Add workaround for SR-15123

[keith]

This works around an issue in Swift 5.5 / Xcode 13.0 beta 5 where
absolute paths to the sources being compiled are always used and
embedded in the case a vfs overlay is used. This tricks the compiler by
using a vfs overlay for the source files as well.

https://bugs.swift.org/browse/SR-15123

[keith]

We might not need this pending swiftlang/llvm-project#3246 but it does appear to work (only with the change being reverted though, otherwise it fails)

[keith]

optimized a bit for revertability here

[acecilia]

I just hit this issue trying to update xcode from 12.5.1 to 13.2.1. In my case, what I observed is that #fileID returns an absolute path that ends up inside the binaries. Before finding this PR I did write a simple reproducible example here

Thanks for the multiple attempts to fix it in different repositories @keith 🙏

How are you currently dealing with this problem? I am not sure how to work around it considering that:

• The bug is still present in the latest Xcode 13.2.1
• This PR is a draft, did not make it to master

Some options on the top of my mind:

• Wait for swift 5.6, which hopefully comes with a fix (this is probably not a good idea, as it may pass some time until next release)
• Stop using vfs overlay
• Maintain my own clone of rules_swift and add this PR to it

[brentleyjones]

Instead of maintaining a fork, you can apply a patch with bazel: https://brentley.dev/patching-bazel-external-dependencies/

[keith]

Yep that's what I would recommend in general, and what we're doing at Lyft. I felt a bit weird about merging this at the time because I wasn't sure if it would have any unexpected side-effects. Realistically we could probably merge it now, but I was hoping that my actual fixes upstream would land soon enough that we wouldn't need too. Unfortunately it looks like they won't be fixed until Swift 5.6, which I assume won't come to Xcode until the spring

[acecilia]

Got it, thank you 🙌

[lyft-lint-bot]

Lyft integration job started: https://buildkite.com/lyft/rules-swift/builds/262 (must be Lyft employee to view)

[brentleyjones]

The linked SR is closed. Is it still a valid bug that should be reopened?

[keith]

needs more investigation, i think there's a new one

[keith]

as of Xcode 14 we have confirmed this is fixed, and don't see any new reason to use this at the moment. I believe it was fixed earlier, with Swift 5.6, but I didn't test before Swift 5.7. Please file an issue if you find any new case that needs this!