async is just a syntactic sugar to transform a function into the state machine you would have otherwise written. If you don't trust the compiler to perform this

async is just a syntactic sugar to transform a function into the state machine you would have otherwise written. If you don't trust the compiler to perform this transform adequately, you can write a Future by hand. There are users using async/await syntax to write software for microcontrollers without dynamic memory allocation, runtimes like tokio are not a hard dependency of async/await syntax or the Future abstraction. C++ coroutines perform the same transform, but they dynamically allocate that state machine for every coroutine (unless the optimizer can eliminate the allocation). In contrast, Rust inlines every coroutine into a single state machine until you explicitly tell it to allocate it. I don't know why C++ chose a less "zero cost" solution; I suspect the fact that they don't have memory safety and so letting you own a coroutine's state is very footgunny in C++ played a part in their choice.