Modernize epoll API

[SUPERCILEX]

This PR:

• Unifies the epoll backends
• Implements the ideas in Weird epoll comment around close #1100, meaning the user can now pass in a Vec and uninits with the same API
• Enables using epoll::wait in non-alloc crates

cc @notgull for extra review

@sunfishcode this buffer trait idea seems pretty cool, maybe we should use this pattern in other areas that take uninits?

All of this should be fully backwards compatible.

[SUPERCILEX]

Ok, I think the remaining questions are:

• Should we clear events in the Vec API? I have a slight preference against this, but I can also see not clearing leading to weird bugs.
• What do we do if the Vec has zero capacity? Do we auto-alloc for the user or let the syscall error out. I'm leaning towards let the syscall error out, but again this could be confusing so I could see wanting to make the API friendlier.

[notgull]

Big fan of the idea here, but a few notes.

[sunfishcode]

I need some more time to think about this.

The EventBuffer trait as a way to migrate away from EventVec and toward just using a plain Vec or array makes sense.

That said, I have reservations about the use of B::Out return value, which means that the value you get back, which determines how you use this function, changes significantly depending on what type of thing you pass in, and it's not clear from the public interface how users would figure out this relationship.

I like how eg. read and read_uninit have intuitive signatures that don't require you to learn about a trait and look up what types implement it, and I like how they give simple error messages if you use the wrong type.

[SUPERCILEX]

Aw, I thought the dependent type theory stuff was the cool part. 😁 I do agree that it's pretty high on the complexity scale though. I've tried tweaking it so the docs are actually usable:

And the error looks like this which is honestly pretty decent IMO:

error[E0277]: the trait bound `{integer}: EventBuffer` is not satisfied
   --> src/event/mod.rs:33:22
    |
33  |     epoll::wait(CWD, 0, 0);
    |     -----------      ^ the trait `EventBuffer` is not implemented for `{integer}`
    |     |
    |     required by a bound introduced by this call
    |
    = help: the following other types implement trait `EventBuffer`:
              &'a mut [Event]
              &'a mut [MaybeUninit<Event>]
              &mut EventVec
              &mut Vec<Event>

[notgull]

cc #908, which implemented a trait similar to EventBuffer but more generic in scope

[SUPERCILEX]

Doh! I'd totally forgotten about that, sorry. And here I thought this was a new idea lol.

So if this space was already explored, then maybe we should just add a wait_uninit method instead and expect people to use spare_capacity_mut? I'm not a huge fan of that approach for Vecs because it means the caller has to take on the unsafety of setting the length if they want to pass the vec along rather than using the returned slice.

If we do want to try the fancy pantsy approach, I can make it look a little more like @notgull's original proposal while maintaining the Internal type so nobody can use it but us.