Apply buffers in ParamSet (#4677)

# Objective

- Fix #4676

## Solution

- Fixes #4676
- I have no reason to think this isn't sound, but `ParamSet` is a bit spooky

Author: DJMcNab

crates/bevy_ecs/macros/src/lib.rs

@@ -250,6 +250,10 @@ pub fn impl_param_set(_input: TokenStream) -> TokenStream {
                         #param.new_archetype(archetype, system_meta);
                     )*
                 }
+
+                fn apply(&mut self, world: &mut World) {
+                    self.0.apply(world)
+                }
             }
 
 

crates/bevy_ecs/src/system/mod.rs

@@ -103,7 +103,7 @@ mod tests {
         query::{Added, Changed, Or, With, Without},
         schedule::{Schedule, Stage, SystemStage},
         system::{
-            IntoExclusiveSystem, IntoSystem, Local, NonSend, NonSendMut, ParamSet, Query,
+            Commands, IntoExclusiveSystem, IntoSystem, Local, NonSend, NonSendMut, ParamSet, Query,
             RemovedComponents, Res, ResMut, System, SystemState,
         },
         world::{FromWorld, World},
@@ -906,4 +906,23 @@ mod tests {
             expected_ids
         );
     }
+
+    #[test]
+    fn commands_param_set() {
+        // Regression test for #4676
+        let mut world = World::new();
+        let entity = world.spawn().id();
+
+        run_system(
+            &mut world,
+            move |mut commands_set: ParamSet<(Commands, Commands)>| {
+                commands_set.p0().entity(entity).insert(A);
+                commands_set.p1().entity(entity).insert(B);
+            },
+        );
+
+        let entity = world.entity(entity);
+        assert!(entity.contains::<A>());
+        assert!(entity.contains::<B>());
+    }
 }