<unknown>:0: error: missing required module '_SwiftSyntaxCShims'

[vsarunas]

Is it reproducible with SwiftPM command-line tools: swift build, swift test, swift package etc?

• Confirmed reproduction steps with SwiftPM CLI. The description text must include reproduction steps with either of command-line SwiftPM commands, swift build, swift test, swift package etc.

Description

We have a build plugin that builds the required products; in CI when a clean repo is checked out with no .build, something will be different and checked out code won't be buildable:

$ swift package --disable-sandbox sdk-build --skip-build --verbose
[...]
Building for debugging...
error: emit-module command failed with exit code 1 (use -v to see invocation)
<unknown>:0: error: missing required module '_SwiftSyntaxCShims'

After that even swift build will fail:

$  swift build --verbose
[...]
/Applications/Xcode-26.0.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-frontend -frontend -c /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/checkouts/swift-collections/Sources/InternalCollectionsUtilities/IntegerTricks/autogenerated/FixedWidthInteger+roundUpToPowerOfTwo.swift /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/checkouts/swift-collections/Sources/InternalCollectionsUtilities/IntegerTricks/autogenerated/Integer rank.swift /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/checkouts/swift-collections/Sources/InternalCollectionsUtilities/IntegerTricks/autogenerated/UInt+first and last set bit.swift /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/checkouts/swift-collections/Sources/InternalCollectionsUtilities/IntegerTricks/autogenerated/UInt+reversed.swift /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/checkouts/swift-collections/Sources/InternalCollectionsUtilities/UnsafeBitSet/autogenerated/_UnsafeBitSet+Index.swift /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/checkouts/swift-collections/Sources/InternalCollectionsUtilities/UnsafeBitSet/autogenerated/_UnsafeBitSet+_Word.swift /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/checkouts/swift-collections/Sources/InternalCollectionsUtilities/UnsafeBitSet/autogenerated/_UnsafeBitSet.swift /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/checkouts/swift-collections/Sources/InternalCollectionsUtilities/_SortedCollection.swift /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/checkouts/swift-collections/Sources/InternalCollectionsUtilities/_UniqueCollection.swift /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/checkouts/swift-collections/Sources/InternalCollectionsUtilities/autogenerated/Debugging.swift /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/checkouts/swift-collections/Sources/InternalCollectionsUtilities/autogenerated/Descriptions.swift /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/checkouts/swift-collections/Sources/InternalCollectionsUtilities/autogenerated/LifetimeOverride.swift /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/checkouts/swift-collections/Sources/InternalCollectionsUtilities/autogenerated/RandomAccessCollection+Offsets.swift -primary-file /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/checkouts/swift-collections/Sources/InternalCollectionsUtilities/autogenerated/UnsafeBufferPointer+Extras.swift /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/checkouts/swift-collections/Sources/InternalCollectionsUtilities/autogenerated/UnsafeMutableBufferPointer+Extras.swift /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/checkouts/swift-collections/Sources/InternalCollectionsUtilities/autogenerated/UnsafeMutableRawBufferPointer+Extras.swift /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/checkouts/swift-collections/Sources/InternalCollectionsUtilities/autogenerated/UnsafeRawBufferPointer+Extras.swift -emit-dependencies-path /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/arm64-apple-macosx/debug/InternalCollectionsUtilities.build/UnsafeBufferPointer+Extras.d -emit-reference-dependencies-path /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/arm64-apple-macosx/debug/InternalCollectionsUtilities.build/UnsafeBufferPointer+Extras.swiftdeps -serialize-diagnostics-path /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/arm64-apple-macosx/debug/InternalCollectionsUtilities.build/UnsafeBufferPointer+Extras.dia -target arm64-apple-macosx10.13 -Xllvm -aarch64-use-tbi -enable-objc-interop -sdk /Applications/Xcode-26.0.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX26.0.sdk -I /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/arm64-apple-macosx/debug/Modules -I /Applications/Xcode-26.0.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/lib -F /Applications/Xcode-26.0.