It's an error if that would be unsafe, or else it compiles fine: the future compiles down to an enum/struct that stores everything inline and it can be returned

It's an error if that would be unsafe, or else it compiles fine: the future compiles down to an enum/struct that stores everything inline and it can be returned/move up the stack/stored in global memory the same way that aggregate values returned without allocation (etc.) in C or C++.