Implement ParJoin for MaybeJoin if the inner type is ParJoin

src/join/mod.rs

@@ -291,6 +291,15 @@ where
     }
 }
 
+// SAFETY: This is safe as long as `T` implements `ParJoin` safely.  `MaybeJoin` relies on `T as
+// Join` for all storage access and safely wraps the inner `Join` API, so it should also be able to
+// implement `ParJoin`.
+#[cfg(feature = "parallel")]
+unsafe impl<T> ParJoin for MaybeJoin<T>
+where
+    T: ParJoin,
+{}
+
 /// `JoinIter` is an `Iterator` over a group of `Storages`.
 #[must_use]
 pub struct JoinIter<J: Join> {

tests/tests.rs

@@ -397,6 +397,77 @@ fn par_join_two_components() {
     );
 }
 
+#[test]
+fn par_join_with_maybe() {
+    use std::sync::{
+        atomic::{AtomicBool, Ordering},
+        Mutex,
+    };
+    let mut world = create_world();
+    world
+        .create_entity()
+        .with(CompInt(1))
+        .with(CompBool(false))
+        .build();
+    world
+        .create_entity()
+        .with(CompInt(2))
+        .with(CompBool(true))
+        .build();
+    world.create_entity().with(CompInt(3)).build();
+    let first = AtomicBool::new(false);
+    let second = AtomicBool::new(false);
+    let third = AtomicBool::new(false);
+    let error = Mutex::new(None);
+    struct Iter<'a>(
+        &'a AtomicBool,
+        &'a AtomicBool,
+        &'a AtomicBool,
+        &'a Mutex<Option<(i8, Option<bool>)>>,
+    );
+    impl<'a, 'b> System<'a> for Iter<'b> {
+        type SystemData = (ReadStorage<'a, CompInt>, ReadStorage<'a, CompBool>);
+
+        fn run(&mut self, (int, boolean): Self::SystemData) {
+            use rayon::iter::ParallelIterator;
+            let Iter(first, second, third, error) = *self;
+            (&int, boolean.maybe()).par_join().for_each(|(int, boolean)| {
+                let boolean = boolean.map(|c| c.0);
+                if !first.load(Ordering::SeqCst) && int.0 == 1 && boolean == Some(false) {
+                    first.store(true, Ordering::SeqCst);
+                } else if !second.load(Ordering::SeqCst) && int.0 == 2 && boolean == Some(true) {
+                    second.store(true, Ordering::SeqCst);
+                } else if !third.load(Ordering::SeqCst) && int.0 == 3 && boolean == None {
+                    third.store(true, Ordering::SeqCst);
+                } else {
+                    *error.lock().unwrap() = Some((int.0, boolean));
+                }
+            });
+        }
+    }
+    let mut dispatcher = DispatcherBuilder::new()
+        .with(Iter(&first, &second, &third, &error), "iter", &[])
+        .build();
+    dispatcher.dispatch(&mut world);
+    assert_eq!(
+        *error.lock().unwrap(),
+        None,
+        "Entity shouldn't be in the join",
+    );
+    assert!(
+        first.load(Ordering::SeqCst),
+        "There should be entity with CompInt(1) and CompBool(false)"
+    );
+    assert!(
+        second.load(Ordering::SeqCst),
+        "There should be entity with CompInt(2) and CompBool(true)"
+    );
+    assert!(
+        third.load(Ordering::SeqCst),
+        "There should be entity with CompInt(3) and no CompBool"
+    );
+}
+
 #[test]
 fn par_join_many_entities_and_systems() {
     use rayon::iter::ParallelIterator;