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/Library/Frameworks -F /Applications/Xcode-26.0.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/Library/PrivateFrameworks -color-diagnostics -Xcc -fcolor-diagnostics -enable-testing -g -debug-info-format=dwarf -dwarf-version=4 -module-cache-path /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/arm64-apple-macosx/debug/ModuleCache -suppress-warnings -swift-version 6 -Onone -D SWIFT_PACKAGE -D DEBUG -D SWIFT_MODULE_RESOURCE_BUNDLE_UNAVAILABLE -D COLLECTIONS_RANDOMIZED_TESTING -new-driver-path /Applications/Xcode-26.0.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-driver -enable-experimental-feature AvailabilityMacro=SwiftStdlib 5.0: macOS 10.14.4, iOS 12.2, watchOS 5.2, tvOS 12.2 -enable-experimental-feature AvailabilityMacro=SwiftStdlib 5.1: macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0 -enable-experimental-feature AvailabilityMacro=SwiftStdlib 5.6: macOS 12.3, iOS 15.4, watchOS 8.5, tvOS 15.4 -enable-experimental-feature AvailabilityMacro=SwiftStdlib 5.8: macOS 13.3, iOS 16.4, watchOS 9.4, tvOS 16.4 -enable-experimental-feature AvailabilityMacro=SwiftStdlib 5.9: macOS 14.0, iOS 17.0, watchOS 10.0, tvOS 17.0 -enable-experimental-feature AvailabilityMacro=SwiftStdlib 5.10: macOS 14.4, iOS 17.4, watchOS 10.4, tvOS 17.4, visionOS 1.1 -enable-experimental-feature AvailabilityMacro=SwiftStdlib 6.0: macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0 -enable-experimental-feature AvailabilityMacro=SwiftStdlib 6.1: macOS 15.4, iOS 18.4, watchOS 11.4, tvOS 18.4, visionOS 2.4 -enable-experimental-feature AvailabilityMacro=SwiftStdlib 6.2: macOS 26.0, iOS 26.0, watchOS 26.0, tvOS 26.0, visionOS 26.0 -enable-upcoming-feature MemberImportVisibility -enable-experimental-feature BuiltinModule -enable-experimental-feature Lifetimes -enable-experimental-feature InoutLifetimeDependence -enable-experimental-feature SuppressedAssociatedTypes -empty-abi-descriptor -plugin-path /Applications/Xcode-26.0.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/host/plugins/testing -user-module-version 1.3.0 -enable-anonymous-context-mangled-names -file-compilation-dir /Users/runner/actions-runner/_work/package-data-model/package-data-model -Xcc -isysroot -Xcc /Applications/Xcode-26.0.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX26.0.sdk -Xcc -F -Xcc /Applications/Xcode-26.0.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/Library/Frameworks -Xcc -F -Xcc /Applications/Xcode-26.0.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/Library/PrivateFrameworks -Xcc -fPIC -Xcc -g -no-auto-bridging-header-chaining -module-name InternalCollectionsUtilities -package-name swift_collections -disable-clang-spi -target-sdk-version 26.0 -target-sdk-name macosx26.0 -external-plugin-path /Applications/Xcode-26.0.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/lib/swift/host/plugins#/Applications/Xcode-26.0.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/bin/swift-plugin-server -external-plugin-path /Applications/Xcode-26.0.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/local/lib/swift/host/plugins#/Applications/Xcode-26.0.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/bin/swift-plugin-server -in-process-plugin-server-path /Applications/Xcode-26.0.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/host/libSwiftInProcPluginServer.dylib -plugin-path /Applications/Xcode-26.0.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/host/plugins -plugin-path /Applications/Xcode-26.0.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/local/lib/swift/host/plugins -parse-as-library -o /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/arm64-apple-macosx/debug/InternalCollectionsUtilities.build/UnsafeBufferPointer+Extras.swift.o -index-store-path /Users/runner/actions-runner/_work/package-data-model/package-data-model/.build/arm64-apple-macosx/debug/index/store -index-system-modules
/Users/runner/actions-runner/_work/package-data-model/package-data-model/Local/OrdoEssentials/Sources/Macros/CFI.swift:1:8: error: missing required module '_SwiftSyntaxCShims'

swift-syntax is correctly extracted:

du -sh .build/prebuilts/swift-syntax/602.0.0/*
 59M	.build/prebuilts/swift-syntax/602.0.0/swiftlang-6.2.0.19.9-MacroSupport-macos_aarch64
 13M	.build/prebuilts/swift-syntax/602.0.0/swiftlang-6.2.0.19.9-MacroSupport-macos_aarch64.zip
 12K	.build/prebuilts/swift-syntax/602.0.0/swiftlang-6.2.0.19.9-manifest.json

du -sk ~/Library/Caches/org.swift.swiftpm/prebuilts/swift-syntax/602.0.0/swiftlang-6.2.0.19.9-*
12908	/Users/runner/Library/Caches/org.swift.swiftpm/prebuilts/swift-syntax/602.0.0/swiftlang-6.2.0.19.9-MacroSupport-macos_aarch64.zip

