Support async/await natively

[benwaffle]

Like https://github.com/scala/scala-async

[Atry]

Dsl.scala is more general than scala.async. I am trying to port compiler plugins in Dsl.scala to Dotty.

There are two compiler plugins in the Dsl.scala project:

1. BangNotation performs function local CPS translation, which originally implemented as an AnalyzerPlugin. Fortunately it can be implemented as an inline macro reset in Dotty.
2. ResetEverywhere added a reset wrapper for every function, in other words, triggering the BangNotation translation for every function. Unfortunately I cannot find an approach to implement it in Dotty because Dotty does not provide an approach to trigger an inline macro in a compiler plugin.

[Atry]

ResetEverywhere enables syntax like Future { !Await(f1); !Await(f2) }. Without ResetEverywhere, you have to write reset(Future { !Await(f1); !Await(f2) }).

Porting ResetEverywhere is blocked by https://github.com/lampepfl/dotty/issues/7160.

[kubukoz]

I'm mildly :-1: on this. async/await in the language would require tying ourselves to a particular implementation of asynchronous effects, which would most likely be Future. If anything, I'd prefer reducing the difference between for { stats } and { stats }, so that we would be able to mix e.g. local defs with generators on anything that provides flatMap.

[Atry]

@kubukoz Not really. A generic language-level solution like Dsl.scala does not bind to any underlying implementation.