C++20 coroutines are not async in the standard. They are just coroutines. Actually they have no implementation. The user has to write classes to implement the promise type and the awaitable type. You could just as easily write a coroutine library wrapping epoll as you could io_uring. The only thing it does behind your back (other than compile to stackless coroutines) is allocate memory, which also goes for a lot of other things.
Is this not also true of Rust? Are you saying Rust in some sense hardcodes an implementation to await in a way C++ doesn't? (I am not a Rust programmer, but I am very very curious about this and would appreciate any insight; I do program in C++ with co_await daily, with my own promise/task classes.)
Rust's async/await support is not intended as a general replacement of coroutines. In fact, async/await is built on top of coroutines (what Rust calls "generators"), but these are not yet stable. https://github.com/rust-lang/rust/issues/43122
Ouch... thanks; I didn't realize the Rust situation was this bad :(. FWIW, I do not look at generators as being what I would want as my interface for working with coroutines, and am very much on board there with the comments from tommythorn. I guess I just have too many decades of experience working with coroutines in various systems I have used :(.
TL;DR: rust makes you bring some sort of executor along. You can write your own, you can use someone else's. I have not done enough of a deep dive into what made it into the standard to give you a great line-by-line comparison.
