchain.resources/ \
          environment \
          --eval \
          --clang-prefix=$DEV_ROOT/clang \
          --sdk-dir=$DEV_ROOT/sdk \
          --stage0=$DEV_ROOT/stage0 \
          --linux-sysroot=$DEV_ROOT/sysroot/linux \
          --linux-riscv64-sysroot=$DEV_ROOT/sysroot/focal \
       | tee
  3. (Optional) Run the following command to tell git to ignore the generated files:

    echo fuchsia-config.toml >> .git/info/exclude
    echo >> .git/info/exclude
  4. (Optional) Customize fuchsia-config.toml.

Build and install Rust

  1. Change into your Rust source directory.
  2. Run the following command to build and install Rust plus the Fuchsia runtimes spec:

    rm -rf install/fuchsia-rust
    mkdir -p install/fuchsia-rust
    # Copy and paste the following subshell to build and install Rust, as needed.
    # The subshell avoids polluting your environment with fuchsia-specific rust settings.
    ( source && ./ install --config fuchsia-config.toml \
      --skip-stage0-validation ) && \
    rm -rf install/fuchsia-rust/lib/.build-id && \
    $DEV_ROOT/infra/fuchsia/prebuilt/tools/vpython3 \
      $DEV_ROOT/infra/fuchsia/recipes/recipes/rust_toolchain.resources/ \
        runtime \
      | $DEV_ROOT/infra/fuchsia/prebuilt/tools/vpython3 \
          $DEV_ROOT/infra/fuchsia/recipes/recipe_modules/toolchain/resources/ \
            --dir install/fuchsia-rust/lib \
            --dist dist \
            --readelf fuchsia-build/host/llvm/bin/llvm-readelf \
            --objcopy fuchsia-build/host/llvm/bin/llvm-objcopy \
      > install/fuchsia-rust/lib/runtime.json

Build only (optional)

If you want to skip the install step, for instance during development of Rust itself, you can do so with the following command.

( source && ./ build --config fuchsia-config.toml \
  --skip-stage0-validation )


If you are getting build errors, try deleting the Rust build directory:

rm -rf fuchsia-build

Then re-run the command to build Rust.

Building Fuchsia with a custom Rust toolchain

With a newly compiled custom Rust toolchain, you're ready to use it to build Fuchsia. Directions on how to do so are available in a dedicated guide.