Remove default implementation of extend_from_iter from RelationshipSourceCollection (#20255)

# Objective

- Remove the default implementation of `extend_from_iter` from the
RelationshipSourceCollection trait to standardize collection types
providing their own implementation.
- Fixes #20253

## Solution

- Removed the default implementation of `extend_from_iter`
- Added `extend_from_iter` implementation for BTreeSet that uses its
native .extend() method
- All other collection types (Vec, SmallVec, EntityHashSet, IndexSet,
EntityIndexSet, Entity) already had their own implementations

Author: muddxyii

crates/bevy_ecs/src/relationship/relationship_source_collection.rs

@@ -78,13 +78,7 @@ pub trait RelationshipSourceCollection {
     /// Add multiple entities to collection at once.
     ///
     /// May be faster than repeatedly calling [`Self::add`].
-    fn extend_from_iter(&mut self, entities: impl IntoIterator<Item = Entity>) {
-        // The method name shouldn't conflict with `Extend::extend` as it's in the rust prelude and
-        // would always conflict with it.
-        for entity in entities {
-            self.add(entity);
-        }
-    }
+    fn extend_from_iter(&mut self, entities: impl IntoIterator<Item = Entity>);
 }
 
 /// This trait signals that a [`RelationshipSourceCollection`] is ordered.
@@ -582,6 +576,10 @@ impl RelationshipSourceCollection for BTreeSet<Entity> {
     fn shrink_to_fit(&mut self) {
         // BTreeSet doesn't have a capacity
     }
+
+    fn extend_from_iter(&mut self, entities: impl IntoIterator<Item = Entity>) {
+        self.extend(entities);
+    }
 }
 
 #[cfg(test)]

release-content/migration-guides/remove_default_extend_from_iter.md

@@ -0,0 +1,27 @@
+---
+title: Remove default implementation of extend_from_iter from RelationshipSourceCollection
+pull_requests: [20255]
+---
+
+The `extend_from_iter` method in the `RelationshipSourceCollection` trait no longer has a default implementation. If you have implemented a custom relationship source collection, you must now provide your own implementation of this method.
+
+```rust
+// Before: method was optional due to default implementation
+impl RelationshipSourceCollection for MyCustomCollection {
+    // ... other required methods
+    // extend_from_iter was automatically provided
+}
+
+// After: method is now required
+impl RelationshipSourceCollection for MyCustomCollection {
+    // ... other required methods
+    fn extend_from_iter(&mut self, entities: impl IntoIterator<Item = Entity>) {
+        // Use your collection's native extend method if available
+        self.extend(entities);
+        // Or implement manually if needed:
+        // for entity in entities {
+        //     self.add(entity);
+        // }
+    }
+}
+```