vfs-stat-cache task not produced when using custom Swift toolchains

[chiragramani]

We've noticed that when using custom Swift toolchains, the vfsstatcache actions are not being produced, whereas everything works as expected with the Xcode default toolchain.

[chiragramani]

After digging into the code, I found that the logic responsible for producing this task is here:

swift-build/Sources/SWBTaskConstruction/TaskProducers/TaskProducer.swift

Line 674 in 172235d

guard let clangInfo = await clangSpec.discoveredCommandLineToolSpecInfo(self, settings.globalScope, delegate) as? DiscoveredClangToolSpecInfo, clangInfo.toolFeatures.has(.vfsstatcache) else { return false }

This section checks for the presence of the vfsstatcache feature. It seems that OSS/custom toolchains don’t include this feature(https://github.com/swiftlang/llvm-project/blob/next/clang/tools/driver/features.json), causing the conditions to fail and the vfs-stat-cache task to not be produced.

When I hardcode the features.json to have vfsstatcache, then I can see the task being produced.

So looks like I should create this issue in https://github.com/swiftlang
llvm-project and close this one.

Thanks again for open-sourcing swift-build - it made finding the root cause so much easier!