Bootstrap rustc with Buck2

NOTE: This is an experimental project

Dependencies

• Buck2 for building: https://github.com/facebookincubator/buck2
  • https://buck2.build/docs/getting_started/
• clang++ and python, as this build assumes those are available on the path
• Reindeer for generating build files: https://github.com/facebookincubator/reindeer
  • Only needed if you wish to make changes to the generated build files

Initial setup

• Caveat: This build makes lots of assumptions about what target it runs on, so it probably only works for x86_64-unknown-linux-gnu (it can be cleaned up)
• To get started run import_rustc.sh. This script will pull in the bootstrap compiler and ci-llvm artifacts that the build depends on.
  • The code under library/src and compiler/src are pulled in from https://github.com/rust-lang/rust with import_rustc.sh --download-source.
  • See rust_src_version to see which version the script was last ran with
• Pull in the prelude git submodule: git submodule update --init prelude or run buck2 init --git
• Run buck2 run //test:hello_world_on_stage2 to run a hello_world with the bootstrapped rustc+libs, while building press p to enable verbose logging with configuration

How it works

• Conceptually, the bootstrap example in buck2 does the same thing as this project: https://github.com/facebookincubator/buck2/tree/main/examples/bootstrap
• library/src/BUCK and compiler/src/BUCK are generated entirely by reindeer
  • See reindeer.toml and fixups under each for reindeer configs
• Both library/stdlib_rule_defs.bzl and compiler/compiler_rule_defs.bzl have light logic around additional dependencies and rustc flags
• Most of the interesting targets to construct the toolchain for each stage live in toolchains/BUCK
• The actual bootstrapping follows what is layed out in the rustc doc: https://rustc-dev-guide.rust-lang.org/building/bootstrapping.html#stages-of-bootstrapping
• Roughly we take these steps:
  1. toolchains//:stage0_rust_toolchain is constructed with the downloaded bootstrap compiler
  2. configure the rust toolchain to use toolchains//:stage0_rust_toolchain for the stage0 platform, defined in bootstrap/platform/BUCK
  3. with the stage0 platform, build libraries under library/src, which contains the standard library and its dependencies
  4. plug in the stage0 standard libraries and build compiler/src, which contains rustc and its dependencies
  5. take the stage0 rustc and construct toolchains//:stage0_compiler_artifacts which is a directory tree that mimics what a normal toolchain looks like. The important thing here is that bin/rustc exists. This will then plug into the toolchain used for stage1. This is used for the stage1 platform.
  6. build the standard libs again on stage1
  7. we now have a freshly built compiler with its standard libraries that can run //test:hello_world_with_bootstrapped_compiler