Improve QueryIter size_hint hints (#4244)

## Objective

This fixes #1686.

`size_hint` can be useful even if a little niche. For example,
`collect::<Vec<_>>()` uses the `size_hint` of Iterator it collects from
to pre-allocate a memory slice large enough to not require re-allocating
when pushing all the elements of the iterator.

## Solution

To this effect I made the following changes:
* Add a `IS_ARCHETYPAL` associated constant to the `Fetch` trait,
  this constant tells us when it is safe to assume that the `Fetch`
  relies exclusively on archetypes to filter queried entities
* Add `IS_ARCHETYPAL` to all the implementations of `Fetch`
* Use that constant in `QueryIter::size_hint` to provide a more useful

## Migration guide

The new associated constant is an API breaking change. For the user,
if they implemented a custom `Fetch`, it means they have to add this
associated constant to their implementation. Either `true` if it doesn't limit
the number of entities returned in a query beyond that of archetypes, or
`false` for when it does.

crates/bevy_ecs/macros/src/fetch.rs

@@ -252,6 +252,8 @@ pub fn derive_world_query_impl(ast: DeriveInput) -> TokenStream {
                         }
                     }
 
+                    const IS_ARCHETYPAL: bool = true #(&& <#field_types as #path::query::WorldQuery>::ReadOnlyFetch::IS_ARCHETYPAL)*;
+
                     const IS_DENSE: bool = true #(&& <#field_types as #path::query::WorldQuery>::ReadOnlyFetch::IS_DENSE)*;
 
                     #[inline]
@@ -303,6 +305,8 @@ pub fn derive_world_query_impl(ast: DeriveInput) -> TokenStream {
                         }
                     }
 
+                    const IS_ARCHETYPAL: bool = true #(&& <#field_types as #path::query::WorldQuery>::#fetch_associated_type::IS_ARCHETYPAL)*;
+
                     const IS_DENSE: bool = true #(&& <#field_types as #path::query::WorldQuery>::#fetch_associated_type::IS_DENSE)*;
 
                     /// SAFETY: we call `set_archetype` for each member that implements `Fetch`

crates/bevy_ecs/src/lib.rs

@@ -51,7 +51,8 @@ mod tests {
         component::{Component, ComponentId},
         entity::Entity,
         query::{
-            Added, ChangeTrackers, Changed, FilterFetch, FilteredAccess, With, Without, WorldQuery,
+            Added, ChangeTrackers, Changed, FilterFetch, FilteredAccess, Or, With, Without,
+            WorldQuery,
         },
         world::{Mut, World},
     };
@@ -1402,6 +1403,40 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_is_archetypal_size_hints() {
+        let mut world = World::default();
+        macro_rules! query_min_size {
+            ($query:ty, $filter:ty) => {
+                world
+                    .query_filtered::<$query, $filter>()
+                    .iter(&world)
+                    .size_hint()
+                    .0
+            };
+        }
+
+        world.spawn().insert_bundle((A(1), B(1), C));
+        world.spawn().insert_bundle((A(1), C));
+        world.spawn().insert_bundle((A(1), B(1)));
+        world.spawn().insert_bundle((B(1), C));
+        world.spawn().insert(A(1));
+        world.spawn().insert(C);
+        assert_eq!(2, query_min_size![(), (With<A>, Without<B>)],);
+        assert_eq!(3, query_min_size![&B, Or<(With<A>, With<C>)>],);
+        assert_eq!(1, query_min_size![&B, (With<A>, With<C>)],);
+        assert_eq!(1, query_min_size![(&A, &B), With<C>],);
+        assert_eq!(4, query_min_size![&A, ()], "Simple Archetypal");
+        assert_eq!(4, query_min_size![ChangeTrackers<A>, ()],);
+        // All the following should set minimum size to 0, as it's impossible to predict
+        // how many entites the filters will trim.
+        assert_eq!(0, query_min_size![(), Added<A>], "Simple Added");
+        assert_eq!(0, query_min_size![(), Changed<A>], "Simple Changed");
+        assert_eq!(0, query_min_size![(&A, &B), Changed<A>],);
+        assert_eq!(0, query_min_size![&A, (Changed<A>, With<B>)],);
+        assert_eq!(0, query_min_size![(&A, &B), Or<(Changed<A>, Changed<B>)>],);
+    }
+
     #[test]
     fn reserve_entities_across_worlds() {
         let mut world_a = World::default();

crates/bevy_ecs/src/query/fetch.rs

@@ -340,6 +340,13 @@ pub trait Fetch<'world, 'state>: Sized {
     /// [`Fetch::archetype_fetch`] will be called for iterators.
     const IS_DENSE: bool;
 
+    /// Returns true if (and only if) this Fetch relies strictly on archetypes to limit which
+    /// components are acessed by the Query.
+    ///
+    /// This enables optimizations for [`crate::query::QueryIter`] that rely on knowing exactly how
+    /// many elements are being iterated (such as `Iterator::collect()`).
+    const IS_ARCHETYPAL: bool;
+
     /// Adjusts internal state to account for the next [`Archetype`]. This will always be called on
     /// archetypes that match this [`Fetch`].
     ///
@@ -458,6 +465,8 @@ impl<'w, 's> Fetch<'w, 's> for EntityFetch {
 
     const IS_DENSE: bool = true;
 
+    const IS_ARCHETYPAL: bool = true;
+
     unsafe fn init(
         _world: &World,
         _state: &Self::State,
@@ -583,6 +592,8 @@ impl<'w, 's, T: Component> Fetch<'w, 's> for ReadFetch<T> {
         }
     };
 
+    const IS_ARCHETYPAL: bool = true;
+
     unsafe fn init(
         world: &World,
         state: &Self::State,
@@ -767,6 +778,8 @@ impl<'w, 's, T: Component> Fetch<'w, 's> for WriteFetch<T> {
         }
     };
 
+    const IS_ARCHETYPAL: bool = true;
+
     unsafe fn init(
         world: &World,
         state: &Self::State,
@@ -873,6 +886,8 @@ impl<'w, 's, T: Component> Fetch<'w, 's> for ReadOnlyWriteFetch<T> {
         }
     };
 
+    const IS_ARCHETYPAL: bool = true;
+
     unsafe fn init(
         world: &World,
         state: &Self::State,
@@ -1002,6 +1017,8 @@ impl<'w, 's, T: Fetch<'w, 's>> Fetch<'w, 's> for OptionFetch<T> {
 
     const IS_DENSE: bool = T::IS_DENSE;
 
+    const IS_ARCHETYPAL: bool = T::IS_ARCHETYPAL;
+
     unsafe fn init(
         world: &World,
         state: &Self::State,
@@ -1212,6 +1229,8 @@ impl<'w, 's, T: Component> Fetch<'w, 's> for ChangeTrackersFetch<T> {
         }
     };
 
+    const IS_ARCHETYPAL: bool = true;
+
     unsafe fn init(
         world: &World,
         state: &Self::State,
@@ -1314,6 +1333,8 @@ macro_rules! impl_tuple_fetch {
 
             const IS_DENSE: bool = true $(&& $name::IS_DENSE)*;
 
+            const IS_ARCHETYPAL: bool = true $(&& $name::IS_ARCHETYPAL)*;
+
             #[inline]
             unsafe fn set_archetype(&mut self, _state: &Self::State, _archetype: &Archetype, _tables: &Tables) {
                 let ($($name,)*) = self;
@@ -1407,6 +1428,8 @@ macro_rules! impl_anytuple_fetch {
 
             const IS_DENSE: bool = true $(&& $name::IS_DENSE)*;
 
+            const IS_ARCHETYPAL: bool = true $(&& $name::IS_ARCHETYPAL)*;
+
             #[inline]
             unsafe fn set_archetype(&mut self, _state: &Self::State, _archetype: &Archetype, _tables: &Tables) {
                 let ($($name,)*) = &mut self.0;
@@ -1512,6 +1535,8 @@ impl<'w, 's, State: FetchState> Fetch<'w, 's> for NopFetch<State> {
 
     const IS_DENSE: bool = true;
 
+    const IS_ARCHETYPAL: bool = true;
+
     #[inline(always)]
     unsafe fn init(
         _world: &World,

crates/bevy_ecs/src/query/filter.rs

@@ -145,6 +145,8 @@ impl<'w, 's, T: Component> Fetch<'w, 's> for WithFetch<T> {
         }
     };
 
+    const IS_ARCHETYPAL: bool = true;
+
     #[inline]
     unsafe fn set_table(&mut self, _state: &Self::State, _table: &Table) {}
 
@@ -280,6 +282,8 @@ impl<'w, 's, T: Component> Fetch<'w, 's> for WithoutFetch<T> {
         }
     };
 
+    const IS_ARCHETYPAL: bool = true;
+
     #[inline]
     unsafe fn set_table(&mut self, _state: &Self::State, _table: &Table) {}
 
@@ -402,6 +406,8 @@ macro_rules! impl_query_filter_tuple {
 
             const IS_DENSE: bool = true $(&& $filter::IS_DENSE)*;
 
+            const IS_ARCHETYPAL: bool = true $(&& $filter::IS_ARCHETYPAL)*;
+
             #[inline]
             unsafe fn set_table(&mut self, state: &Self::State, table: &Table) {
                 let ($($filter,)*) = &mut self.0;
@@ -578,6 +584,8 @@ macro_rules! impl_tick_filter {
                 }
             };
 
