Assuming I handle them correctly, what closure attributes can I safely bit cast in and away?

Using Swift
1

As the title says, what closure attributes can I safely bit cast in and away?

  • @escaping
  • @Sendable
  • @isolated(any)
  • any explicit isolation

And do any of these have irreversible drawbacks?
For example, does casting @isolated(any) away and in sequentially restore the stored isolation, or is it permanantly gone?

2

Well to not just blindly ask, I do know that @isolated(any) is bit cast able out and any explicit isolation is bit cast able both ways. I added them to the question to help other finders.

3

I don’t believe bitcasting closures is ever safe.

The language has some implicit conversions, like @escaping to non-@escaping and @Sendable to non-@Sendable for the safe combinations, so there is never a reason to unsafely bitcast closures.

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4

MainActor.assumeIsolated does it, after checking its invariants: swift/stdlib/public/Concurrency/MainActor.swift at main · swiftlang/swift · GitHub

So presumably, at least casting away global actor annotations is safe so long as you enforce them manually.

5

You should not do things just because standard library code does them. The standard library is co-developed with the compiler, and it is not at all uncommon that we have to change that code in order to model something more explicitly.

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