1# Chapter 29: toolchains, which compiler, and why always `-O3`
2
3buck2 does not ship compilers. A buck2 project names, in a *toolchains*
4cell, which compiler each kind of rule uses, and this folder is that cell.
5Every toolchain here is one of the prelude's `system_*` ones, which means
6"whatever is on `PATH`". Inside `nix develop`, `PATH` is the devshell, so in
7practice the flake decides the versions: rustc from fenix, clang and lld
8from nixpkgs.
9
10The interesting line is one flag on the Rust toolchain:
11
12```python
13rustc_flags = ["-Copt-level=3", "-Cdebuginfo=1"],
14```
15
16Every Rust target in the repository is built optimised. Not as a release
17mode you switch to: always.
18
19> **Aside: a game you cannot play in debug.** The prelude's default is
20> rustc's, opt-level 0, and a cycle-accurate SNES core at opt-level 0 runs
21> *below* real time: 24 to 31 frames a second against the console's 60
22> (measured 2026-09-20). So an unoptimised build is not a slower version of
23> the same program; the game cannot be played in it. jgenesis' own
24> `Cargo.toml` says the same ("most of the emulators are unusably slow
25> without optimizations"). Debug info stays on: it costs nothing at run
26> time and keeps backtraces readable.
27
28`test` and `remote_test_execution` exist because `rust_test` asks for both.
29Tests here are plain binaries run where they were built, so neither
30configures anything; the prelude ships a no-op for the first and not the
31second, so `local_test_execution.bzl` is the smallest thing that satisfies
32it.
33
34> **Try it.** `nix develop -c buck2 test //...` runs every test target with
35> these toolchains. Outside the devshell the same command uses whatever
36> rustc the host has, or none.
37
38## For the people who maintain it
39
40### In this folder
41
42| Path | What |
43| --- | --- |
44| [BUCK](BUCK) | The cell: `rust` (edition 2024, `-Copt-level=3 -Cdebuginfo=1`), `cxx`, `python_bootstrap`, `genrule`, the no-op `test` toolchain and `remote_test_execution`. |
45| [local_test_execution.bzl](local_test_execution.bzl) | The minimal `RemoteTestExecutionToolchainInfo` rule for purely local test runs. |
46| [README.md](README.md) | This chapter. |
47| [CLAUDE.md](CLAUDE.md) | Do not add a debug build, for agents. |
48
49← Previous: [Chapter 28, nix/](../nix/) · Up: [jevsnes](../) · Next: [Chapter 30, platforms/](../platforms/) →