Improve safety comment, don't box unnecessarily

crates/bevy_ecs/src/removal_detection.rs

@@ -12,9 +12,10 @@ use bevy_ecs_macros::SystemParam;
 
 use std::{
     fmt::Debug,
-    iter::{Cloned, Flatten},
+    iter,
     marker::PhantomData,
     ops::{Deref, DerefMut},
+    option,
 };
 
 /// Wrapper around a [`ManualEventReader<Entity>`] so that we
@@ -117,7 +118,8 @@ pub struct RemovedComponents<'w, 's, T: Component> {
     event_sets: &'w RemovedComponentEvents,
 }
 
-type RemovedIter<'a> = Flatten<std::option::IntoIter<Cloned<ManualEventIterator<'a, Entity>>>>;
+type RemovedIter<'a> =
+    iter::Flatten<option::IntoIter<iter::Cloned<ManualEventIterator<'a, Entity>>>>;
 
 impl<'w, 's, T: Component> RemovedComponents<'w, 's, T> {
     pub fn iter(&mut self) -> RemovedIter<'_> {

crates/bevy_ecs/src/system/system_param.rs

@@ -1322,7 +1322,7 @@ impl<'a> SystemParam for &'a RemovedComponentEvents {
     type State = RemovedComponentEventsState;
 }
 
-// SAFETY: Only reads World components
+// SAFETY: Only reads World removed component events
 unsafe impl<'a> ReadOnlySystemParam for &'a RemovedComponentEvents {}
 
 /// The [`SystemParamState`] of [`RemovedComponentEvents`].

crates/bevy_ecs/src/world/mod.rs

@@ -742,25 +742,25 @@ impl World {
 
     /// Returns an iterator of entities that had components of type `T` removed
     /// since the last call to [`World::clear_trackers`].
-    pub fn removed<T: Component>(&self) -> Box<dyn Iterator<Item = Entity> + '_> {
-        if let Some(component_id) = self.components.get_id(TypeId::of::<T>()) {
-            Box::new(self.removed_with_id(component_id))
-        } else {
-            Box::new(std::iter::empty())
-        }
+    pub fn removed<T: Component>(&self) -> impl DoubleEndedIterator<Item = Entity> + '_ {
+        self.components
+            .get_id(TypeId::of::<T>())
+            .map(|component_id| self.removed_with_id(component_id))
+            .into_iter()
+            .flatten()
     }
 
     /// Returns an iterator of entities that had components with the given `component_id` removed
     /// since the last call to [`World::clear_trackers`].
     pub fn removed_with_id(
         &self,
         component_id: ComponentId,
-    ) -> Box<dyn Iterator<Item = Entity> + '_> {
-        if let Some(removed) = self.removed_components.get(component_id) {
-            Box::new(removed.iter_current_update_events().cloned())
-        } else {
-            Box::new(std::iter::empty())
-        }
+    ) -> impl DoubleEndedIterator<Item = Entity> + '_ {
+        self.removed_components
+            .get(component_id)
+            .map(|removed| removed.iter_current_update_events().cloned())
+            .into_iter()
+            .flatten()
     }
 
     /// Inserts a new resource with standard starting values.