Add `as_pin_ref` and `as_pin_mut` to `Either`

[thomaseizinger]

Over at either, we are discussing to add structural projection.

Would that be a safe thing to do? What are the reasons why it is not public here in futures::Either?

Any advice is greatly appreciated.

[taiki-e]

Option has as_pin_ref and as_pin_mut for this purpose. It's okay to provide Either::{as_pin_ref, as_pin_mut}, as these are sound unless you write other (wrong) unsafe code.

[thomaseizinger]

Thank you for answering so quickly and thoroughly :)

This is resolved from my end unless you want to keep it open to maybe also expose futures::Either::project publicly?

[taiki-e]

Re-opening -- I think it is reasonable to have such methods in futures.

[thomaseizinger]

Would it make sense to depend on either instead?

[cuviper]

It's possible, but that would be a breaking change to combine them, as one can currently have both impl LocalTrait for either::Either and for futures::future::Either.

[taiki-e]

Would it make sense to depend on either instead?

In addition to it being a breaking change, we are also implementing stream and async io traits on our Either type, so we do not plan to depend on either crate. (e.g., it makes #2362 impossible)