Advice regarding Sendable constraints on generic async APIs

Yes it’s a hole in the model. Continuations can slip through not sendable values across async boundaries today.

We’re working on fixing it in Swift 6, “transferring” annotations on parameters is what will be the solution here probably. There may be a tradeoff where the unsafe api maybe remains actually unsafe still or something like that — to be discussed I think.

@Michael_Gottesman is working on those.