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.

Prerequisites

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

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 https://github.com/rust-lang/rust.git $DEV_ROOT/rust
```
Run the following command to install cmake and ninja:
```
sudo apt-get install cmake ninja-build
```

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 \
      https://fuchsia.googlesource.com/manifest && \
  jiri update \
)

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:

DEV_ROOT=DEV_ROOT
HOST_TRIPLE=x86_64-unknown-linux-gnu
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
EOF
STAGE0_DATE=$(sed -nr 's/^compiler_date=(.*)/\1/p' ${DEV_ROOT}/rust/src/stage0)
STAGE0_VERSION=$(sed -nr 's/^compiler_version=(.*)/\1/p' ${DEV_ROOT}/rust/src/stage0)
STAGE0_COMMIT_HASH=$( \
  curl -s "https://static.rust-lang.org/dist/${STAGE0_DATE}/channel-rust-${STAGE0_VERSION}.toml" \
  | python3 -c 'import tomllib, sys; print(tomllib.load(sys.stdin.buffer)["pkg"]["rust"]["git_commit_hash"])')
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 generate_config.py below.

Configure Rust for Fuchsia

Change into your Rust directory.

Run the following command to generate a configuration for the Rust toolchain:

DEV_ROOT=DEV_ROOT
$DEV_ROOT/infra/fuchsia/prebuilt/tools/vpython3 \
  $DEV_ROOT/infra/fuchsia/recipes/recipes/rust_toolchain.resources/generate_config.py \
    config_toml \
    --clang-prefix=$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 \
    $DEV_ROOT/infra/fuchsia/recipes/recipes/rust_toolchain.resources/generate_config.py \
      environment \
      --eval \
      --clang-prefix=$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 fuchsia-env.sh

(Optional) Run the following command to tell git to ignore the generated files:

echo fuchsia-config.toml >> .git/info/exclude
echo fuchsia-env.sh >> .git/info/exclude

(Optional) Customize fuchsia-config.toml.

Build and install Rust

Change into your Rust source directory.

Run the following command to build and install Rust plus the Fuchsia runtimes spec:

DEV_ROOT=DEV_ROOT
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 fuchsia-env.sh && ./x.py 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/generate_config.py \
    runtime \
  | $DEV_ROOT/infra/fuchsia/prebuilt/tools/vpython3 \
      $DEV_ROOT/infra/fuchsia/recipes/recipe_modules/toolchain/resources/runtimes.py \
        --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 fuchsia-env.sh && ./x.py build --config fuchsia-config.toml \
  --skip-stage0-validation )

Troubleshooting

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.