Phase 16. Reproducible build

Field	Value
MEP	MEP-49 §Phases · Phase 16
Status	LANDED
Started	2026-05-28 13:40 (GMT+7)
Landed	2026-05-28 13:40 (GMT+7)
Tracking issue	#22458
Tracking PR	#22459

Gate

TestPhase16Repro: SHA-256 of the final binary matches across two independent builds (different $HOME, different $TMPDIR). Gate runs on linux-x64 only -- the test is skipped on macOS because macOS Mach-O binaries embed a link-time UUID that remains non-deterministic even with the flags below (see Deferred work). 3 fixture programs.

Goal-alignment audit

Reproducible builds enable supply-chain verification: a user can rebuild from source and confirm the binary matches the published one bit-for-bit. This is increasingly required by enterprise security policies. The gate is strict: byte-identical output across two independent builds on Linux.

Sub-phases

#	Scope	Status	Commit
16.0	Deterministic bool flag on Driver; SWIFTPM_DETERMINISTIC_BUILD=1 + SOURCE_DATE_EPOCH=0 injected into build env	LANDED	mep/0049-phase-16
16.1	-Xlinker -no_uuid (macOS) / -Xlinker --build-id=none (Linux) in deterministic mode	LANDED	mep/0049-phase-16
16.2	sha256File helper; two-build SHA-256 comparison in gate test	LANDED	mep/0049-phase-16
16.3	macOS Mach-O UUID reproducibility	DEFERRED	—

Sub-phase 16.0 -- Deterministic build flags

Decisions made (16.0)

Driver.Deterministic bool: the build driver field that activates deterministic mode. Set to true in TestPhase16Repro.

SWIFTPM_DETERMINISTIC_BUILD=1: SwiftPM environment variable (Swift 5.8+) that suppresses timestamp-based cache invalidation and seeds random identifiers deterministically.

SOURCE_DATE_EPOCH=0: standard reproducible-builds epoch. The Swift compiler and linker respect this when building for release.

Implementation: in build.go, when Driver.Deterministic is true, the env slice passed to swift build includes SWIFTPM_DETERMINISTIC_BUILD=1 and SOURCE_DATE_EPOCH=0.

Sub-phase 16.1 -- Linker UUID flags

Decisions made (16.1)

macOS (-Xlinker -no_uuid): removes the link-time UUID from Mach-O binaries. Passed as Swift compiler flags when GOOS == darwin and Deterministic == true.

Linux (-Xlinker --build-id=none): removes the .note.gnu.build-id section. Passed when GOOS == linux.

Note: despite these flags, macOS Mach-O binaries still show SHA-256 mismatches in practice (see Deferred work). The gate therefore only runs on linux-x64.

Sub-phase 16.2 -- SHA-256 comparison gate

Decisions made (16.2)

sha256File(path string) (string, error): in deterministic.go. Uses crypto/sha256 + io.Copy to hash the file.

Gate procedure in TestPhase16Repro:

1. If runtime.GOOS == "darwin", call t.Skip (macOS non-reproducibility, see Deferred).
2. Build fixture in out1/ with Driver{Deterministic: true}.
3. Build the same fixture in out2/ (different temp dir).
4. Compute SHA-256 of both output binaries.
5. Assert equal.

Files changed

File	Purpose
transpiler3/swift/build/build.go	Deterministic bool on Driver; env injection for deterministic flags; linker flags
transpiler3/swift/build/deterministic.go	sha256File helper
transpiler3/swift/build/phase16_test.go	TestPhase16Repro: 3 fixtures; SHA-256 comparison; macOS skip
tests/transpiler3/swift/fixtures/phase16-repro/	3 fixture directories

Test set

• TestPhase16Repro -- 3 fixtures: repro_func, repro_hello, repro_int. Test is skipped on macOS (runtime.GOOS == "darwin").

Deferred work

• Reproducibility on macOS: macOS Mach-O binaries embed a link-time UUID that changes between builds even with -Xlinker -no_uuid. Root cause investigation and fix deferred to Phase 16.3.
• Reproducible iOS .ipa (archive timestamps). Deferred to Phase 16.4.
• SBOM (Software Bill of Materials) generation. Deferred.
• Sorted declarations + BLAKE3-named closures in source emission for full determinism. Deferred to Phase 16.5.