Remove ChildOf::get and Deref impl (#18080)

# Objective

There are currently three ways to access the parent stored on a ChildOf
relationship:

1. `child_of.parent` (field accessor)
2. `child_of.get()` (get function)
3. `**child_of` (Deref impl)

I will assert that we should only have one (the field accessor), and
that the existence of the other implementations causes confusion and
legibility issues. The deref approach is heinous, and `child_of.get()`
is significantly less clear than `child_of.parent`.

## Solution

Remove `impl Deref for ChildOf` and `ChildOf::get`.

The one "downside" I'm seeing is that:

```rust
entity.get::<ChildOf>().map(ChildOf::get)
```
Becomes this:

```rust
entity.get::<ChildOf>().map(|c| c.parent)
```

I strongly believe that this is worth the increased clarity and
consistency. I'm also not really a huge fan of the "pass function
pointer to map" syntax. I think most people don't think this way about
maps. They think in terms of a function that takes the item in the
Option and returns the result of some action on it.

## Migration Guide

```rust
// Before
**child_of
// After
child_of.parent

// Before
child_of.get()
// After
child_of.parent

// Before
entity.get::<ChildOf>().map(ChildOf::get)
// After
entity.get::<ChildOf>().map(|c| c.parent)
```

crates/bevy_ecs/src/hierarchy.rs

@@ -99,22 +99,6 @@ pub struct ChildOf {
     pub parent: Entity,
 }
 
-impl ChildOf {
-    /// Returns the parent entity, which is the "target" of this relationship.
-    pub fn get(&self) -> Entity {
-        self.parent
-    }
-}
-
-impl Deref for ChildOf {
-    type Target = Entity;
-
-    #[inline]
-    fn deref(&self) -> &Self::Target {
-        &self.parent
-    }
-}
-
 // TODO: We need to impl either FromWorld or Default so ChildOf can be registered as Reflect.
 // This is because Reflect deserialize by creating an instance and apply a patch on top.
 // However ChildOf should only ever be set with a real user-defined entity.  Its worth looking into
@@ -280,7 +264,7 @@ pub fn validate_parent_has_component<C: Component>(
         return;
     };
     if !world
-        .get_entity(child_of.get())
+        .get_entity(child_of.parent)
         .is_ok_and(|e| e.contains::<C>())
     {
         // TODO: print name here once Name lives in bevy_ecs

crates/bevy_input_focus/src/lib.rs

@@ -151,17 +151,17 @@ impl<E: Event + Clone> Event for FocusedInput<E> {
 #[derive(QueryData)]
 /// These are for accessing components defined on the targeted entity
 pub struct WindowTraversal {
-    parent: Option<&'static ChildOf>,
+    child_of: Option<&'static ChildOf>,
     window: Option<&'static Window>,
 }
 
 impl<E: Event + Clone> Traversal<FocusedInput<E>> for WindowTraversal {
     fn traverse(item: Self::Item<'_>, event: &FocusedInput<E>) -> Option<Entity> {
-        let WindowTraversalItem { parent, window } = item;
+        let WindowTraversalItem { child_of, window } = item;
 
         // Send event to parent, if it has one.
-        if let Some(parent) = parent {
+        if let Some(child_of) = child_of {
-            return Some(parent.get());
+            return Some(child_of.parent);
         };
 
         // Otherwise, send it to the window entity (unless this is a window entity).
@@ -334,7 +334,7 @@ impl IsFocused for World {
             if e == entity {
                 return true;
             }
-            if let Some(parent) = self.entity(e).get::<ChildOf>().map(ChildOf::get) {
+            if let Some(parent) = self.entity(e).get::<ChildOf>().map(|c| c.parent) {
                 e = parent;
             } else {
                 return false;

crates/bevy_picking/src/events.rs

@@ -79,7 +79,7 @@ pub struct Pointer<E: Debug + Clone + Reflect> {
 /// propagates to the pointer's window and stops there.
 #[derive(QueryData)]
 pub struct PointerTraversal {
-    parent: Option<&'static ChildOf>,
+    child_of: Option<&'static ChildOf>,
     window: Option<&'static Window>,
 }
 
@@ -88,11 +88,11 @@ where
     E: Debug + Clone + Reflect,
 {
     fn traverse(item: Self::Item<'_>, pointer: &Pointer<E>) -> Option<Entity> {
-        let PointerTraversalItem { parent, window } = item;
+        let PointerTraversalItem { child_of, window } = item;
 
         // Send event to parent, if it has one.
-        if let Some(parent) = parent {
+        if let Some(child_of) = child_of {
-            return Some(parent.get());
+            return Some(child_of.parent);
         };
 
         // Otherwise, send it to the window entity (unless this is a window entity).

