Async successors

[felipesere]

This doesn't compile at the moment, but I felt it was easier to get pointers on code than hypotheticals in Discord.

The current compile error is:

error[E0599]: no method named `poll` found for type `Fut` in the current scope
  --> src/stream/stream/successor.rs:51:52
   |
51 |         match self.as_mut().successor()(self.next).poll(cx) {
   |                                                    ^^^^ method not found in `Fut`
   |
   = help: items from traits can only be used if the type parameter is bounded by the trait
help: the following trait defines an item `poll`, perhaps you need to restrict type parameter `Fut` with it:
   |
41 | impl <F, Fut: core::future::future::Future, T> Stream for Successor<F, Fut, T>
   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

which confuses me because I thought I had that trait bound on the where?

[ghost]

I think the error is saying you're calling .poll(cx) on a Fut whereas you should be calling it on a Pin<&mut Fut>, i.e. the future is not pinned.

We should store the future returned by successor(next) into the Successor struct and then poll it. Note that if polling returns Poll::Pending, later on we'll want to poll that same future again instead of asking successor(next) to give a fresh future.

[felipesere]

Anything I’d we would need to do to get this merged? 🤩

[yoshuawuyts]

Looks great; thanks heaps!

changes implemented

[yoshuawuyts]

Created a backup of the async impl here for future reference (e.g. when we get async closures): https://gist.github.com/yoshuawuyts/32fb44959c0bd66a683008ee1d540500. Thanks!