Add a method iter_combinations on query to iterate over combination…

…s of query results (bevyengine#1763)

Related to [discussion on discord](https://discord.com/channels/691052431525675048/742569353878437978/824731187724681289)

With const generics, it is now possible to write generic iterator over multiple entities at once.

This enables patterns of query iterations like

```rust
for [e1, e2, e3] in query.iter_combinations() {
   // do something with relation of all three entities
}
```

The compiler is able to infer the correct iterator for given size of array, so either of those work
```rust
for [e1, e2] in query.iter_combinations()  { ... }
for [e1, e2, e3] in query.iter_combinations()  { ... }
```

This feature can be very useful for systems like collision detection.

When you ask for permutations of size K of N entities:
- if K == N, you get one result of all entities
- if K < N, you get all possible subsets of N with size K, without repetition
- if K > N, the result set is empty (no permutation of size K exist)

Co-authored-by: Carter Anderson <[email protected]>

Cargo.toml

@@ -267,6 +267,10 @@ path = "examples/ecs/fixed_timestep.rs"
 name = "hierarchy"
 path = "examples/ecs/hierarchy.rs"
 
+[[example]]
+name = "iter_combinations"
+path = "examples/ecs/iter_combinations.rs"
+
 [[example]]
 name = "parallel_query"
 path = "examples/ecs/parallel_query.rs"

crates/bevy_ecs/src/query/fetch.rs

@@ -279,6 +279,18 @@ pub struct ReadFetch<T> {
     sparse_set: *const ComponentSparseSet,
 }
 
+impl<T> Clone for ReadFetch<T> {
+    fn clone(&self) -> Self {
+        Self {
+            storage_type: self.storage_type,
+            table_components: self.table_components,
+            entity_table_rows: self.entity_table_rows,
+            entities: self.entities,
+            sparse_set: self.sparse_set,
+        }
+    }
+}
+
 /// SAFETY: access is read only
 unsafe impl<T> ReadOnlyFetch for ReadFetch<T> {}
 
@@ -382,6 +394,21 @@ pub struct WriteFetch<T> {
     change_tick: u32,
 }
 
+impl<T> Clone for WriteFetch<T> {
+    fn clone(&self) -> Self {
+        Self {
+            storage_type: self.storage_type,
+            table_components: self.table_components,
+            table_ticks: self.table_ticks,
+            entities: self.entities,
+            entity_table_rows: self.entity_table_rows,
+            sparse_set: self.sparse_set,
+            last_change_tick: self.last_change_tick,
+            change_tick: self.change_tick,
+        }
+    }
+}
+
 /// The [`FetchState`] of `&mut T`.
 pub struct WriteState<T> {
     component_id: ComponentId,