Ensure that the closure outlives the generator [breaking-change]

[edef1c]

Previously, it was possible to cause unsafety by having the closure refer to values with lifetimes that weren't enclosed by the generator's lifetime.

[Amanieu]

The only part I'm still not sure about is whether this handles variance correctly...

[Amanieu]

So I had a look at the variance, and here is what I think it should be:

• Generator should be variant over 'a and Output, but invariant over Input.
• Yielder should be variant over Input but invariant over Output.

This means that you be using the following PhantomData types:

• Generator: PhantomData<(&'a (), *mut Input, *const Output)>
• Yielder: PhantomData<(*const Input, *mut Output)>

Additionally, you should only provide an immutable reference to Yielder in the generator function. A mutable reference would allow calling mem::swap on the yielder and replacing it with a different one. Yielder::suspend only requires an immutable reference anyways.

[Amanieu]

And finally, you can add Send and Sync for Generator (neither are applicable to Yielder). Sync should always be available, while Send should only be available if Stack: Send.