Implement IntoIterator for ECS wrapper types. (bevyengine#5096)

# Objective

Improve ergonomics by passing on the `IntoIterator` impl of the underlying type to wrapper types.

## Solution

Implement `IntoIterator` for ECS wrapper types (Mut, Local, Res, etc.).

Co-authored-by: devil-ira <justthecooldude@gmail.com>

crates/bevy_ecs/src/change_detection.rs

@@ -245,6 +245,31 @@ pub struct ResMut<'a, T: ?Sized + Resource> {
     pub(crate) ticks: Ticks<'a>,
 }
 
+impl<'w, 'a, T: Resource> IntoIterator for &'a ResMut<'w, T>
+where
+    &'a T: IntoIterator,
+{
+    type Item = <&'a T as IntoIterator>::Item;
+    type IntoIter = <&'a T as IntoIterator>::IntoIter;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.value.into_iter()
+    }
+}
+
+impl<'w, 'a, T: Resource> IntoIterator for &'a mut ResMut<'w, T>
+where
+    &'a mut T: IntoIterator,
+{
+    type Item = <&'a mut T as IntoIterator>::Item;
+    type IntoIter = <&'a mut T as IntoIterator>::IntoIter;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.set_changed();
+        self.value.into_iter()
+    }
+}
+
 change_detection_impl!(ResMut<'a, T>, T, Resource);
 impl_methods!(ResMut<'a, T>, T, Resource);
 impl_debug!(ResMut<'a, T>, Resource);
@@ -298,6 +323,31 @@ pub struct Mut<'a, T: ?Sized> {
     pub(crate) ticks: Ticks<'a>,
 }
 
+impl<'w, 'a, T> IntoIterator for &'a Mut<'w, T>
+where
+    &'a T: IntoIterator,
+{
+    type Item = <&'a T as IntoIterator>::Item;
+    type IntoIter = <&'a T as IntoIterator>::IntoIter;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.value.into_iter()
+    }
+}
+
+impl<'w, 'a, T> IntoIterator for &'a mut Mut<'w, T>
+where
+    &'a mut T: IntoIterator,
+{
+    type Item = <&'a mut T as IntoIterator>::Item;
+    type IntoIter = <&'a mut T as IntoIterator>::IntoIter;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.set_changed();
+        self.value.into_iter()
+    }
+}
+
 change_detection_impl!(Mut<'a, T>, T,);
 impl_methods!(Mut<'a, T>, T,);
 impl_debug!(Mut<'a, T>,);

crates/bevy_ecs/src/system/system_param.rs

@@ -344,6 +344,18 @@ impl<'w, T: Resource> From<ResMut<'w, T>> for Res<'w, T> {
     }
 }
 
+impl<'w, 'a, T: Resource> IntoIterator for &'a Res<'w, T>
+where
+    &'a T: IntoIterator,
+{
+    type Item = <&'a T as IntoIterator>::Item;
+    type IntoIter = <&'a T as IntoIterator>::IntoIter;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.value.into_iter()
+    }
+}
+
 /// The [`SystemParamState`] of [`Res<T>`].
 #[doc(hidden)]
 pub struct ResState<T> {
@@ -719,6 +731,30 @@ impl<'a, T: FromWorld + Send + Sync + 'static> DerefMut for Local<'a, T> {
     }
 }
 
+impl<'w, 'a, T: FromWorld + Send + 'static> IntoIterator for &'a Local<'w, T>
+where
+    &'a T: IntoIterator,
+{
+    type Item = <&'a T as IntoIterator>::Item;
+    type IntoIter = <&'a T as IntoIterator>::IntoIter;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.0.into_iter()
+    }
+}
+
+impl<'w, 'a, T: FromWorld + Send + 'static> IntoIterator for &'a mut Local<'w, T>
+where
+    &'a mut T: IntoIterator,
+{
+    type Item = <&'a mut T as IntoIterator>::Item;
+    type IntoIter = <&'a mut T as IntoIterator>::IntoIter;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.0.into_iter()
+    }
+}
+
 /// The [`SystemParamState`] of [`Local<T>`].
 #[doc(hidden)]
 pub struct LocalState<T: Send + 'static>(pub(crate) SyncCell<T>);

crates/bevy_pbr/src/light.rs

@@ -971,7 +971,7 @@ pub(crate) fn assign_lights_to_clusters(
 
         if config.dynamic_resizing() {
             let mut cluster_index_estimate = 0.0;
-            for light in lights.iter() {
+            for light in &lights {
                 let light_sphere = light.sphere();
 
                 // Check if the light is within the view frustum
@@ -1126,7 +1126,7 @@ pub(crate) fn assign_lights_to_clusters(
         }
 
         let mut update_from_light_intersections = |visible_lights: &mut Vec<Entity>| {
-            for light in lights.iter() {
+            for light in &lights {
                 let light_sphere = light.sphere();
 
                 // Check if the light is within the view frustum