[SwiftBuild] Propagate unit-test linker flags to SwiftPM's synthetic test runner

[kateinoigakukun]

Description

Follow-up from #773.

For Swift SDK / WASI test builds, the actual runnable output is a SwiftPM-synthesized test runner target. If linker flags remain only on the unit test product and are not copied to the synthetic runner, the produced .wasm executable can miss required linker settings.

The minimum reproducer has:

• a header-only system library target, CHelper, exposing helper_value()
• Vendor/helper.c, which the repro script builds into a WASI archive at Vendor/libhelper.a
• Lib, which calls helper_value()
• LibTests, which supplies -L <Vendor> -lhelper through test-target linkerSettings

Updated reproducer zip: https://gist.githubusercontent.com/kateinoigakukun/ba21c654a675032aa77906877c906e53/raw/e95f44931cc10b5f6197d93f42cffc9991faa478/updated-777-test-runner-ldflags-linux-2026-06-12.zip

The updated zip preserves the original repro package and original repro.sh, and also includes repro-linux-2026-06-12.sh plus adaptation notes for running the same reproducer with the Linux June 12 snapshot toolchain and Swift SDK.

Expected behavior

The generated SwiftPM test runner should inherit the relevant linker flags from the unit test product in both Debug and Release, including platform-specific linker settings.

For this reproducer, the synthetic LibTests-test-runner.wasm link should include the test target's -L <Vendor> -lhelper flags and should resolve helper_value.

Actual behavior

With the June 12, 2026 development snapshot and swift-DEVELOPMENT-SNAPSHOT-2026-06-12-a_wasm, the native backend succeeds and SwiftBuild still reproduces the original failure.

Observed in the latest verification:

native exit:     0
swiftbuild exit: 1
native -lhelper count:     3
swiftbuild -lhelper count: 0

The SwiftBuild link step for the synthetic runner:

Link LibTests-test-runner.wasm (wasm32)

does not include -lhelper, then fails with:

wasm-ld: error: .../libLibTests.a(Lib.o): undefined symbol: helper_value

Steps to reproduce

1. Download and unzip https://gist.githubusercontent.com/kateinoigakukun/ba21c654a675032aa77906877c906e53/raw/e95f44931cc10b5f6197d93f42cffc9991faa478/updated-777-test-runner-ldflags-linux-2026-06-12.zip.

2. Enter the package directory:

   cd 777-test-runner-ldflags

3. On this Linux verification machine, run the adapted script:

   ./repro-linux-2026-06-12.sh

   The adapted script uses:

   /home/ubuntu/.local/share/swiftly/toolchains/main-snapshot-2026-06-12/usr/bin/swift
   swift-DEVELOPMENT-SNAPSHOT-2026-06-12-a_wasm
   

   It also builds Vendor/libhelper.a from the original Vendor/helper.c before running the build-system comparison.

4. On a machine matching the original macOS-style repro assumptions, the original script is still available:

   ./repro.sh

5. The script runs the same comparison for both build systems:

   swift build --package-path "$ROOT" --scratch-path "$WORK/native" --swift-sdk "$SWIFT_SDK_ID" --build-tests --build-system native -v
   swift build --package-path "$ROOT" --scratch-path "$WORK/swiftbuild" --swift-sdk "$SWIFT_SDK_ID" --build-tests --build-system swiftbuild -v

6. Expected reproducer result for the current bug: native exits 0; SwiftBuild exits nonzero while linking LibTests-test-runner.wasm with undefined symbol: helper_value.