This machine has:

sha256sum ~/Library/Caches/org.swift.swiftpm/prebuilts/swift-syntax/602.0.0/swiftlang-6.2.0.19.9-*
a1cd8f0289fd157e86316246d4e6160229c6c2cfeb06b25af6043a6a49573044  /Users/runner/Library/Caches/org.swift.swiftpm/prebuilts/swift-syntax/602.0.0/swiftlang-6.2.0.19.9-MacroSupport-macos_aarch64.zip
d9719016ca233fd8667dc87bc36460089913d98a5f54105749ae059b8e8f2235  /Users/runner/Library/Caches/org.swift.swiftpm/prebuilts/swift-syntax/602.0.0/swiftlang-6.2.0.19.9-manifest.json

My machine has:

49f745a25a972123fd17b72599a305efbabe6986b848e5cecd16acd9c9140b75  /Users/sarunas/Library/Caches/org.swift.swiftpm/prebuilts/swift-syntax/602.0.0/swiftlang-6.2.0.19.9-MacroSupport-macos_aarch64.zip
6ca25fbdfa7d71302bbef6e61eb731b66c36119afa33912e0b403fe7b99265f4  /Users/sarunas/Library/Caches/org.swift.swiftpm/prebuilts/swift-syntax/602.0.0/swiftlang-6.2.0.19.9-manifest.json

If I remove the cache, new files are downloaded with matching checksums, but I'm able to build fine locally.
Why do the checksums change for the same version?

a1cd8f0289fd157e86316246d4e6160229c6c2cfeb06b25af6043a6a49573044  swiftlang-6.2.0.19.9-MacroSupport-macos_aarch64.zip
d9719016ca233fd8667dc87bc36460089913d98a5f54105749ae059b8e8f2235  swiftlang-6.2.0.19.9-manifest.json

On the CI machine if I trigger SPM update, it will compile fine, but SwiftSyntax will be built:

rm -rf .build
swift package update
swift package --disable-sandbox sdk-build --skip-build --verbose
[....]
[65/111] Compiling SwiftSyntax SyntaxNodeStructure.swift

The compilation error started to happen a few days ago.

Expected behavior

Use SwiftSyntax pre-compiled and do not fail to build

Actual behavior

Compilation error

Steps to reproduce

No response

Swift Package Manager version/commit hash

No response

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

swift-driver version: 1.127.14.1 Apple Swift version 6.2 (swiftlang-6.2.0.19.9 clang-1700.3.19.1)
Target: arm64-apple-macosx26.0

Darwin mini7-osx-ci.local 25.0.0 Darwin Kernel Version 25.0.0: Mon Aug 25 21:17:53 PDT 2025; root:xnu-12377.1.9~3/RELEASE_ARM64_T8112 arm64

[dschaefer2]

First, yes, the checksum shouldn't be changing. I'm digging into that.

One other thing to consider for CI is that we cache the prebuilts in the cache directory. That defaults to ~/.swiftpm/cache. That might be bringing in old prebuilts if the user directory is shared between builds.

[vsarunas]

It is a symlink:

$ ls -l ~/.swiftpm/cache
lrwxr-xr-x  1 runner  admin  46 Oct 31  2024 /Users/runner/.swiftpm/cache -> /Users/runner/Library/Caches/org.swift.swiftpm

The directory only contained the mentioned files only.

How to understand when SwiftSyntax will be built and why the prebuilt one is not being used?

[dschaefer2]

I would use the --cache-path option to swift build and avoid using the user cache directory when running builds in CI. Or make sure it's cleared out. I'm just wondering if it has out of date files in it.

You should see the following messages if the prebuilts are being used.

Downloading package prebuilt https://download.swift.org/prebuilts/swift-syntax/602.0.0/swiftlang-6.2.0.19.9-MacroSupport-macos_aarch64.zip
Downloaded https://download.swift.org/prebuilts/swift-syntax/602.0.0/swiftlang-6.2.0.19.9-MacroSupport-macos_aarch64.zip (0.39s)

You only get the first one if there was a checksum mismatch.