[dylan-conway]

rustc's frontend (parse, macro expansion, typeck, borrowck) is single-threaded by default — only the LLVM backend fans out via codegen-units. The Rust crate graph's critical path is a serial chain ending in bun_runtime, so during the tail of every bun bd one core does ~20s of work while the rest idle.

-Zthreads=8 enables rustc's parallel-frontend query pool. It shares cargo's jobserver, so concurrent rustcs don't oversubscribe — each frontend thread acquires a -j token before running. Gated on !cfg.ci so PR/release builds keep deterministic diagnostic ordering and stay off the nightly -Z surface.

Why 8 and not nproc: scaling on bun_runtime (32-core box, debug, no incremental cache):

-Zthreads	wall
1	20.2s
4	13.2s
8	10.6s
16	9.6s
0 (=nproc=32)	10.9s

Returns flatten past 8 (macro expansion is inherently serial; the query DAG has its own critical path), and 0 is marginally worse than 8 from sharded-lock contention. 8 is also the upstream proposal for the eventual default (rust-lang/compiler-team#681), so this becomes a no-op when that lands.

Verified: flag present in build.ninja for local configure, absent with --ci=true; bun bd --revision works.

[robobun]

^{Updated 4:44 PM PT - May 19th, 2026}

❌ @dylan-conway, your commit 715a175 has 1 failures in Build #56243 (All Failures):

• test/regression/issue/03830.test.ts - code 1 on 🐧 13 x64-asan

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🧪   To try this PR locally:

bunx bun-pr 31096

That installs a local version of the PR into your bun-31096 executable, so you can run:

bun-31096 --bun

[coderabbitai]

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• scripts/build/rust.ts

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Walkthrough

This PR conditionally enables Rust's parallel frontend compilation for local development builds. The change appends -Zthreads=8 to the Rust compiler flags only when building outside CI, preserving deterministic behavior in CI and release pipelines while improving build times locally.

Changes

Parallel Rust Frontend Optimization

Layer / File(s)	Summary
Conditional parallel rustc frontend for local builds scripts/build/rust.ts	A !cfg.ci guard appends -Zthreads=8 to CARGO_ENCODED_RUSTFLAGS to enable parallel rustc frontend processing in local builds. Extensive inline comments document why 8 threads are chosen, how jobserver interaction works, and explain why CI and release builds remain on the serial path for determinism and byte-identical outputs.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: enabling rustc's parallel frontend with -Zthreads=8 for local builds, which is the core focus of this PR.
Description check	✅ Passed	The PR description covers both required template sections: it explains what the PR does (parallel frontend enablement, why 8 threads, performance data) and how it was verified (flag presence in build.ninja, CI gating confirmation, successful builds).
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[github-actions]

This PR may be a duplicate of:

1. build: enable parallel rustc frontend via -Zthreads=8 #30761 - Also adds -Zthreads=8 to rustflags in scripts/build/rust.ts to enable the rustc parallel frontend

🤖 Generated with Claude Code

[claude]

LGTM — local-only build flag with clear rationale and CI gating.

Extended reasoning...

Overview

This PR adds a single conditional rustflags.push("-Zthreads=8") to scripts/build/rust.ts, gated behind !cfg.ci. It enables rustc's parallel frontend for local developer builds to speed up the critical-path crate compilation, with a detailed code comment explaining the choice of 8 threads and why CI/release builds are excluded.

Security risks

None. This is a build-script change that only adds a compiler optimization flag for local builds. It does not touch runtime code, auth, crypto, or any user-facing surface.

Level of scrutiny

Low. The change is confined to the build pipeline and explicitly excluded from CI/release via !cfg.ci, so shipped binaries are unaffected. The file already uses several nightly -Z flags (-Zsanitizer, -Zbuild-std, -Zsplit-lto-unit, -Zlocation-detail), and the toolchain is pinned to nightly per existing comments, so adding another -Z flag follows the established pattern. Worst-case failure mode is a local build error, which is easily reverted.

Other factors

The PR description includes benchmark data justifying the thread count, references the upstream compiler-team proposal that will eventually make this the default, and notes manual verification that the flag appears in local build.ninja but not under --ci=true. The change is small (one conditional push + comment), self-contained, and matches the surrounding code style. No bugs were found by the bug-hunting system.