Swift Package Manager version/commit hash

Swift Package Manager - Swift 6.5.0-dev

Swift & OS version (output of swift --version ; uname -a)

Swift version 6.5-dev (LLVM 18d2bfb70c14d89, Swift c13d82e5987aecb)
Target: x86_64-unknown-linux-gnu
Build config: +assertions
Linux swift-dev.fugu.katei.dev 6.8.0-94-generic #96-Ubuntu SMP PREEMPT_DYNAMIC Fri Jan  9 20:36:55 UTC 2026 x86_64 x86_64 x86_64 GNU/Linux

[kateinoigakukun]

Minimum reproducer zip is here:

• 777-test-runner-ldflags.zip
• Direct zip URL: https://gist.githubusercontent.com/kateinoigakukun/ba21c654a675032aa77906877c906e53/raw/192c4f8a859ffb915a1c0a7e7648e353f0e42b22/777-test-runner-ldflags.zip
• Secret gist index: https://gist.github.com/kateinoigakukun/ba21c654a675032aa77906877c906e53

How to reproduce:

curl -L -O https://gist.githubusercontent.com/kateinoigakukun/ba21c654a675032aa77906877c906e53/raw/192c4f8a859ffb915a1c0a7e7648e353f0e42b22/777-test-runner-ldflags.zip
unzip 777-test-runner-ldflags.zip
cd 777-test-runner-ldflags
./repro.sh

Environment used to validate these repros:

• Host: macOS 26.5.1 (arm64)
• Toolchain: ~/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2026-05-27-a.xctoolchain
• TOOLCHAINS=org.swift.64202605271a
• swift --version: Apple Swift version 6.5-dev (LLVM 8f81a64f6a8bcf6, Swift 4d0c97fa5b05711)
• Swift SDK: DEVELOPMENT-SNAPSHOT-2026-05-27-a-wasm32-unknown-wasip1
• Each repro.sh uses the explicit snapshot toolchain above, not the global swift on PATH.

Observed on this environment:

• --build-system native: passed (Build complete!) and kept the test target linker flags on the runnable output
• --build-system swiftbuild: synthetic LibTests-test-runner.wasm link failed with undefined symbol: helper_value

[kateinoigakukun]

Verified with the minimum reproducer from the issue gist on Linux using the requested June 12 snapshot.

• Workdir: /workspace/swiftpm-swiftbuild-issues/swb-jskit-777
• Reproducer zip: 777-test-runner-ldflags.zip
• Command run: cd /workspace/swiftpm-swiftbuild-issues/swb-jskit-777/777-test-runner-ldflags && ./repro-linux-2026-06-12.sh 2>&1 | tee ../repro.log
• Swift: /home/ubuntu/.local/share/swiftly/toolchains/main-snapshot-2026-06-12/usr/bin/swift
• Swift version: Swift version 6.5-dev (LLVM 18d2bfb70c14d89, Swift c13d82e5987aecb)
• Swift SDK: swift-DEVELOPMENT-SNAPSHOT-2026-06-12-a_wasm
• Adaptation: copied the original repro.sh to repro-linux-2026-06-12.sh and only replaced the macOS .xctoolchain/Info.plist/PlistBuddy lookup with the requested Linux Swift binary plus the requested SDK ID/sysroot. clang and llvm-ar are derived from the requested Swift binary directory. The package, helper source/archive flow, and native/swiftbuild build commands were left unchanged.

Result:

• --build-system native: exit 0
• --build-system swiftbuild: exit 1
• Native log contains -lhelper 3 times; swiftbuild log contains -lhelper 0 times.
• The swiftbuild log contains the original failure mode while linking the synthetic runner:
  • Link LibTests-test-runner.wasm (wasm32)
  • wasm-ld: error: .../libLibTests.a(Lib.o): undefined symbol: helper_value
• Log paths:
  • /workspace/swiftpm-swiftbuild-issues/swb-jskit-777/repro.log
  • /workspace/swiftpm-swiftbuild-issues/swb-jskit-777/777-test-runner-ldflags/native.log
  • /workspace/swiftpm-swiftbuild-issues/swb-jskit-777/777-test-runner-ldflags/swiftbuild.log
  • /workspace/swiftpm-swiftbuild-issues/swb-jskit-777/adaptation-notes.md

Conclusion: still reproducible with the original failure mode, so I am leaving this issue open.

[kateinoigakukun]

swiftlang/swift-package-manager#10225