[SR-4127] swiftc shouldn't rebuild downstream dependencies when private API changes

[ddunbar]

Previous ID	SR-4127
Radar	None
Original Reporter	@ddunbar
Type	Bug

Attachment: Download

Environment

macOS

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, IncrementalBuild
Assignee	None
Priority	Medium

md5: 1f0e2aac39013f6dc1f73cf999e85e6f

Issue Description:

In the following test case, the second build should not recompile or relink bazTests, because the additional function added to the source does not change the public API of baz.

This requires a couple things to be fixed:

1. The baz.swiftmodule currently does change in the test case.
2. The baz.o file is rewritten, even though its contents are unchanged.

Test case is attached, steps to reproduce:

$ rm -rf .build
$ swift build -c release --build-tests
Compile Swift Module 'baz' (1 sources)
Compile Swift Module 'bazTests' (1 sources)
Linking ./.build/release/bazPackageTests.xctest/Contents/MacOS/bazPackageTests
$ (for f in .build/release/baz.{swiftmodule,build/baz.swift.o}; do printf "%s -- stat hash: %s -- content hash: %s\n" ${f} $(stat ${f} | md5) $(md5 < ${f}); done)
.build/release/baz.swiftmodule -- stat hash: 51b581c0e5b03d1ac6d76d6536b6c657 -- content hash: 2a13cb88f1d5100364fe882eae166d6c
.build/release/baz.build/baz.swift.o -- stat hash: 034c99c73f4ca317f8c2177f9bbe6321 -- content hash: ae38b556a84069586ff9c359d4749915
$ printf 'private func f%d() {}\n' $(wc -c < Sources/baz.swift) >> Sources/baz.swift 
$ swift build -c release --build-tests
Compile Swift Module 'baz' (1 sources)
Compile Swift Module 'bazTests' (1 sources)
$ (for f in .build/release/baz.{swiftmodule,build/baz.swift.o}; do printf "%s -- stat hash: %s -- content hash: %s\n" ${f} $(stat ${f} | md5) $(md5 < ${f}); done)
.build/release/baz.swiftmodule -- stat hash: 8654cc82d490935f8b0781ecff4e2078 -- content hash: 6f24a013ba83f16864ad410211b4caf5
.build/release/baz.build/baz.swift.o -- stat hash: 156d28f073b62c27f66d0eb2add5b60a -- content hash: ae38b556a84069586ff9c359d4749915

[belkadan]

In order to do this we need to stop putting non-public things into the swiftmodule.