WIP Async2

[majocha]

Currently just replaces cancellable.
Features: cold start, multi-start, cancellable, trampolined binds for safe non-tail recursion, fast tail-recursion with ReturnFromFinal, exceptions cached in ExceptionDispatchInfo for fast recovery from deep call stacks.

Synchronous executions should be quite fast and low alloc when it's done, but there are some fixes needed:
Currently complex mut rec expressions are not statically compiled, see the nowarn in CheckDeclarations.fs.

Very initial work.

[github-actions]

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@majocha,

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[vzarytovskii]

To avoid confusion - it's not the same "async2" as in runtime-supported async. It's just different re-implementation of async, right?

[majocha]

To avoid confusion - it's not the same "async2" as in runtime-supported async. It's just different re-implementation of async, right?

Nope, no. I wasn't even aware this was a thing until now :) The goal is to possibly replace all the internal use of async and cancellable with one reasonably efficient resumable CE. I went with async2 name after the example in resumable code RFC.

LOL, probably before I ever finish this, the runtime async model will switch to https://github.com/dotnet/runtimelab/blob/feature/async2-experiment/docs/design/features/runtime-handled-tasks.md

I guess this is the fate, always catching up :)

[majocha]

Implementing fast tail call with ReturnFromFinal is tricky While we can abandon intermediate computations in a tail call chain, we cannot abandon the first one, we must return to it, because it is the one actually awaited.

One possible solution is to thread a IValueTaskSource context between tail calls. We can bind to a ValueTask in the initial ReturnFromFinal, set the value in the last call of the chain, dropping all the intermediate computations.

[majocha]

I went simply with TaskCompletionSource as a tail-call context. With ReturnFromFinal the overhead is now significantly lower.