[SwiftBuild] Stop rewriting `SDKROOT=auto` for Swift SDK destinations in SwiftBuild

[kateinoigakukun]

Description

Follow-up from #773.

SwiftBuild currently rewrites SDKROOT=auto to a platform SDK name, but that is incorrect for Swift SDK destinations. Swift SDK destinations derive their SDK identity from the Swift SDK manifest, and rewriting SDKROOT=auto to something like webassembly can fail because there is no separately registered platform SDK with that name.

The attached reproducer is JavaScriptKit-agnostic and exercises this through a command plugin that calls packageManager.build(.product("SDKRootAutoReproPackageTests"), ...) while targeting a wasm Swift SDK destination.

Expected behavior

When the active run destination is a Swift SDK destination, the existing SDKROOT=auto rewrite path should not run.

Swift SDK destinations should retain their manifest-defined SDK identity, and wasm Swift SDK builds should not fail because of a rewritten platform SDK name.

Actual behavior

With swift-DEVELOPMENT-SNAPSHOT-2026-06-12-a_wasm, the native/control path succeeds and builds SDKRootAutoReproPackageTests.

The same reproducer with --build-system swiftbuild fails with:

succeeded: false
error: unable to find sdk 'webassembly'
error: Build failed

This matches the original failure mode.

Steps to reproduce

Updated reproducer zip: https://gist.githubusercontent.com/kateinoigakukun/ba21c654a675032aa77906877c906e53/raw/4fb7bfb28eecb50f20973c4c3299e5e82ba222c1/780-sdkroot-auto-linux-2026-06-12.zip

The updated zip preserves the original repro.sh and includes repro-linux-2026-06-12.sh plus adaptation notes for the Linux snapshot run.

unzip 780-sdkroot-auto.zip
cd 780-sdkroot-auto

TOOLCHAIN=/home/ubuntu/.local/share/swiftly/toolchains/main-snapshot-2026-06-12 \
SWIFT_SDK=swift-DEVELOPMENT-SNAPSHOT-2026-06-12-a_wasm \
./repro-linux-2026-06-12.sh

On macOS with the original 2026-05-27 snapshot setup described in the original reproducer, ./repro.sh preserves the original invocation shape.

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:

• 780-sdkroot-auto.zip
• Direct zip URL: https://gist.githubusercontent.com/kateinoigakukun/ba21c654a675032aa77906877c906e53/raw/231f32428deb3a43d7df12d66163e8894cc5ad1f/780-sdkroot-auto.zip
• Secret gist index: https://gist.github.com/kateinoigakukun/ba21c654a675032aa77906877c906e53

How to reproduce:

curl -L -O https://gist.githubusercontent.com/kateinoigakukun/ba21c654a675032aa77906877c906e53/raw/231f32428deb3a43d7df12d66163e8894cc5ad1f/780-sdkroot-auto.zip
unzip 780-sdkroot-auto.zip
cd 780-sdkroot-auto
./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: passes and builds SDKRootAutoReproPackageTests
• --build-system swiftbuild: reproduces the bad SDKROOT=auto rewrite and fails with error: unable to find sdk 'webassembly'

[kateinoigakukun]

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

Toolchain:

/home/ubuntu/.local/share/swiftly/toolchains/main-snapshot-2026-06-12/usr/bin/swift --version
Swift version 6.5-dev (LLVM 18d2bfb70c14d89, Swift c13d82e5987aecb)
Target: x86_64-unknown-linux-gnu
Build config: +assertions

Swift SDK: swift-DEVELOPMENT-SNAPSHOT-2026-06-12-a_wasm

Workdir: /workspace/swiftpm-swiftbuild-issues/swb-jskit-780/780-sdkroot-auto

I preserved the original repro.sh and ran a copied script, repro-linux-2026-06-12.sh, with only the Linux/snapshot setup adapted: Linux toolchain path, requested Swift SDK name, and a fallback TOOLCHAINS=swift-DEVELOPMENT-SNAPSHOT-2026-06-12-a because this Linux toolchain has no macOS Info.plist.

Command:

TOOLCHAIN=/home/ubuntu/.local/share/swiftly/toolchains/main-snapshot-2026-06-12 \
SWIFT_SDK=swift-DEVELOPMENT-SNAPSHOT-2026-06-12-a_wasm \
./repro-linux-2026-06-12.sh > repro-run.log 2>&1

Result:

• native/control path succeeded: succeeded: true, and Build of product 'SDKRootAutoReproPackageTests' complete!
• --build-system swiftbuild failed with the original failure mode:

succeeded: false
error: unable to find sdk 'webassembly'
error: Build failed

Conclusion: still reproducible with the requested 2026-06-12 snapshot and SDK. Leaving this issue open.

Logs:

• /workspace/swiftpm-swiftbuild-issues/swb-jskit-780/780-sdkroot-auto/repro-run.log
• /workspace/swiftpm-swiftbuild-issues/swb-jskit-780/780-sdkroot-auto/native.log
• /workspace/swiftpm-swiftbuild-issues/swb-jskit-780/780-sdkroot-auto/swiftbuild.log
• /workspace/swiftpm-swiftbuild-issues/swb-jskit-780/780-sdkroot-auto/adaptation-notes.md

[kateinoigakukun]

swiftlang/swift-package-manager#10228