Never vs. PhantomData

[cramertj]

An RFC was recently approved for the ! (Never) type, which represents the type of a value that can never exist.

Several structs in of futures-rs use PhantomData to mark the type of values that can't exist, such as the Error type of Finished, the Item type of Failed, and the Item and Error types of Empty. I believe that, once this PR lands (and then makes its way to stable), these structs should be rewritten to use the Never type. Empty, for example, could be written as follows:

pub struct Empty {}

pub fn empty() -> Empty { // Is this even necessary any more?
    Empty {}
}

impl Future for Empty {
    type Item = !;
    type Error = !;

    fn poll(&mut self, _: &mut Task) -> Poll<Self::Item, Self::Error> {
        Poll::NotReady
    }

    fn schedule(&mut self, task: &mut Task) {
        drop(task);
    }
}

Unfortunately, this feature has yet to land on nightly, let alone stable. Therefore, a transition to Never is likely a ways off. As this library is already getting lots of attention, I believe there should be a plan for how and if to structure the library so that a future (hehe 😃 ) transition to Never is possible.

In order to be more consistent with terminology, I also think that Empty should be renamed to Never. ! is called Never rather than Empty or Void as a result of the discussion at the end of the Never PR. The gist of it is that Empty isn't a future for an Empty value, it's a future for a value that can Never exist (i.e. it Never completes).

P.S. Great work on this library, @alexcrichton & company! I'm excited to use this in my future Rust projects.

[alexcrichton]

Oh man I'd love to make use of !! This seems like a super slick way to use that type, and I know I personally find it annoying that sometimes I have to specify the other type parameter of failed, empty, etc.

Right now though this library does compile on stable Rust, and I'd personally prefer to require that, but as soon as ! lands on nightly then I'd love to add a nightly feature to this crate which adds versions of these combinators. Eventually once ! is stable we can just deprecate all the old combinators in favor of the ones using !.

How's that sound?

[cramertj]

Sounds great! I'll keep an eye out on the ! PR and follow up once it lands on nightly.

Did you have any thoughts about changing the name of Empty? Perhaps NeverFuture? I feel a bit anxious about Never since it isn't really equivalent to !-- it's a Future that yields !.

[cramertj]

The ergonomics of this feature didn't work out due to restrictions around default type parameters and missing trait impls for !. I'm closing for now with the intent to reopen later once ! and default type parameters have had a chance to develop.