Revert "Recalculate entity aabbs when meshes change (bevyengine#4944)" (bevyengine#5489)

# Objective

Sadly, bevyengine#4944 introduces a serious exponential despawn behavior, which cannot be included in 0.8. [Handling AABBs properly is a controversial topic](bevyengine#5423 (comment)) and one that deserves more time than the day we have left before release.

## Solution

This reverts commit c2b332f.

Author: cart

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

@@ -1,18 +1,15 @@
 mod render_layers;
 
-use smallvec::SmallVec;
-
 pub use render_layers::*;
 
 use bevy_app::{CoreStage, Plugin};
-use bevy_asset::{AssetEvent, Assets, Handle};
+use bevy_asset::{Assets, Handle};
 use bevy_ecs::prelude::*;
 use bevy_hierarchy::{Children, Parent};
 use bevy_reflect::std_traits::ReflectDefault;
 use bevy_reflect::Reflect;
 use bevy_transform::components::GlobalTransform;
 use bevy_transform::TransformSystem;
-use bevy_utils::HashMap;
 use std::cell::Cell;
 use thread_local::ThreadLocal;
 
@@ -130,11 +127,6 @@ pub struct VisibilityBundle {
 #[derive(Component)]
 pub struct NoFrustumCulling;
 
-/// Use this component to opt-out of built-in [`Aabb`] updating for Mesh entities (i.e. to opt out
-/// of [`update_bounds`]).
-#[derive(Component)]
-pub struct NoAabbUpdate;
-
 /// Collection of entities visible from the current view.
 ///
 /// This component contains all entities which are visible from the currently
@@ -168,59 +160,9 @@ impl VisibleEntities {
     }
 }
 
-/// Tracks which [`Entities`](Entity) have which meshes for entities whose [`Aabb`]s are managed by
-/// the [`calculate_bounds`] and [`update_bounds`] systems. This is needed because `update_bounds`
-/// recomputes `Aabb`s for entities whose mesh has been mutated. These mutations are visible via
-/// [`AssetEvent<Mesh>`](AssetEvent) which tells us which mesh was changed but not which entities
-/// have that mesh.
-#[derive(Debug, Default, Clone)]
-pub struct EntityMeshMap {
-    entities_with_mesh: HashMap<Handle<Mesh>, SmallVec<[Entity; 1]>>,
-    mesh_for_entity: HashMap<Entity, Handle<Mesh>>,
-}
-
-impl EntityMeshMap {
-    /// Register the passed `entity` as having the passed `mesh_handle`.
-    fn register(&mut self, entity: Entity, mesh_handle: &Handle<Mesh>) {
-        // Note that this list can have duplicates if an entity is registered for a mesh multiple
-        // times. This should be rare and only cause an additional `Aabb.clone()` in
-        // `update_bounds` so it is preferable to a `HashSet` for now.
-        self.entities_with_mesh
-            .entry(mesh_handle.clone_weak())
-            .or_default()
-            .push(entity);
-        self.mesh_for_entity
-            .insert(entity, mesh_handle.clone_weak());
-    }
-
-    /// Deregisters the mapping between an `Entity` and `Mesh`. Used so [`update_bounds`] can
-    /// track which mappings are still active so `Aabb`s are updated correctly.
-    fn deregister(&mut self, entity: Entity) {
-        let mut inner = || {
-            let mesh = self.mesh_for_entity.remove(&entity)?;
-
-            // This lookup failing is _probably_ an error.
-            let entities = self.entities_with_mesh.get_mut(&mesh)?;
-
-            // There could be duplicate entries in here if an entity was registered with a mesh
-            // multiple times. It's important to remove all references so that if an entity gets a
-            // new mesh and its old mesh is mutated, the entity doesn't get its old mesh's new
-            // `Aabb`. Note that there _should_ only be one entity.
-            for i in (0..entities.len()).rev() {
-                if entities[i] == entity {
-                    entities.swap_remove(i);
-                }
-            }
-            Some(())
-        };
-        inner();
-    }
-}
-
 #[derive(Debug, Hash, PartialEq, Eq, Clone, SystemLabel)]
 pub enum VisibilitySystems {
     CalculateBounds,
-    UpdateBounds,
     UpdateOrthographicFrusta,
     UpdatePerspectiveFrusta,
     UpdateProjectionFrusta,
@@ -236,121 +178,60 @@ impl Plugin for VisibilityPlugin {
     fn build(&self, app: &mut bevy_app::App) {
         use VisibilitySystems::*;
 
