Chapter 29: toolchains, which compiler, and why always -O3

buck2 does not ship compilers. A buck2 project names, in a toolchains cell, which compiler each kind of rule uses, and this folder is that cell. Every toolchain here is one of the prelude's system_* ones, which means "whatever is on PATH". Inside nix develop, PATH is the devshell, so in practice the flake decides the versions: rustc from fenix, clang and lld from nixpkgs.

The interesting line is one flag on the Rust toolchain:

rustc_flags = ["-Copt-level=3", "-Cdebuginfo=1"],

Every Rust target in the repository is built optimised. Not as a release mode you switch to: always.

Aside: a game you cannot play in debug. The prelude's default is rustc's, opt-level 0, and a cycle-accurate SNES core at opt-level 0 runs below real time: 24 to 31 frames a second against the console's 60 (measured 2026-09-20). So an unoptimised build is not a slower version of the same program; the game cannot be played in it. jgenesis' own Cargo.toml says the same ("most of the emulators are unusably slow without optimizations"). Debug info stays on: it costs nothing at run time and keeps backtraces readable.

test and remote_test_execution exist because rust_test asks for both. Tests here are plain binaries run where they were built, so neither configures anything; the prelude ships a no-op for the first and not the second, so local_test_execution.bzl is the smallest thing that satisfies it.

Try it. nix develop -c buck2 test //... runs every test target with these toolchains. Outside the devshell the same command uses whatever rustc the host has, or none.

For the people who maintain it

In this folder

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