Implement RelationshipSourceCollection for BTreeSet (#18469)

# Objective

`BTreeSet` doesn't implement `RelationshipSourceCollection`.

## Solution

Implement it.

## Testing

`cargo clippy`

---

## Showcase

You can now use `BTreeSet` in a `RelationshipTarget`

---------

Co-authored-by: Chris Russell <8494645+chescock@users.noreply.github.com>
Co-authored-by: François Mockers <mockersf@gmail.com>

crates/bevy_ecs/src/entity/map_entities.rs

@@ -6,9 +6,12 @@ use crate::{
     world::World,
 };
 
-use alloc::{collections::VecDeque, vec::Vec};
+use alloc::{
+    collections::{BTreeSet, VecDeque},
+    vec::Vec,
+};
 use bevy_platform::collections::HashSet;
-use core::hash::BuildHasher;
+use core::{hash::BuildHasher, mem};
 use smallvec::SmallVec;
 
 /// Operation to map all contained [`Entity`] fields in a type to new values.
@@ -72,6 +75,16 @@ impl<S: BuildHasher + Default> MapEntities for HashSet<Entity, S> {
         *self = self.drain().map(|e| entity_mapper.get_mapped(e)).collect();
     }
 }
+
+impl MapEntities for BTreeSet<Entity> {
+    fn map_entities<E: EntityMapper>(&mut self, entity_mapper: &mut E) {
+        *self = mem::take(self)
+            .into_iter()
+            .map(|e| entity_mapper.get_mapped(e))
+            .collect();
+    }
+}
+
 impl MapEntities for Vec<Entity> {
     fn map_entities<E: EntityMapper>(&mut self, entity_mapper: &mut E) {
         for entity in self.iter_mut() {
@@ -95,6 +108,7 @@ impl<A: smallvec::Array<Item = Entity>> MapEntities for SmallVec<A> {
         }
     }
 }
+
 /// An implementor of this trait knows how to map an [`Entity`] into another [`Entity`].
 ///
 /// Usually this is done by using an [`EntityHashMap<Entity>`] to map source entities

crates/bevy_ecs/src/relationship/relationship_source_collection.rs

@@ -1,3 +1,5 @@
+use alloc::collections::{btree_set, BTreeSet};
+
 use crate::entity::{hash_set::EntityHashSet, Entity};
 use alloc::vec::Vec;
 use smallvec::SmallVec;
@@ -445,6 +447,47 @@ impl<const N: usize> OrderedRelationshipSourceCollection for SmallVec<[Entity; N
     }
 }
 
+impl RelationshipSourceCollection for BTreeSet<Entity> {
+    type SourceIter<'a> = core::iter::Copied<btree_set::Iter<'a, Entity>>;
+
+    fn new() -> Self {
+        BTreeSet::new()
+    }
+
+    fn with_capacity(_: usize) -> Self {
+        // BTreeSet doesn't have a capacity
+        Self::new()
+    }
+
+    fn reserve(&mut self, _: usize) {
+        // BTreeSet doesn't have a capacity
+    }
+
+    fn add(&mut self, entity: Entity) -> bool {
+        self.insert(entity)
+    }
+
+    fn remove(&mut self, entity: Entity) -> bool {
+        self.remove(&entity)
+    }
+
+    fn iter(&self) -> Self::SourceIter<'_> {
+        self.iter().copied()
+    }
+
+    fn len(&self) -> usize {
+        self.len()
+    }
+
+    fn clear(&mut self) {
+        self.clear();
+    }
+
+    fn shrink_to_fit(&mut self) {
+        // BTreeSet doesn't have a capacity
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;