Like a stack frame, but allocated once of a fixed size, instead of being LIFO allocated in a reserved region (which itself must be allocated upfront, when you don't know how big you're going to end up).

The difference being: if your tasks need 192B of memory each, and you spawn 10 of each, you just consumed a little less than 2kB. With green threads, you have 10 times the starting size of your stack (generally a few kB). That makes a big difference if you don't have much memory.

Kinda like a stack frame, but more compact and allocated in one go beforehand.

They can, but the function itself cannot be suspended by calling something else (i.e. await being a keyword enforces this), so any function that is called can use the original OS thread stack. Any called function can in turn be an async function, and will return a future[1] that in turn capture that function stack. So yes, a chain of suspended async functions sort of looks like a stack, but its size is known a compile time [2].

[1] I'm not familiar with rust semantics here, just making educated guesses.

[2] Not sure how rust deals with recursion in this case. I assume you get a compilation error because it will fail to deduce the return value of the function, and you'll have to explicitly box the future: the "stack" would then look a linked list of activation records.

Async function don't really call anything by themselves: the executor does, and all function called in the context of an async function is called on the executor's stack. You just end up with a fixed number of executors running on their own OS thread with a normal stack.