[gpderetta]

Async annotations in the vast majority of languages (possibly outside of JS and maybe rust) don't protect any useful invariant. You can still block in async code by calling any already existing blocking function not marked async, and async code can run in parallel when you have multiple underlying executors and shared memory.

If you design a language form scratch, async could be useful, but even then there would no need to annotate functions as they could be inferred. Generally effect systems seem a superior solution anyway.