A clarification: Rust async/await stacks do not move or change size. They are fixed to the size of a static call graph and are pinned in place. (Going beyond that can be done with explicit heap allocation, but it's extremely rare in practice.)

This is, from what I can tell, their main advantage in performance over other implementations. (In Rust, at least- the performance story may easily be different in garbage collected languages like Go or Java.)

Another caveat: Rust's claims to have implemented M:N in the past must be tempered a bit by two factors. First, Rust had to use segmented stacks, which Go moved away from and which Loom seems to be avoiding, precisely because they're so expensive. Second, Rust had to dispatch all IO APIs dynamically based on the currently selected runtime, which Go avoids by not having two runtimes, and which Loom can probably avoid via its scheduler implementation and/or JIT devirtualization.