Phase 16. Reproducible build gate

Field	Value
MEP	MEP-53 §Phases
Status	LANDED
Started	2026-05-29 (GMT+7)
Landed	2026-05-29 (GMT+7)
Tracking issue	#22613
Tracking PR	#22499
Commit	7e013a1d1a

Gate

TestPhase16Repro walks tests/transpiler3/rust/fixtures/phase16-repro/ (7 fixtures), builds each fixture twice with Driver{Deterministic: true, NoCache: true}, and SHA-256 compares the two resulting binaries. On macOS, the gate platform-skips: the Mach-O LC_UUID load command is randomised per link by ld64 and cannot be neutralised from rustc-side flags.

Lowering decisions

Driver.Deterministic=true triggers three changes to the cargo invocation:

1. SOURCE_DATE_EPOCH=0 in the env, so cargo respects a fixed timestamp for CARGO_PKG_* env vars and any build-time chrono::Utc::now() (cargo embeds the build timestamp into the binary in certain configurations).
2. RUSTFLAGS=-C strip=symbols, which strips symbol tables (carrying path-dependent debug info and build IDs) post-link.
3. --locked, which forces cargo to use Cargo.lock exactly and refuses to update any dependency.

Combined, these three flags produce SHA-256-byte-identical binaries on Linux across two builds with the same inputs.

On macOS, the LC_UUID load command in Mach-O binaries is computed from a hash of the file's contents at link time, then re-randomised across re-links. Apple's ld64 (the system linker) does not expose a --no-uuid flag (unlike lld, which does). The MEP rejected using lld on darwin because that would require an extra toolchain install for users who otherwise want Apple's stock toolchain. The gate platform-skips on darwin and asserts only on Linux.

The fixture set (7 fixtures) is small to keep CI time reasonable — building 7 fixtures twice each is ~30s warm. Coverage: hello world, arithmetic, lists, records, sums, closures, and a 200-LOC composite test.

Files changed

File	Purpose
transpiler3/rust/build/build.go	Wire Deterministic env / RUSTFLAGS / --locked
transpiler3/rust/build/phase16_test.go	7-fixture double-build + SHA-256 compare gate, platform-skip on darwin
tests/transpiler3/rust/fixtures/phase16-repro/*.mochi + .out	7 fixtures

Test set

• TestPhase16Repro/<fixture> for each .mochi in the fixture directory (7 fixtures).

Closeout notes

Reproducibility under cargo is a moving target. Rust 1.84 (December 2024) made symbol-table emission deterministic by default; before that, RUSTFLAGS=-C strip=symbols was needed for symbols too. The MEP-53 implementation always sets the strip flag for safety against older rustc versions a user might pin.

A future sub-phase 16.1 could exercise the same gate under cargo-zigbuild on x86_64-unknown-linux-musl and aarch64-unknown-linux-musl. Currently the cross-build path is exercised in Driver.Build but not gated per-fixture in CI.

The macOS skip is documented in the test source for clarity:

if runtime.GOOS == "darwin" {
    t.Skip("LC_UUID randomised per link; reproducibility cannot be enforced on darwin without lld")
}