To use your analogy, I feel like you're saying that only humans that have legs are human. I'm saying that they're all human.

This analogy is weird.

Furthermore, because closures aren't inherently boxed, there might not even be a stack frame to begin with. Closures are syntax sugar for a struct + a trait implementation on that struct, and said call is then very inline-able.

This is also where I'm getting at with the single invocation thing: with these futures, all of these closures are going to be inlined into one big state machine, which is then itself eventually put on the heap. But that heap allocation has nothing to do with the closure, and more to do with the future wanting to not tie its data down to a specific stack frame.