Bootstrapping0 doesn't exist when building stdlib with Swift 5.6.0

Development Compiler
1 
~/Development/AppleLLVM/5.6.0/swift/utils/build-toolchain com.apple

gets me

FAILED: libswift/SIL.o $BUILD_ROOT/AppleLLVM/5.6.0/buildbot_osx/swift-macosx-arm64/libswift/SIL.o
cd $SRC_ROOT/5.6.0/swift/libswift && $BUILD_ROOT/AppleLLVM/5.6.0/buildbot_osx/swift-macosx-arm64/bin/swiftc -c -o $BUILD_ROOT/AppleLLVM/5.6.0/buildbot_osx/swift-macosx-arm64/libswift/SIL.o -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.3.sdk -resource-dir $BUILD_ROOT/AppleLLVM/5.6.0/buildbot_osx/swift-macosx-arm64/bin/../bootstrapping0/lib/swift -target arm64-apple-macosx10.15 -module-name SIL -emit-module -emit-module-path $BUILD_ROOT/AppleLLVM/5.6.0/buildbot_osx/swift-macosx-arm64/libswift/SIL.swiftmodule -parse-as-library $SRC_ROOT/5.6.0/swift/libswift/Sources/SIL/ApplySite.swift $SRC_ROOT/5.6.0/swift/libswift/Sources/SIL/Argument.swift $SRC_ROOT/5.6.0/swift/libswift/Sources/SIL/BasicBlock.swift $SRC_ROOT/5.6.0/swift/libswift/Sources/SIL/Builder.swift $SRC_ROOT/5.6.0/swift/libswift/Sources/SIL/Function.swift $SRC_ROOT/5.6.0/swift/libswift/Sources/SIL/GlobalVariable.swift $SRC_ROOT/5.6.0/swift/libswift/Sources/SIL/Instruction.swift $SRC_ROOT/5.6.0/swift/libswift/Sources/SIL/Location.swift $SRC_ROOT/5.6.0/swift/libswift/Sources/SIL/Operand.swift $SRC_ROOT/5.6.0/swift/libswift/Sources/SIL/Registration.swift $SRC_ROOT/5.6.0/swift/libswift/Sources/SIL/Type.swift $SRC_ROOT/5.6.0/swift/libswift/Sources/SIL/Utils.swift $SRC_ROOT/5.6.0/swift/libswift/Sources/SIL/StackList.swift $SRC_ROOT/5.6.0/swift/libswift/Sources/SIL/Value.swift -wmo -Xfrontend -validate-tbd-against-ir=none -Xfrontend -enable-cxx-interop -Xcc -UIBOutlet -Xcc -UIBAction -Xcc -UIBInspectable -O -cross-module-optimization -I $SRC_ROOT/5.6.0/swift/include/swift -I $SRC_ROOT/5.6.0/swift/include -I $BUILD_ROOT/AppleLLVM/5.6.0/buildbot_osx/swift-macosx-arm64/libswift
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.3.sdk/usr/include/module.modulemap:95:11: error: header 'stdarg.h' not found
   header "stdarg.h" // note: supplied by the compiler

Which is because -resource-dir $BUILD_ROOT/AppleLLVM/5.6.0/buildbot_osx/swift-macosx-arm64/bin/../bootstrapping0/lib/swift doesn't exist

Removing that part builds fine

2

Seems the build script treats macosx-arm64 as a cross-compile target, which raises a ton of issues too long to be all documented here. I had to spawn an x86_64 shell with arch -arch x86_64 zsh