Coroutines are one means of implementing async; the way they are implemented in Rust is more like building a state machine out of the async function. It can still be mapped to coroutines, and there are probably crates that use async and macros to make coroutines usable on stable, but the effort to have a stable language feature is still ongoing, with https://lang-team.rust-lang.org/design_notes/general_coroutines.html giving the overview.