They don't need to mark functions as needing special compilation / rewriting to support asynchronous use since the runtime provides the necessary mechanism to pause and resume (e.g. stackful continuations). So 'async' is unnecessary and 'await' can be a library function (or remain a keyword of course).

No joke at all, not that I said "like implementation", not that they have async/await.

https://github.com/dotnet/runtimelab/issues/2398

Be sure to read all the way to the bottom,