[Merged by Bors] - exact sized event iterators

[mockersf]

Objective

• Remove Resource binding on events, introduce a new Event trait
• Ensure event iterators are ExactSizeIterator

Solution

• Builds on [Merged by Bors] - Increment last event count on next instead of iter #2382 and [Merged by Bors] - Implement len and is_empty for EventReaders #2969

Changelog

• Events, EventWriter, EventReader and so on now require that the underlying type is Event, rather than Resource. Both of these are trivial supertraits of Send + Sync + 'static with universal blanket implementations: this change is currently purely cosmetic.
• Event reader iterators now implement ExactSizeIterator

[alice-i-cecile]

Yes please to an event trait. IMO we may want to remove the blanket impl later, but that can be done when we need it.

[alice-i-cecile]

Related to #2073, which also introduces and uses an Event trait.

[mockersf]

@alice-i-cecile why the C-Breaking-Change? I don't think there's anything breaking here?

[alice-i-cecile]

The Resource bound is changing to Event. This will break external code that abstracts over events. The fix is completely trivial though.

[mockersf]

Nop, everything that impl Resource also impl Event, the two traits are equivalent (at least for now). I copied the declaration and the blanket impl...

[alice-i-cecile]

So, consider this extension method on World from #3839, which I stole from my private code base:

    pub fn send_event<E: Resource>(&mut self, event: E) {
        let mut events: Mut<Events<E>> = self.get_resource_mut()
        .expect("The specified event resource was not found in the world. Did you forget to call `app.add_event::<E>()`?");

        events.send(event);
    }

This can be written entirely in end user code, and will cease to compile after this change. Every possible use of it will still be valid, but the type signature itself is now wrong because of the bound here:

bevy/crates/bevy_ecs/src/event.rs

Line 359 in ddd900c

impl<T: Event> Events<T> {

.

I 100% think this is the right call, but the change is technically breaking 🤷🏽

EDIT: no, this will still compile, because Rust can infer that all Resource will fulfill Event. You were right.

[alice-i-cecile]

@mockersf can you write a changelog for this when you get a moment? Stealing from #4685 should cover most of it.

[alice-i-cecile]

bors r+

[bors]

Pull request successfully merged into main.

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