Implement FusedIterator for eligible Iterator types (#4942)

# Objective
Most of our `Iterator` impls satisfy the requirements of `std::iter::FusedIterator`, which has internal specialization that optimizes `Interator::fuse`. The std lib iterator combinators do have a few that rely on `fuse`, so this could optimize those use cases. I don't think we're using any of them in the engine itself, but beyond a light increase in compile time, it doesn't hurt to implement the trait.

## Solution
Implement the trait for all eligible iterators in first party crates. Also add a missing `ExactSizeIterator` on an iterator that could use it.

Author: james7132

crates/bevy_ecs/src/entity/mod.rs

@@ -196,6 +196,7 @@ impl<'a> Iterator for ReserveEntitiesIterator<'a> {
 }
 
 impl<'a> core::iter::ExactSizeIterator for ReserveEntitiesIterator<'a> {}
+impl<'a> core::iter::FusedIterator for ReserveEntitiesIterator<'a> {}
 
 #[derive(Debug, Default)]
 pub struct Entities {

crates/bevy_ecs/src/query/iter.rs

@@ -5,7 +5,7 @@ use crate::{
     query::{Fetch, QueryState, WorldQuery},
     storage::{TableId, Tables},
 };
-use std::{borrow::Borrow, marker::PhantomData, mem::MaybeUninit};
+use std::{borrow::Borrow, iter::FusedIterator, marker::PhantomData, mem::MaybeUninit};
 
 use super::{QueryFetch, QueryItem, ReadOnlyFetch};
 
@@ -72,6 +72,12 @@ where
     }
 }
 
+// This is correct as [`QueryIter`] always returns `None` once exhausted.
+impl<'w, 's, Q: WorldQuery, QF, F: WorldQuery> FusedIterator for QueryIter<'w, 's, Q, QF, F> where
+    QF: Fetch<'w, State = Q::State>
+{
+}
+
 /// An [`Iterator`] over query results of a [`Query`](crate::system::Query).
 ///
 /// This struct is created by the [`Query::iter_many`](crate::system::Query::iter_many) method.
@@ -359,6 +365,15 @@ where
     }
 }
 
+// This is correct as [`QueryCombinationIter`] always returns `None` once exhausted.
+impl<'w, 's, Q: WorldQuery, F: WorldQuery, const K: usize> FusedIterator
+    for QueryCombinationIter<'w, 's, Q, F, K>
+where
+    QueryFetch<'w, Q>: Clone + ReadOnlyFetch,
+    QueryFetch<'w, F>: Clone + ReadOnlyFetch,
+{
+}
+
 struct QueryIterationCursor<'w, 's, Q: WorldQuery, QF: Fetch<'w, State = Q::State>, F: WorldQuery> {
     table_id_iter: std::slice::Iter<'s, TableId>,
     archetype_id_iter: std::slice::Iter<'s, ArchetypeId>,

crates/bevy_ecs/src/world/spawn_batch.rs

@@ -3,6 +3,7 @@ use crate::{
     entity::Entity,
     world::World,
 };
+use std::iter::FusedIterator;
 
 pub struct SpawnBatchIter<'w, I>
 where
@@ -84,3 +85,10 @@ where
         self.inner.len()
     }
 }
+
+impl<I, T> FusedIterator for SpawnBatchIter<'_, I>
+where
+    I: FusedIterator<Item = T>,
+    T: Bundle,
+{
+}

crates/bevy_reflect/src/array.rs

@@ -1,7 +1,7 @@
 use crate::{serde::Serializable, Reflect, ReflectMut, ReflectRef};
-use std::fmt::Debug;
 use std::{
     any::Any,
+    fmt::Debug,
     hash::{Hash, Hasher},
 };
 

crates/bevy_render/src/mesh/mesh/mod.rs

@@ -14,7 +14,7 @@ use bevy_ecs::system::{lifetimeless::SRes, SystemParamItem};
 use bevy_math::*;
 use bevy_reflect::TypeUuid;
 use bevy_utils::Hashed;
-use std::{collections::BTreeMap, hash::Hash};
+use std::{collections::BTreeMap, hash::Hash, iter::FusedIterator};
 use thiserror::Error;
 use wgpu::{
     util::BufferInitDescriptor, BufferUsages, IndexFormat, VertexAttribute, VertexFormat,
@@ -749,8 +749,18 @@ impl Iterator for IndicesIter<'_> {
             IndicesIter::U32(iter) => iter.next().map(|val| *val as usize),
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        match self {
+            IndicesIter::U16(iter) => iter.size_hint(),
+            IndicesIter::U32(iter) => iter.size_hint(),
+        }
+    }
 }
 
+impl<'a> ExactSizeIterator for IndicesIter<'a> {}
+impl<'a> FusedIterator for IndicesIter<'a> {}
+
 impl From<&Indices> for IndexFormat {
     fn from(indices: &Indices) -> Self {
         match indices {

crates/bevy_render/src/render_resource/pipeline_cache.rs

@@ -13,7 +13,7 @@ use bevy_asset::{AssetEvent, Assets, Handle};
 use bevy_ecs::event::EventReader;
 use bevy_ecs::system::{Res, ResMut};
 use bevy_utils::{default, tracing::error, Entry, HashMap, HashSet};
-use std::{hash::Hash, mem, ops::Deref, sync::Arc};
+use std::{hash::Hash, iter::FusedIterator, mem, ops::Deref, sync::Arc};
 use thiserror::Error;
 use wgpu::{PipelineLayoutDescriptor, ShaderModule, VertexBufferLayout as RawVertexBufferLayout};
 
@@ -672,3 +672,5 @@ impl<'a> Iterator for ErrorSources<'a> {
         current
     }
 }
+
+impl<'a> FusedIterator for ErrorSources<'a> {}