Add subtrait for reborrowing QueryData types

[ecoskey]

Objective

Allow reborrowing for QueryData types. The end goal here is to work towards a fast System::run_iter api, avoiding the overhead of get_param with each iteration. Funnily enough the only system param that actually needs this is Single, but it seems worthwhile to have anyways

Next steps:

1. ReborrowSystemParam
2. SystemIter

Solution

• Add ReborrowQueryData, an optional subtrait (though almost all QueryData types should implement this in practice)

/// A [`QueryData`] that's able to be reborrowed, converting a reference into
/// an owned struct with a shorter lifetime.
pub trait ReborrowQueryData: QueryData {
    /// Returns a `QueryData` item with a smaller lifetime.
    fn reborrow<'wlong: 'short, 'slong: 'short, 'short>(
        item: &'short mut Self::Item<'wlong, 'slong>,
    ) -> Self::Item<'short, 'short>;
}

[NicoZweifel]

This is pretty neat! This makes total sense to me in terms of utility functions and the like. I suggested a test that tests exactly this use case.

Other than that there are only a few minor things:

• For the EntityRef types, we can dereference and use the Copy implementations instead of manually creating the struct again, which is a bit more maintainable.

• The implemention of Ref<> can also be slightly simplified since ComponentTicksRef implements Clone.

• I noticed a few missing square brackets in the doc comments.

[NicoZweifel]

Suggested change

	}
	}
	#[test]
	fn test_reborrow_allows_reuse() {
	#[derive(Component)]
	struct A(f32);
	fn increment(mut component: Mut<A>) {
	component.0 += 1.;
	}
	let mut world = World::new();
	world.spawn(A(0.));
	let mut query = world.query::<&mut A>();
	let mut component = query.single_mut(&mut world).unwrap();
	increment(component.reborrow());
	component.0 += 1.;
	assert_eq!(component.0, 2.0);
	}

[NicoZweifel]

Suggested change

	/// Returns an `EntityRef<>` with a smaller lifetime.
	/// This is useful if you have `&EntityRef`, but you need an `EntityRef`.
	/// Returns an [`EntityRef<>`] with a smaller lifetime.
	/// This is useful if you have &[`EntityRef`], but you need an [`EntityRef`].

[NicoZweifel]

Suggested change

	/// Returns an `EntityRefExcept<>` with a smaller lifetime.
	/// This is useful if you have `&EntityRefExcept`, but you need an `EntityRefExcept`.
	/// Returns an [`EntityRefExcept<>`] with a smaller lifetime.
	/// This is useful if you have &[`EntityRefExcept`], but you need an [`EntityRefExcept`].

[NicoZweifel]

Suggested change

	/// Returns an `FilteredEntityRef<>` with a smaller lifetime.
	/// This is useful if you have `&FilteredEntityRef`, but you need an `FilteredEntityRef`.
	/// Returns an [`FilteredEntityRef<>`] with a smaller lifetime.
	/// This is useful if you have &[`FilteredEntityRef`], but you need an [`FilteredEntityRef`].

[NicoZweifel]

Suggested change

	/// Returns a `Mut<>` with a smaller lifetime.
	/// This is useful if you have `&Ref<T>`, but you need a `Ref<T>`.
	/// Returns a [`Mut<>`] with a smaller lifetime.
	/// This is useful if you have &[`Ref<T>`], but you need a [`Ref<T>`].

[NicoZweifel]

Suggested change

	/// Returns a `QueryData` item with a smaller lifetime.
	/// Returns a [`QueryData`] item with a smaller lifetime.

[NicoZweifel]

Suggested change

	EntityRefExcept {
	entity: self.entity,
	access: self.access,
	phantom: PhantomData,
	}
	*self

[NicoZweifel]

Suggested change

	EntityRef { cell: self.cell }
	*self

[NicoZweifel]

Suggested change

	FilteredEntityRef {
	entity: self.entity,
	access: self.access,
	}
	*self

[NicoZweifel]

Suggested change

	Ref {
	value: self.value,
	ticks: ComponentTicksRef {
	added: self.ticks.added,
	changed: self.ticks.changed,
	changed_by: self.ticks.changed_by,
	last_run: self.ticks.last_run,
	this_run: self.ticks.this_run,
	},
	}
	Ref {
	ticks: self.ticks.clone(),
	..*self
	}