[Merged by Bors] - [ecs] implement is_empty for queries

crates/bevy_ecs/src/system/mod.rs

@@ -438,6 +438,21 @@ mod tests {
         assert_eq!(conflicts, vec![b_id, d_id]);
     }
 
+    #[test]
+    fn query_is_empty() {
+        fn sys(empty: Query<&A>, not_empty: Query<&B>) {
+            assert!(empty.is_empty());
+            assert!(!not_empty.is_empty());
+        }
+
+        let mut world = World::default();
+        world.spawn().insert(B);
+
+        let mut sys = sys.system();
+        sys.initialize(&mut world);
+        sys.run((), &mut world);
+    }
+
     #[test]
     #[allow(clippy::too_many_arguments)]
     fn can_have_16_parameters() {

crates/bevy_ecs/src/system/query.rs

@@ -543,6 +543,18 @@ where
             >())),
         }
     }
+
+    /// Returns true if this query contains no elements.
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        // SAFE: system runs without conflicts with other systems.
+        // same-system queries have runtime borrow checks when they conflict
+        let mut iter = unsafe {
+            self.state
+                .iter_unchecked_manual(self.world, self.last_change_tick, self.change_tick)
+        };
+        iter.next().is_none()
+    }
 }
 
 /// An error that occurs when retrieving a specific [`Entity`]'s component from a [`Query`]