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Trying it out
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Thinking ahead a bit, what will happen for an AsyncStream conformer that implements back-pressure? i.e. doesn't necessarily sit waiting in the upstream next all the time; how will it support cancellation if it can no longer throw CancellationError (because the type is no longer necessarily the unlimited any Error)? Is it forced to just quietly return nil from its iterator's next?
Definitely +1 on this proposal. This will resolve a longstanding issue with AsyncSequence and finally make it more usable. One thing I wonder is with this proposal, what is the future of @rethrows protocols? They were never officially put through Evolution, but they exist in the language today. Should we deprecate or remove them?