QueryIter implements ExactSizeIterator, but doesnt provide an exact size_hint

[MinerSebas]

Bevy version

cd4c684

What you did

I read the documentation of ExactSizeIterator, and notice that this requirement isn't true:

When implementing an ExactSizeIterator, you must also implement Iterator.
When doing so, the implementation of Iterator::size_hint must return the exact size of the iterator.

In the standard implementation this is checked with an assertion.

Instead of providing an exact size_hint, Bevy overrides that function, which removes the need for that invariant to be true.

While it isn't a problem here, it would still be nice if QueryIter would provide an exact size_hint if it can.

[mockersf]

This may not be possible currently in rust without trait specialisation: rust-lang/rust#31844

size_hint comes from Iterator impl, which is:

bevy/crates/bevy_ecs/src/query/iter.rs

Lines 48 to 50 in cd4c684

	impl<'w, 's, Q: WorldQuery, F: WorldQuery> Iterator for QueryIter<'w, 's, Q, F>
	where
	F::Fetch: FilterFetch,

but ExactSizeIterator impl fixes the F type param to ()

bevy/crates/bevy_ecs/src/query/iter.rs

Line 118 in cd4c684

impl<'w, 's, Q: WorldQuery> ExactSizeIterator for QueryIter<'w, 's, Q, ()> {

so without a specialisation of Iterator for QueryIter<'w, 's, Q, ()> we can't have a different size_hint for that case.

[MinerSebas]

Playing around I got this almost working:

    fn size_hint(&self) -> (usize, Option<usize>) {
        if TypeId::of::<F>() == TypeId::of::<()>() {
            let temp = self
                .query_state
                .matched_archetypes
                .ones()
                .map(|index| self.world.archetypes[ArchetypeId::new(index)].len())
                .sum();

            return (temp, Some(temp));
        }

        (0, None)
    }

But then F needs to be 'static and naively adding that constraint creates even more demands for 'static.

Edit: Annotating the other locations with 'static allows Bevy to compile (with all tests passing), but I don't know if this is the proper solution.

[cart]

If we can't meet the ExactSizeIter requirements for all cases, maybe we're better of removing ExactSizeIterator in favor of a manual len() impl for unfiltered queries.

[james7132]

This was resolved in #4244. It's currently inaccurate for partially iterated iterators, which is documented in #5149 with a fix in #5214. Closing this.