Error on bootstrap script

I'm trying to build the package manager using the instruction here under "Using the Trunk Snapshot". When it gets to the stage: --bootstrap: note: building self-hosted 'swift-build' it throws this error:

.../swift-source/swiftpm: error: manifest parse error(s):
<unknown>:0: error: unknown argument: '-package-description-version'

This happens in this context:

[18/18] Linking CXX CFBundle shared module perftests/Xcode/PerfTests/XcodePerfTests.xctest/Contents/MacOS/XcodePerfTests
--- bootstrap: note: building stage1
--- bootstrap: note: building self-hosted 'swift-build': env SWIFT_EXEC=/../Swift/swift-source/swiftpm/.build/x86_64-apple-macosx/debug/swiftc SWIFTPM_BUILD_DIR=/.../Swift/swift-source/swiftpm/.build SWIFTPM_BOOTSTRAP=1 SDKROOT=/Applications/ /.../Swift/swift-source/swiftpm/.build/x86_64-apple-macosx/debug/swift-build-stage1 --disable-sandbox -Xlinker -rpath -Xlinker @executable_path/../lib/swift/macosx -Xswiftc -Xcc -Xswiftc -DSPM_BUILD_IDENT=431e5eea -Xswiftc -I/.../Swift/swift-source/swiftpm/.build/x86_64-apple-macosx/debug/llbuild/products/llbuildSwift -Xswiftc -I/.../Swift/swift-source/swiftpm/../llbuild/products/libllbuild/include -Xlinker -rpath -Xlinker @executable_path/../lib/swift/pm/llbuild -Xlinker -L/./Swift/swift-source/swiftpm/.build/x86_64-apple-macosx/debug/llbuild/lib -Xlinker -rpath -Xlinker /../Swift/swift-source/swiftpm/.build/x86_64-apple-macosx/debug/llbuild/lib --build-tests
/Swift/swift-source/swiftpm: **error:** manifest parse error(s):
<unknown>:0: error: unknown argument: '-package-description-version'
--- bootstrap: error: build failed with exit status 1

What is this error and how do I mitigate it?

That is the same error I suddenly started getting here a few weeks ago.

I think that means a newer version of SwiftPM (your checkout?) is trying to run and call into parts of an older, mismatched Swift (system install?). Maybe that means it is not finding its corresponding parts in neighbour directories and falling back to the system install. Maybe it means that the neighbour directory is checked out at a mismatched revision. Or maybe it means not quite either, but still something along those general lines.

You need the latest Xcode 11 Beta or a recent toolchain. Here are the development docs: