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:

Run the following command to install cmake:
```
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, and a recent version of clang:

DEV_ROOT=DEV_ROOT
cat <<"EOF" > cipd.ensure
@Subdir sdk
fuchsia/sdk/core/${platform} latest
@Subdir sysroot/linux
fuchsia/third_party/sysroot/linux latest
@Subdir clang
fuchsia/third_party/clang/${platform} integration
EOF
$DEV_ROOT/infra/fuchsia/prebuilt/tools/cipd ensure --root . --ensure-file cipd.ensure

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.
```
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=DEV_ROOT
$DEV_ROOT/infra/fuchsia/prebuilt/tools/vpython3 \
  $DEV_ROOT/infra/fuchsia/recipes/recipes/contrib/rust_toolchain.resources/generate_config.py \
    config_toml \
    --clang-prefix=$DEV_ROOT/clang \
    --host-sysroot=$DEV_ROOT/sysroot/linux \
    --prefix=$(pwd)/install/fuchsia-rust \
   | tee fuchsia-config.toml
$DEV_ROOT/infra/fuchsia/prebuilt/tools/vpython3 \
    $DEV_ROOT/infra/fuchsia/recipes/recipes/contrib/rust_toolchain.resources/generate_config.py \
      environment \
      --eval \
      --clang-prefix=$DEV_ROOT/clang \
      --sdk-dir=$DEV_ROOT/sdk \
      --linux-amd64-sysroot=$DEV_ROOT/sysroot/linux \
      --linux-arm64-sysroot=$DEV_ROOT/sysroot/linux \
   | 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 ) && \
rm -rf install/fuchsia-rust/lib/.build-id && \
$DEV_ROOT/infra/fuchsia/prebuilt/tools/vpython3 \
  $DEV_ROOT/infra/fuchsia/recipes/recipes/contrib/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 \
        --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.

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.