> require the STL

That it has to heap-allocate if non-inlined is a misconception. This is only the default behavior.

One can define:

void *operator new(size_t sz, Foo &foo)

in the coro's promise type, and this:

- removes the implicitly-defined operator new

- forces the coro's signature to be CoroType f(Foo &foo), and forwards arguments to the "operator new" one defined

Therefore, it's pretty trivial to support coroutines even when heap cannot be used, especially in the non-recursive case.

Yes, green threads ("stackful coroutines") are more straightforward to use, however:

- they can't be arbitrarily destroyed when suspended (this would require stack unwinding support and/or active support from the green thread runtime)

- they are very ABI dependent. Among the "few registers" one has to save FPU registers. Which, in the case of older Arm architectures, and codegen options similar to -mgeneral-regs-only (for code that runs "below" userspace). Said FPU registers also take a lot of space in the stack frame, too

Really, stackless coros are just FSM generators (which is obvious if one looks at disasm)