A zero cost abstraction is just an abstraction you couldn’t write any better yourself, not one that is actually free. If you’re lookint for an item in a hashmap, you still have to pay the cost of looking up the key. The promise of zero-cost abstractions is that you’re paying the lowest possible cost to do that, with a nice interface that you didn’t have to write from scratch.

Agreed, that's why I said might. Whether rust async is the best possible abstraction probably depends on the event loop, and your code. I could be wrong but I think if you write a trivial async function it will still have spinach for the async even if there are no yield points and it's effectively a non async function