+            const IS_ARCHETYPAL: bool = false;
+
             unsafe fn set_table(&mut self, state: &Self::State, table: &Table) {
                 self.table_ticks = table
                     .get_column(state.component_id).unwrap()

crates/bevy_ecs/src/query/iter.rs

@@ -138,9 +138,6 @@ where
         }
     }
 
-    // NOTE: For unfiltered Queries this should actually return a exact size hint,
-    // to fulfil the ExactSizeIterator invariant, but this isn't practical without specialization.
-    // For more information see Issue #1686.
     fn size_hint(&self) -> (usize, Option<usize>) {
         let max_size = self
             .query_state
@@ -149,7 +146,9 @@ where
             .map(|index| self.world.archetypes[ArchetypeId::new(index)].len())
             .sum();
 
-        (0, Some(max_size))
+        let archetype_query = F::Fetch::IS_ARCHETYPAL && QF::IS_ARCHETYPAL;
+        let min_size = if archetype_query { max_size } else { 0 };
+        (min_size, Some(max_size))
     }
 }
 
@@ -297,9 +296,6 @@ where
         unsafe { QueryCombinationIter::fetch_next_aliased_unchecked(self) }
     }
 
-    // NOTE: For unfiltered Queries this should actually return a exact size hint,
-    // to fulfil the ExactSizeIterator invariant, but this isn't practical without specialization.
-    // For more information see Issue #1686.
     fn size_hint(&self) -> (usize, Option<usize>) {
         if K == 0 {
             return (0, Some(0));
@@ -324,15 +320,18 @@ where
                 n / k_factorial
             });
 
-        (0, max_combinations)
+        let archetype_query = F::Fetch::IS_ARCHETYPAL && QF::IS_ARCHETYPAL;
+        let min_combinations = if archetype_query { max_size } else { 0 };
+        (min_combinations, max_combinations)
     }
 }
 
 // NOTE: We can cheaply implement this for unfiltered Queries because we have:
 // (1) pre-computed archetype matches
 // (2) each archetype pre-computes length
 // (3) there are no per-entity filters
-// TODO: add an ArchetypeOnlyFilter that enables us to implement this for filters like With<T>
+// TODO: add an ArchetypeOnlyFilter that enables us to implement this for filters like With<T>.
+// This would need to be added to all types that implement Filter with Filter::IS_ARCHETYPAL = true
 impl<'w, 's, Q: WorldQuery, QF> ExactSizeIterator for QueryIter<'w, 's, Q, QF, ()>
 where
     QF: Fetch<'w, 's, State = Q::State>,

crates/bevy_ecs/src/query/mod.rs

@@ -54,38 +54,38 @@ mod tests {
         let mut a_query = world.query::<&A>();
         assert_eq!(a_query.iter_combinations::<0>(&world).count(), 0);
         assert_eq!(
-            a_query.iter_combinations::<0>(&world).size_hint(),
-            (0, Some(0))
+            a_query.iter_combinations::<0>(&world).size_hint().1,
+            Some(0)
         );
         assert_eq!(a_query.iter_combinations::<1>(&world).count(), 4);
         assert_eq!(
-            a_query.iter_combinations::<1>(&world).size_hint(),
-            (0, Some(4))
+            a_query.iter_combinations::<1>(&world).size_hint().1,
+            Some(4)
         );
         assert_eq!(a_query.iter_combinations::<2>(&world).count(), 6);
         assert_eq!(
-            a_query.iter_combinations::<2>(&world).size_hint(),
-            (0, Some(6))
+            a_query.iter_combinations::<2>(&world).size_hint().1,
+            Some(6)
         );
         assert_eq!(a_query.iter_combinations::<3>(&world).count(), 4);
         assert_eq!(
-            a_query.iter_combinations::<3>(&world).size_hint(),
-            (0, Some(4))
+            a_query.iter_combinations::<3>(&world).size_hint().1,
+            Some(4)
         );
         assert_eq!(a_query.iter_combinations::<4>(&world).count(), 1);
         assert_eq!(
-            a_query.iter_combinations::<4>(&world).size_hint(),
-            (0, Some(1))
+            a_query.iter_combinations::<4>(&world).size_hint().1,
+            Some(1)
         );
         assert_eq!(a_query.iter_combinations::<5>(&world).count(), 0);
         assert_eq!(
-            a_query.iter_combinations::<5>(&world).size_hint(),
-            (0, Some(0))
+            a_query.iter_combinations::<5>(&world).size_hint().1,
+            Some(0)
         );
         assert_eq!(a_query.iter_combinations::<1024>(&world).count(), 0);
         assert_eq!(
-            a_query.iter_combinations::<1024>(&world).size_hint(),
-            (0, Some(0))
+            a_query.iter_combinations::<1024>(&world).size_hint().1,
+            Some(0)
         );
 
         let values: Vec<[&A; 2]> = world.query::<&A>().iter_combinations(&world).collect();