Stack generator safety suggestion

[vlad9486]

I think it is possible to make stack generator safe. The Co object is not Clone, therefore the only possibility to outlive the Gen is to return it from the coroutine, am I right? It is even impossible to yield it. Type system can require the coroutine return the Co object along with its return value.

pub struct Gen<'s, Y, R, F: Future> {

will be

pub struct Gen<'s, Y, R, Ret, F: Future<Output = (Co<'s, Y, R>, Ret)>> {

and

    pub fn resume_with(&mut self, arg: R) -> GeneratorState<Y, F::Output> {

will be

    pub fn resume_with(&mut self, arg: R) -> GeneratorState<Y, Ret> {

The user is forced to return the Co, and they cannot clone it, so it will be the same Co, and Ret cannot contain one more.