Swift 6 boundary issue fix

[hallee]

There's currently an error in release builds related to isolation of operation:

482 |   @_transparent
483 |   private func isSetting<R>(
    |                          `- note: consider making generic parameter 'R' conform to the 'Sendable' protocol
484 |     _ value: Bool,
485 |     isolation: isolated (any Actor)? = #isolation,
    :
489 |       try await DependencyValues.$isSetting.withValue(value, operation: operation)
490 |     #else
491 |       try await operation()
    |                 `- error: non-sendable type 'R' returned by implicitly asynchronous call to nonisolated function cannot cross actor boundary
492 |     #endif
493 |   }

I believe the correct fix for this is marking operation with the sending keyword. Here's the Swift proposal for that keyword, maybe someone who better understands concurrency should confirm that my understanding is correct: https://github.com/swiftlang/swift-evolution/blob/main/proposals/0430-transferring-parameters-and-results.md

Fixes #255

[cuteiosdev]

Have tested this change and it addresses issue #255. Not sure if there are other concurrency concerns related to the fix though.

[stephencelis]

I think the only question here is around Swift 6 snapshots being a rapidly moving target at the moment. While we're definitely open to merging this if it fixes the problem, I think we'd also like the Swift team to acknowledge if this is an expected requirement or if its boundary-checking system should infer sending here. Do either of you want to ask on the forums or open a Swift issue? Or would you like us to do so?

[cuteiosdev]

I was hopeful that we would get a new Xcode beta build today and see if that was fixed, but that didn't happen. For iOS / macOS apps it only manifests on archive builds so I'm inclined to be cautious. I'll ask over in the swift forums about this and see if there's any advice / direction to go with.

[stephencelis]

@cuteiosdev Thanks! Please keep us posted! If you reply with the forum thread here I'll be sure to follow it

[cuteiosdev]

Discussion has been raised over on the Swift Forum at https://forums.swift.org/t/sendable-error-when-creating-archive-build-and-swift-6/73726

[cuteiosdev]

@stephencelis it looks like the fix here appears to be the right one from discussion in the Swift forums and also on mastodon.

[Andrewangeta]

This would also address #253