-        app.init_resource::<EntityMeshMap>()
-            .add_system_to_stage(
-                CoreStage::PostUpdate,
-                calculate_bounds.label(CalculateBounds),
-            )
-            .add_system_to_stage(CoreStage::PostUpdate, update_bounds.label(UpdateBounds))
-            .add_system_to_stage(
-                CoreStage::PostUpdate,
-                update_frusta::<OrthographicProjection>
-                    .label(UpdateOrthographicFrusta)
-                    .after(TransformSystem::TransformPropagate),
-            )
-            .add_system_to_stage(
-                CoreStage::PostUpdate,
-                update_frusta::<PerspectiveProjection>
-                    .label(UpdatePerspectiveFrusta)
-                    .after(TransformSystem::TransformPropagate),
-            )
-            .add_system_to_stage(
-                CoreStage::PostUpdate,
-                update_frusta::<Projection>
-                    .label(UpdateProjectionFrusta)
-                    .after(TransformSystem::TransformPropagate),
-            )
-            .add_system_to_stage(
-                CoreStage::PostUpdate,
-                visibility_propagate_system.label(VisibilityPropagate),
-            )
-            .add_system_to_stage(
-                CoreStage::PostUpdate,
-                check_visibility
-                    .label(CheckVisibility)
-                    .after(CalculateBounds)
-                    .after(UpdateBounds)
-                    .after(UpdateOrthographicFrusta)
-                    .after(UpdatePerspectiveFrusta)
-                    .after(UpdateProjectionFrusta)
-                    .after(VisibilityPropagate)
-                    .after(TransformSystem::TransformPropagate),
-            );
+        app.add_system_to_stage(
+            CoreStage::PostUpdate,
+            calculate_bounds.label(CalculateBounds),
+        )
+        .add_system_to_stage(
+            CoreStage::PostUpdate,
+            update_frusta::<OrthographicProjection>
+                .label(UpdateOrthographicFrusta)
+                .after(TransformSystem::TransformPropagate),
+        )
+        .add_system_to_stage(
+            CoreStage::PostUpdate,
+            update_frusta::<PerspectiveProjection>
+                .label(UpdatePerspectiveFrusta)
+                .after(TransformSystem::TransformPropagate),
+        )
+        .add_system_to_stage(
+            CoreStage::PostUpdate,
+            update_frusta::<Projection>
+                .label(UpdateProjectionFrusta)
+                .after(TransformSystem::TransformPropagate),
+        )
+        .add_system_to_stage(
+            CoreStage::PostUpdate,
+            visibility_propagate_system.label(VisibilityPropagate),
+        )
+        .add_system_to_stage(
+            CoreStage::PostUpdate,
+            check_visibility
+                .label(CheckVisibility)
+                .after(CalculateBounds)
+                .after(UpdateOrthographicFrusta)
+                .after(UpdatePerspectiveFrusta)
+                .after(UpdateProjectionFrusta)
+                .after(VisibilityPropagate)
+                .after(TransformSystem::TransformPropagate),
+        );
     }
 }
 
-/// Calculates [`Aabb`]s for [`Entities`](Entity) with [`Mesh`]es.
 pub fn calculate_bounds(
     mut commands: Commands,
     meshes: Res<Assets<Mesh>>,
     without_aabb: Query<(Entity, &Handle<Mesh>), (Without<Aabb>, Without<NoFrustumCulling>)>,
-    mut entity_mesh_map: ResMut<EntityMeshMap>,
 ) {
     for (entity, mesh_handle) in &without_aabb {
         if let Some(mesh) = meshes.get(mesh_handle) {
             if let Some(aabb) = mesh.compute_aabb() {
-                entity_mesh_map.register(entity, mesh_handle);
                 commands.entity(entity).insert(aabb);
             }
         }
     }
 }
 
-/// Updates [`Aabb`]s for [`Entities`](Entity) with [`Mesh`]es. This includes `Entities` that have
-/// been assigned new `Mesh`es as well as `Entities` whose `Mesh` has been directly mutated.
-///
-/// To opt out of bound calculation for an `Entity`, give it the [`NoAabbUpdate`] component.
-///
-/// NOTE: This system needs to remove entities from their collection in
-/// [`EntityMeshMap`] whenever a mesh handle is reassigned or an entity's mesh handle is
-/// removed. This may impact performance if meshes with many entities are frequently
-/// reassigned/removed.
-pub fn update_bounds(
-    mut commands: Commands,
-    meshes: Res<Assets<Mesh>>,
-    mut mesh_reassigned: Query<
-        (Entity, &Handle<Mesh>, &mut Aabb),
-        (
-            Changed<Handle<Mesh>>,
-            Without<NoFrustumCulling>,
-            Without<NoAabbUpdate>,
-        ),
-    >,
-    mut entity_mesh_map: ResMut<EntityMeshMap>,
-    mut mesh_events: EventReader<AssetEvent<Mesh>>,
-    entities_lost_mesh: RemovedComponents<Handle<Mesh>>,
-) {
-    for entity in entities_lost_mesh.iter() {
-        entity_mesh_map.deregister(entity);
-    }
-
-    for (entity, mesh_handle, mut aabb) in mesh_reassigned.iter_mut() {
-        entity_mesh_map.deregister(entity);
-        if let Some(mesh) = meshes.get(mesh_handle) {
-            if let Some(new_aabb) = mesh.compute_aabb() {
-                entity_mesh_map.register(entity, mesh_handle);
-                *aabb = new_aabb;
-            }
-        }
-    }
-
-    let to_update = |event: &AssetEvent<Mesh>| {
-        let handle = match event {
-            AssetEvent::Modified { handle } => handle,
-            _ => return None,
-        };
-        let mesh = meshes.get(handle)?;
-        let entities_with_handle = entity_mesh_map.entities_with_mesh.get(handle)?;
-        let aabb = mesh.compute_aabb()?;
-        Some((aabb, entities_with_handle))
-    };
-    for (aabb, entities_with_handle) in mesh_events.iter().filter_map(to_update) {
-        for entity in entities_with_handle {
-            commands.entity(*entity).insert(aabb.clone());
-        }
-    }
-}
-
 pub fn update_frusta<T: Component + CameraProjection + Send + Sync + 'static>(
     mut views: Query<(&GlobalTransform, &T, &mut Frustum)>,
 ) {