The goal of doing this is to reduce the amount of boilerplate and repeated code w.r.t. Continuation. Having just added `resume()` in four places, I got the sense that there was a lot of common code that was being duplicated. I removed the Throwing variants of these types (they can be expressed as Continuation<T, E:Error> instead of ThrowingContinuation<E>) and I broke out a significant amount of common code between CheckedContinuation and UnsafeContinuation into an implementation-only protocol to avoid repeating it. D.R.Y.
This change resolves rdar://74154769.
This change implements the changes proposed in swift-evolution PR #1264.
Existing test coverage should be sufficient here since the added function
simply calls into the existing `resume(returning:)` function.
This change resolves rdar://74031110.
The `try await` ordering is both easier to read and indicates the order
of operations better, because the suspension point occurs first and
then one can observe a thrown error.
We somehow ended up with a set hidden in `Task` as well as a set at top level. SILGen currently
hooks into the top-level ones, so shed the `Task`-namespaced versions for now.
`get_async_continuation[_addr]` begins a suspend operation by accessing the continuation value that can resume
the task, which can then be used in a callback or event handler before executing `await_async_continuation` to
suspend the task.
The experimental concurrency model will require a supporting runtime
and possibly end-user-visible library constructs. Introduce a stub of
such a library, enabled by a new `build-script` option
`--enable-experimental-concurrency`, so we have a place to put this
work.
