Chapter 26: toolchains, the compilers buck uses, all from nix

buck2 does not bring its own compilers. It asks a toolchain target "what is your C compiler, your Rust compiler, your Python?" and runs whatever it is told. This folder answers, and every answer is a path into the nix store.

They are the buck prelude's own system_demo_toolchains, copied and configured as that macro's documentation asks, with one change in where the paths come from: not from PATH, which buck's C shim never searches (it execs by absolute path), but from .buckconfig.local, which the devshell writes on every entry (chapter 16). So a build sees exactly the tools the flake pins, and nothing else on the machine.

flowchart LR
  F["flake.nix devshell"] -->|"shellHook writes"| L[".buckconfig.local: [nix] cc, cxx, ar, python, [rust] rustc_identity"]
  L -->|"read_root_config"| T["toolchains/BUCK: cxx, python, rust, genrule, test, zip_file"]
  T --> B["every buck action"]

The Rust toolchain carries three settings worth knowing about. Optimisation is explicit (-Copt-level=3), because build scripts read their OPT_LEVEL from it. Debug assertions stay on for development and tests (the executor's thread checks are debug_asserts), and the nix package turns them off for release. And every compile carries an inert --cfg rustc_identity="…" with the compiler's store path.

Aside: a flag that does nothing, on purpose. No code reads rustc_identity. It is there because the prelude's toolchain invokes the bare name rustc, so buck's cache key never recorded which rustc compiled a crate. With a build cache shared across repositories, a library compiled by another repository's older rustc was served back and failed with E0514, "compiled by an incompatible version of rustc". Putting the compiler's path in a flag puts it in every key, and the collision is gone.

Try it. cat .buckconfig.local inside nix develop: every path in it is the one this folder hands to buck.

For the people who maintain it

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