[ecs] implement is_empty for queries (#2271)

## Problem
- The `Query` struct does not provide an easy way to check if it is empty. 
- Specifically, users have to use `.iter().peekable()` or `.iter().next().is_none()` which is not very ergonomic. 
- Fixes: #2270 

## Solution
- Implement an `is_empty` function for queries to more easily check if the query is empty.

crates/bevy_ecs/src/query/iter.rs

@@ -69,6 +69,62 @@ where
             current_index: 0,
         }
     }
+
+    /// Consumes `self` and returns true if there were no elements remaining in this iterator.
+    #[inline(always)]
+    pub(crate) fn none_remaining(mut self) -> bool {
+        // NOTE: this mimics the behavior of `QueryIter::next()`, except that it
+        // never gets a `Self::Item`.
+        unsafe {
+            if self.is_dense {
+                loop {
+                    if self.current_index == self.current_len {
+                        let table_id = match self.table_id_iter.next() {
+                            Some(table_id) => table_id,
+                            None => return true,
+                        };
+                        let table = &self.tables[*table_id];
+                        self.filter.set_table(&self.query_state.filter_state, table);
+                        self.current_len = table.len();
+                        self.current_index = 0;
+                        continue;
+                    }
+
+                    if !self.filter.table_filter_fetch(self.current_index) {
+                        self.current_index += 1;
+                        continue;
+                    }
+
+                    return false;
+                }
+            } else {
+                loop {
+                    if self.current_index == self.current_len {
+                        let archetype_id = match self.archetype_id_iter.next() {
+                            Some(archetype_id) => archetype_id,
+                            None => return true,
+                        };
+                        let archetype = &self.archetypes[*archetype_id];
+                        self.filter.set_archetype(
+                            &self.query_state.filter_state,
+                            archetype,
+                            self.tables,
+                        );
+                        self.current_len = archetype.len();
+                        self.current_index = 0;
+                        continue;
+                    }
+
+                    if !self.filter.archetype_filter_fetch(self.current_index) {
+                        self.current_index += 1;
+                        continue;
+                    }
+
+                    return false;
+                }
+            }
+        }
+    }
 }
 
 impl<'w, 's, Q: WorldQuery, F: WorldQuery> Iterator for QueryIter<'w, 's, Q, F>

crates/bevy_ecs/src/query/state.rs

@@ -68,6 +68,16 @@ where
         state
     }
 
+    #[inline]
+    pub fn is_empty(&self, world: &World, last_change_tick: u32, change_tick: u32) -> bool {
+        // SAFE: the iterator is instantly consumed via `none_remaining` and the implementation of
+        // `QueryIter::none_remaining` never creates any references to the `<Q::Fetch as Fetch<'w>>::Item`.
+        unsafe {
+            self.iter_unchecked_manual(world, last_change_tick, change_tick)
+                .none_remaining()
+        }
+    }
+
     pub fn validate_world_and_update_archetypes(&mut self, world: &World) {
         if world.id() != self.world_id {
             panic!("Attempted to use {} with a mismatched World. QueryStates can only be used with the World they were created from.",

crates/bevy_ecs/src/system/mod.rs

@@ -438,6 +438,30 @@ mod tests {
         assert_eq!(conflicts, vec![b_id, d_id]);
     }
 
+    #[test]
+    fn query_is_empty() {
+        fn without_filter(not_empty: Query<&A>, empty: Query<&B>) {
+            assert!(!not_empty.is_empty());
+            assert!(empty.is_empty());
+        }
+
+        fn with_filter(not_empty: Query<&A, With<C>>, empty: Query<&A, With<D>>) {
+            assert!(!not_empty.is_empty());
+            assert!(empty.is_empty());
+        }
+
+        let mut world = World::default();
+        world.spawn().insert(A).insert(C);
+
+        let mut without_filter = without_filter.system();
+        without_filter.initialize(&mut world);
+        without_filter.run((), &mut world);
+
+        let mut with_filter = with_filter.system();
+        with_filter.initialize(&mut world);
+        with_filter.run((), &mut world);
+    }
+
     #[test]
     #[allow(clippy::too_many_arguments)]
     fn can_have_16_parameters() {

crates/bevy_ecs/src/system/query.rs

@@ -543,6 +543,15 @@ where
             >())),
         }
     }
+
+    /// Returns true if this query contains no elements.
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        // TODO: This code can be replaced with `self.iter().next().is_none()` if/when
+        // we sort out how to convert "write" queries to "read" queries.
+        self.state
+            .is_empty(self.world, self.last_change_tick, self.change_tick)
+    }
 }
 
 /// An error that occurs when retrieving a specific [`Entity`]'s component from a [`Query`]