crates/bevy_render/src/view/visibility/mod.rs

@@ -405,13 +405,13 @@ fn visibility_propagate_system(
     mut visibility_query: Query<(&Visibility, &mut InheritedVisibility)>,
     children_query: Query<&Children, (With<Visibility>, With<InheritedVisibility>)>,
 ) {
-    for (entity, visibility, parent, children) in &changed {
+    for (entity, visibility, child_of, children) in &changed {
         let is_visible = match visibility {
             Visibility::Visible => true,
             Visibility::Hidden => false,
             // fall back to true if no parent is found or parent lacks components
-            Visibility::Inherited => parent
+            Visibility::Inherited => child_of
-                .and_then(|p| visibility_query.get(p.get()).ok())
+                .and_then(|c| visibility_query.get(c.parent).ok())
                 .is_none_or(|(_, x)| x.get()),
         };
         let (_, mut inherited_visibility) = visibility_query

crates/bevy_scene/src/dynamic_scene.rs

@@ -316,7 +316,7 @@ mod tests {
                 .unwrap()
                 .get::<ChildOf>()
                 .unwrap()
-                .get(),
+                .parent,
             "something about reloading the scene is touching entities with the same scene Ids"
         );
         assert_eq!(
@@ -326,7 +326,7 @@ mod tests {
                 .unwrap()
                 .get::<ChildOf>()
                 .unwrap()
-                .get(),
+                .parent,
             "something about reloading the scene is touching components not defined in the scene but on entities defined in the scene"
         );
         assert_eq!(
@@ -336,7 +336,7 @@ mod tests {
                 .unwrap()
                 .get::<ChildOf>()
                 .expect("something is wrong with this test, and the scene components don't have a parent/child relationship")
-                .get(),
+                .parent,
             "something is wrong with this test or the code reloading scenes since the relationship between scene entities is broken"
         );
     }

crates/bevy_text/src/text.rs

@@ -508,14 +508,14 @@ pub fn detect_text_needs_rerender<Root: Component>(
     // - Span component changed.
     // - Span TextFont changed.
     // - Span children changed (can include additions and removals).
-    for (entity, maybe_span_parent, has_text_block) in changed_spans.iter() {
+    for (entity, maybe_span_child_of, has_text_block) in changed_spans.iter() {
         if has_text_block {
             once!(warn!("found entity {} with a TextSpan that has a TextLayout, which should only be on root \
                 text entities (that have {}); this warning only prints once",
                 entity, core::any::type_name::<Root>()));
         }
 
-        let Some(span_parent) = maybe_span_parent else {
+        let Some(span_child_of) = maybe_span_child_of else {
             once!(warn!(
                 "found entity {} with a TextSpan that has no parent; it should have an ancestor \
                 with a root text component ({}); this warning only prints once",
@@ -524,13 +524,13 @@ pub fn detect_text_needs_rerender<Root: Component>(
             ));
             continue;
         };
-        let mut parent: Entity = span_parent.get();
+        let mut parent: Entity = span_child_of.parent;
 
         // Search for the nearest ancestor with ComputedTextBlock.
         // Note: We assume the perf cost from duplicate visits in the case that multiple spans in a block are visited
         // is outweighed by the expense of tracking visited spans.
         loop {
-            let Ok((maybe_parent, maybe_computed, has_span)) = computed.get_mut(parent) else {
+            let Ok((maybe_child_of, maybe_computed, has_span)) = computed.get_mut(parent) else {
                 once!(warn!("found entity {} with a TextSpan that is part of a broken hierarchy with a ChildOf \
                     component that points at non-existent entity {}; this warning only prints once",
                     entity, parent));
@@ -546,7 +546,7 @@ pub fn detect_text_needs_rerender<Root: Component>(
                     entity, parent));
                 break;
             }
-            let Some(next_parent) = maybe_parent else {
+            let Some(next_child_of) = maybe_child_of else {
                 once!(warn!(
                     "found entity {} with a TextSpan that has no ancestor with the root text \
                     component ({}); this warning only prints once",
@@ -555,7 +555,7 @@ pub fn detect_text_needs_rerender<Root: Component>(
                 ));
                 break;
             };
-            parent = next_parent.get();
+            parent = next_child_of.parent;
         }
     }
 }

crates/bevy_transform/src/systems.rs

@@ -52,8 +52,8 @@ pub fn compute_transform_leaves(
 ) {
     leaves
         .par_iter_mut()
-        .for_each(|(transform, mut global_transform, parent)| {
+        .for_each(|(transform, mut global_transform, child_of)| {
-            let Ok(parent_transform) = parents.get(parent.get()) else {
+            let Ok(parent_transform) = parents.get(child_of.parent) else {
                 return;
             };
             if parent_transform.is_changed()
@@ -102,7 +102,7 @@ mod serial {
             (Ref<Transform>, &mut GlobalTransform, Option<&Children>),
             (With<ChildOf>, With<Children>),
         >,
-        parent_query: Query<(Entity, Ref<ChildOf>), With<GlobalTransform>>,
+        child_query: Query<(Entity, Ref<ChildOf>), With<GlobalTransform>>,
         mut orphaned_entities: Local<Vec<Entity>>,
     ) {
         orphaned_entities.clear();
@@ -115,9 +115,9 @@ mod serial {
                 *global_transform = GlobalTransform::from(*transform);
             }
 
