add missing Iterator::size_hint overrides, most of which cause ExactSizeIterator::len to panic (#22035)

# Objective

- Fixes `ExactSizeIterator::len` panic in a few of these iterators.
Note: the std default impl of `ExactSizeIterator::len` has
`assert_eq!(Some(lower), upper);`. I am facing this issue myself with
`<index_set::Iter as ExactSizeIterator>::len`.
- Should slightly improve performance in some cases, where the more
precise `size_hint` causes less allocations (for example when collecting
into a `Vec`)

## Solution

- Add `size_hint` overrides that forward to inner iterators or an
`ExactSizeIterator::len` override in one specific case.

## Testing

- Since the overrides are pretty simple, I mostly didn't add any tests.
- I didn't feel 100% confident in the `WriteBatchIds::size_hint`
implementation, so I added a test there.

Author: JonasJebing

crates/bevy_ecs/src/entity/hash_map.rs

@@ -176,6 +176,10 @@ impl<'a, V> Iterator for Keys<'a, V> {
     fn next(&mut self) -> Option<Self::Item> {
         self.0.next()
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
 }
 
 impl<V> ExactSizeIterator for Keys<'_, V> {}
@@ -234,6 +238,10 @@ impl<V> Iterator for IntoKeys<V> {
     fn next(&mut self) -> Option<Self::Item> {
         self.0.next()
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
 }
 
 impl<V> ExactSizeIterator for IntoKeys<V> {}

crates/bevy_ecs/src/entity/hash_set.rs

@@ -238,6 +238,10 @@ impl<'a> Iterator for Iter<'a> {
     fn next(&mut self) -> Option<Self::Item> {
         self.0.next()
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
 }
 
 impl ExactSizeIterator for Iter<'_> {}
@@ -293,6 +297,10 @@ impl Iterator for IntoIter {
     fn next(&mut self) -> Option<Self::Item> {
         self.0.next()
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
 }
 
 impl ExactSizeIterator for IntoIter {}
@@ -345,6 +353,10 @@ impl<'a> Iterator for Drain<'a> {
     fn next(&mut self) -> Option<Self::Item> {
         self.0.next()
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
 }
 
 impl ExactSizeIterator for Drain<'_> {}
@@ -394,6 +406,10 @@ impl<'a, F: FnMut(&Entity) -> bool> Iterator for ExtractIf<'a, F> {
     fn next(&mut self) -> Option<Self::Item> {
         self.0.next()
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
 }
 
 impl<F: FnMut(&Entity) -> bool> FusedIterator for ExtractIf<'_, F> {}

crates/bevy_ecs/src/entity/index_map.rs

@@ -861,6 +861,10 @@ impl<'a, V> Iterator for Iter<'a, V> {
     fn next(&mut self) -> Option<Self::Item> {
         self.0.next()
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
 }
 
 impl<V> DoubleEndedIterator for Iter<'_, V> {
@@ -934,6 +938,10 @@ impl<'a, V> Iterator for IterMut<'a, V> {
     fn next(&mut self) -> Option<Self::Item> {
         self.0.next()
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
 }
 
 impl<V> DoubleEndedIterator for IterMut<'_, V> {
@@ -1004,6 +1012,10 @@ impl<V> Iterator for IntoIter<V> {
     fn next(&mut self) -> Option<Self::Item> {
         self.0.next()
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
 }
 
 impl<V> DoubleEndedIterator for IntoIter<V> {
@@ -1072,6 +1084,10 @@ impl<V> Iterator for Drain<'_, V> {
     fn next(&mut self) -> Option<Self::Item> {
         self.0.next()
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
 }
 
 impl<V> DoubleEndedIterator for Drain<'_, V> {
@@ -1120,6 +1136,10 @@ impl<'a, V> Iterator for Keys<'a, V> {
     fn next(&mut self) -> Option<Self::Item> {
         self.0.next()
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
 }
 
 impl<V> DoubleEndedIterator for Keys<'_, V> {
@@ -1188,6 +1208,10 @@ impl<V> Iterator for IntoKeys<V> {
     fn next(&mut self) -> Option<Self::Item> {
         self.0.next()
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
 }
 
 impl<V> DoubleEndedIterator for IntoKeys<V> {

crates/bevy_ecs/src/entity/index_set.rs

@@ -582,6 +582,10 @@ impl<'a> Iterator for Iter<'a> {
     fn next(&mut self) -> Option<Self::Item> {
         self.0.next()
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
 }
 
 impl DoubleEndedIterator for Iter<'_> {
@@ -651,6 +655,10 @@ impl Iterator for IntoIter {
     fn next(&mut self) -> Option<Self::Item> {
         self.0.next()
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
 }
 
 impl DoubleEndedIterator for IntoIter {
@@ -723,6 +731,10 @@ impl<'a> Iterator for Drain<'a> {
     fn next(&mut self) -> Option<Self::Item> {
         self.0.next()
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
 }
 
 impl DoubleEndedIterator for Drain<'_> {

crates/bevy_ecs/src/message/messages.rs

@@ -382,6 +382,11 @@ impl<E: Message> Iterator for WriteBatchIds<E> {
 
         result
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let len = <Self as ExactSizeIterator>::len(self);
+        (len, Some(len))
+    }
 }
 
 impl<E: Message> ExactSizeIterator for WriteBatchIds<E> {
@@ -425,4 +430,19 @@ mod tests {
         assert_eq!(test_messages.len(), 2); // Messages are double-buffered, so we see 2 + 0 = 2
         assert_eq!(test_messages.iter_current_update_messages().count(), 0);
     }
+
+    #[test]
+    fn write_batch_iter_size_hint() {
+        #[derive(Message, Clone, Copy)]
+        struct TestMessage;
+
+        let mut test_messages = Messages::<TestMessage>::default();
+        let write_batch_ids = test_messages.write_batch([TestMessage; 4]);
+        let expected_len = 4;
+        assert_eq!(write_batch_ids.len(), expected_len);
+        assert_eq!(
+            write_batch_ids.size_hint(),
+            (expected_len, Some(expected_len))
+        );
+    }
 }

crates/bevy_mesh/src/index.rs

@@ -33,6 +33,15 @@ where
             FourIterators::Fourth(iter) => iter.next(),
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        match self {
+            FourIterators::First(iter) => iter.size_hint(),
+            FourIterators::Second(iter) => iter.size_hint(),
+            FourIterators::Third(iter) => iter.size_hint(),
+            FourIterators::Fourth(iter) => iter.size_hint(),
+        }
+    }
 }
 
 /// An error that occurred while trying to invert the winding of a [`Mesh`](super::Mesh).