Build Fuchsia with a custom Rust toolchain

This guide explains how to build a Rust compiler for use with the Fuchsia build and configure a Fuchsia build to use it. This is useful if you need to build Fuchsia with a patched compiler, or a compiler built with custom options.

Use another version of Rust

If all you need to do is use a different version than the one currently being used, most of this doc is not necessary. Fuchsia builders build Rust after every change to Rust's main branch.

Find the commit hash you want to use.

Run the following commands from your Fuchsia directory:

# Replace COMMIT with the full Rust commit hash.
# This command updates the manifests in the integration repo, which you can
# then commit or revert as necessary.
fx roll-compiler --package rust git_revision:{{ '<var>' }}COMMIT{{ '</var>' }}

# Optional: Update developer tools.
fx roll-compiler --package rust_tools git_revision:{{ '<var>' }}COMMIT{{ '</var>' }} --platforms linux-amd64 --platforms mac-amd64

# Fetch the package versions you specified and install them in `prebuilt/`.
jiri fetch-packages -local-manifest

Run the following command to build Fuchsia:
```
fx build
```
The Fuchsia build now uses the updated compiler.
Note: A clean build is not necessary; the build automatically detects the new compiler version.

Prerequisites

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

Note: These instructions are for Debian-based systems, but you should use the correct package manager for your machine.

Run the following command to install cmake:
```
sudo apt-get install cmake ninja-build
```
Complete the following guide to download the Fuchsia source: Get Fuchsia source code. To confirm that jiri is in your PATH run jiri -help.
Note: The below commands assume DEV_ROOT is set to the parent directory of your Fuchsia checkout.

Run the following command to obtain the infra sources:

DEV_ROOT={{ '<var>' }}DEV_ROOT{{ '</var> '}} # parent of your fuchsia directory
mkdir -p $DEV_ROOT/infra && \
( \

  builtin cd $DEV_ROOT/infra && \
  jiri init && \
  jiri import -overwrite -name=fuchsia/prebuilt infra \
      https://fuchsia.googlesource.com/manifest && \
  jiri update \
)

Note: Running jiri update from the infra directory ensures that you have the most recent configurations and tools.

Download and extract the Fuchsia core IDK to $DEV_ROOT/sdk. For more information, see Download the Fuchsia IDK.

Run the following command to use cipd to get the linux sysroot package for your host platform:

# You may want to: rm -rf $DEV_ROOT/sysroot
mkdir -p $DEV_ROOT/sysroot
cipd install fuchsia/sysroot/linux-amd64 latest -root $DEV_ROOT/sysroot/linux-x64
cipd install fuchsia/sysroot/linux-arm64 latest -root $DEV_ROOT/sysroot/linux-arm64

If you haven't already, clone the Rust source.

git clone https://github.com/rust-lang/rust.git $DEV_ROOT/rust

Configure Rust for Fuchsia

Change into your Rust directory.

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

DEV_ROOT={{ '<var>' }}DEV_ROOT{{ '</var>' }}

$DEV_ROOT/infra/fuchsia/prebuilt/tools/vpython \
  $DEV_ROOT/infra/fuchsia/recipes/recipes/rust_toolchain.resources/generate_config.py \
    config_toml \
    --clang-prefix=$DEV_ROOT/fuchsia/prebuilt/third_party/clang/linux-x64 \
    --host-sysroot=$DEV_ROOT/fuchsia/prebuilt/third_party/sysroot/linux \
    --prefix=$(pwd)/install/fuchsia-rust \
   | tee fuchsia-config.toml

$DEV_ROOT/infra/fuchsia/prebuilt/tools/vpython \
    $DEV_ROOT/infra/fuchsia/recipes/recipes/rust_toolchain.resources/generate_config.py \
      environment \
      --eval \
      --clang-prefix=$DEV_ROOT/fuchsia/prebuilt/third_party/clang/linux-x64 \
      --sdk-dir=$DEV_ROOT/sdk \
      --linux-amd64-sysroot=$DEV_ROOT/sysroot/linux-x64 \
      --linux-arm64-sysroot=$DEV_ROOT/sysroot/linux-arm64 \
   | 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={{ '<var>' }}DEV_ROOT{{ '</var>' }}

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 ) && \
rm -rf install/fuchsia-rust/lib/.build-id && \
$DEV_ROOT/infra/fuchsia/prebuilt/tools/vpython \
  $DEV_ROOT/infra/fuchsia/recipes/recipes/rust_toolchain.resources/generate_config.py \
    runtime \
  | $DEV_ROOT/infra/fuchsia/prebuilt/tools/vpython \
      $DEV_ROOT/infra/fuchsia/recipes/recipe_modules/toolchain/resources/runtimes.py \
        --dir install/fuchsia-rust/lib \
        --readelf fuchsia-build/*/llvm/bin/llvm-readelf \
        --objcopy fuchsia-build/*/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 )

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.

Build Fuchsia with your custom Rust toolchain

Change into your Fuchsia directory.

Run the following command to use the newly built toolchain:

DEV_ROOT={{ '<var>' }}DEV_ROOT{{ '</var>' }}

fx set core.x64 \
  --args=rustc_prefix="\"$DEV_ROOT/rust/install/fuchsia-rust/bin\"" \
  --args=rustc_version_string='"1"'
# plus other settings such as:
#   --with //bundles:kitchen_sink
#   --variant=coverage-rust  # to enable coverage profiling of fuchsia binaries
#   --variant=host_coverage-rust  # to enable coverage profiling of host binaries

Note: rustc_version_string can be any string, and it’s used to force a recompile after a custom toolchain changes. If you rebuild the toolchain, change the value so Rust targets get rebuilt.

Run the following command to rebuild Fuchsia:
```
fx build
```