-            for (child, actual_parent) in parent_query.iter_many(children) {
+            for (child, child_of) in child_query.iter_many(children) {
                 assert_eq!(
-                    actual_parent.get(), entity,
+                    child_of.parent, entity,
                     "Malformed hierarchy. This probably means that your hierarchy has been improperly maintained, or contains a cycle"
                 );
                 // SAFETY:
@@ -137,9 +137,9 @@ mod serial {
                     propagate_recursive(
                         &global_transform,
                         &transform_query,
-                        &parent_query,
+                        &child_query,
                         child,
-                        changed || actual_parent.is_changed(),
+                        changed || child_of.is_changed(),
                     );
                 }
             }
@@ -170,7 +170,7 @@ mod serial {
             (Ref<Transform>, &mut GlobalTransform, Option<&Children>),
             (With<ChildOf>, With<Children>),
         >,
-        parent_query: &Query<(Entity, Ref<ChildOf>), With<GlobalTransform>>,
+        child_query: &Query<(Entity, Ref<ChildOf>), With<GlobalTransform>>,
         entity: Entity,
         mut changed: bool,
     ) {
@@ -215,9 +215,9 @@ mod serial {
         };
 
         let Some(children) = children else { return };
-        for (child, actual_parent) in parent_query.iter_many(children) {
+        for (child, child_of) in child_query.iter_many(children) {
             assert_eq!(
-            actual_parent.get(), entity,
+            child_of.parent, entity,
             "Malformed hierarchy. This probably means that your hierarchy has been improperly maintained, or contains a cycle"
         );
             // SAFETY: The caller guarantees that `transform_query` will not be fetched for any
@@ -229,9 +229,9 @@ mod serial {
                 propagate_recursive(
                     global_matrix.as_ref(),
                     transform_query,
-                    parent_query,
+                    child_query,
                     child,
-                    changed || actual_parent.is_changed(),
+                    changed || child_of.is_changed(),
                 );
             }
         }
@@ -463,7 +463,7 @@ mod parallel {
             let mut last_child = None;
             let new_children = children_iter.map(
                 |(child, (transform, mut global_transform), (children, child_of))| {
-                    assert_eq!(child_of.get(), parent);
+                    assert_eq!(child_of.parent, parent);
                     if p_global_transform.is_changed()
                         || transform.is_changed()
                         || global_transform.is_added()

crates/bevy_ui/src/layout/mod.rs

@@ -127,12 +127,12 @@ pub fn ui_layout_system(
 
     computed_node_query
         .iter()
-        .for_each(|(entity, maybe_parent)| {
+        .for_each(|(entity, maybe_child_of)| {
-            if let Some(parent) = maybe_parent {
+            if let Some(child_of) = maybe_child_of {
                 // Note: This does not cover the case where a parent's Node component was removed.
                 // Users are responsible for fixing hierarchies if they do that (it is not recommended).
                 // Detecting it here would be a permanent perf burden on the hot path.
-                if parent.is_changed() && !ui_children.is_ui_node(parent.get()) {
+                if child_of.is_changed() && !ui_children.is_ui_node(child_of.parent) {
                     warn!(
                         "Node ({entity}) is in a non-UI entity hierarchy. You are using an entity \
 with UI components as a child of an entity without UI components, your UI layout may be broken."

crates/bevy_winit/src/accessibility.rs

@@ -227,9 +227,9 @@ fn update_adapter(
 ) -> TreeUpdate {
     let mut to_update = vec![];
     let mut window_children = vec![];
-    for (entity, node, children, parent) in &nodes {
+    for (entity, node, children, child_of) in &nodes {
         let mut node = (**node).clone();
-        queue_node_for_update(entity, parent, &node_entities, &mut window_children);
+        queue_node_for_update(entity, child_of, &node_entities, &mut window_children);
         add_children_nodes(children, &node_entities, &mut node);
         let node_id = NodeId(entity.to_bits());
         to_update.push((node_id, node));
@@ -258,7 +258,7 @@ fn queue_node_for_update(
     window_children: &mut Vec<NodeId>,
 ) {
     let should_push = if let Some(child_of) = child_of {
-        !node_entities.contains(child_of.get())
+        !node_entities.contains(child_of.parent)
     } else {
         true
     };

examples/tools/scene_viewer/animation_plugin.rs

@@ -109,7 +109,7 @@ fn assign_clips(
                 }
 
                 // Go to the next parent.
-                current = children.get(entity).ok().map(ChildOf::get);
+                current = children.get(entity).ok().map(|c| c.parent);
             }
         }
 

examples/ui/ghost_nodes.rs

@@ -108,9 +108,9 @@ fn button_system(
     mut counter_query: Query<&mut Counter>,
 ) {
     // Update parent counter on click
-    for (interaction, parent) in &mut interaction_query {
+    for (interaction, child_of) in &mut interaction_query {
         if matches!(interaction, Interaction::Pressed) {
-            let mut counter = counter_query.get_mut(parent.get()).unwrap();
+            let mut counter = counter_query.get_mut(child_of.parent).unwrap();
             counter.0 += 1;
         }
     }