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build: enable rustc parallel frontend (-Zthreads=8) for local builds #31096

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Merged
dylan-conway merged 1 commit into from
May 19, 2026
Merged

build: enable rustc parallel frontend (-Zthreads=8) for local builds #31096

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19 changes: 19 additions & 0 deletions scripts/build/rust.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -387,6 +387,25 @@ export function emitRust(n: Ninja, cfg: Config, inputs: RustBuildInputs): string
// (flags.ts:293-301). Needed so profilers and crash backtraces walk Rust
// frames the same as the Zig binary did.
rustflags.push("-Cforce-frame-pointers=yes");
// Parallel frontend: rustc's default is single-threaded for parse / macro
// expansion / typeck / borrowck, so the critical-path crate (`bun_runtime`)
// sits on one core while the rest idle. With this, independent compiler
// queries run on a rayon pool and the long pole roughly halves. The pool
// shares cargo's jobserver, so N rustcs × 8 doesn't oversubscribe — each
// thread acquires a `-j` token before doing work.
//
// Why 8, not nproc: returns flatten past ~8 (the query DAG has its own
// serial spine — macro expansion in particular), and `-Zthreads=0` (= nproc)
// measured marginally *worse* on a 32-core box from sharded-lock contention.
// 8 is also the upstream proposal for the eventual default
// (rust-lang/compiler-team#681).
//
// Local-only: CI/release builds want byte-identical output across runs, and
// the parallel frontend can reorder diagnostics (and is still nightly
// `-Z`-gated). The shipped binaries stay on the serial path.
if (!cfg.ci) {
rustflags.push("-Zthreads=8");
}
// rustc does not emit `.llvm_addrsig` by default on *any* target (verified
// empirically — Linux-gnu, musl, darwin, msvc all missing it). lld's
// `--icf=safe` (flags.ts:960) and lld-link's `/OPT:SAFEICF` (flags.ts:778)
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