Make MPSCAsyncChannel #isolation methods use nonisolated(nonsending)`.
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We can perform more aggressive optimizer optimizations with nonisolated(nonsending) than #isolation. So use that instead to get some easy perf wins.
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I don't think we can change this one since this is a protocol requirement of AsyncSequence unless nonisolated(nonsending) methods are capable of fulfilling isolated protocol requirement
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| func suspendNext(isolation: isolated (any Actor)? = #isolation) async throws -> Element? { | ||
| nonisolated(nonsending) func suspendNext() async throws -> Element? { |
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in my recent testing, it seems nonisolated(nonsending) does not grant the ability to actually procure a reference to the isolated parameter in a way that allows isolating closures in the same manner that capturing an explicit isolated parameter does. i suppose in this case it doesn't matter since the parameter isn't being captured by the body closures, but that does make me wonder if those closures are actually not being isolated in the intended manner currently.
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Could you briefly expand what closures you are worried are not isolated in particular?
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so, the concern would be that even though the method inherits isolation by default (in the original formulation), the isolated parameter is unused within the body closures. this is true in the prior version with an isolated parameter, and seems to also be true even after converting to implicit caller isolation inheritance. this means the actual 'work' invoked by the with* runtime methods isn't isolated, so IIUC it's going to require an executor hop if the caller is actor-isolated. this can be seen in this sort of example: https://swift.godbolt.org/z/GP4dPhfKv.
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Ah so you are talking about the withTaskCancellationHandler and withCheckedContinuation handler. Both of them have open PRs to address these issues swiftlang/swift#85191 & swiftlang/swift#85518
Edit: I see that you left a comment on the latter PR. The hop should be eliminated and I think @gottesmm is looking into that.
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We can perform more aggressive optimizer optimizations with nonisolated(nonsending) than #isolation. So use that instead to get some easy perf wins.