Optional .system(), part 3 (#2422)

# Objective

- Continue work of #2398 and #2403.
- Make `.system()` syntax optional when using `.config()` API.

## Solution

- Introduce new prelude trait, `ConfigurableSystem`, that shorthands `my_system.system().config(...)` as `my_system.config(...)`.
- Expand `configure_system_local` test to also cover the new syntax.

crates/bevy_ecs/src/lib.rs

@@ -29,8 +29,8 @@ pub mod prelude {
             Schedule, Stage, StageLabel, State, SystemLabel, SystemSet, SystemStage,
         },
         system::{
-            Commands, In, IntoChainSystem, IntoExclusiveSystem, IntoSystem, Local, NonSend,
-            NonSendMut, Query, QuerySet, RemovedComponents, Res, ResMut, System,
+            Commands, ConfigurableSystem, In, IntoChainSystem, IntoExclusiveSystem, IntoSystem,
+            Local, NonSend, NonSendMut, Query, QuerySet, RemovedComponents, Res, ResMut, System,
         },
         world::{FromWorld, Mut, World},
     };

crates/bevy_ecs/src/system/function_system.rs

@@ -263,6 +263,42 @@ impl<In, Out, Param: SystemParam, Marker, F> FunctionSystem<In, Out, Param, Mark
         self
     }
 }
+
+/// Provides `my_system.config(...)` API.
+pub trait ConfigurableSystem<In, Out, Param: SystemParam, Marker>:
+    IntoSystem<In, Out, (IsFunctionSystem, Param, Marker)>
+{
+    /// See [`FunctionSystem::config()`](crate::system::FunctionSystem::config).
+    fn config(
+        self,
+        f: impl FnOnce(&mut <Param::Fetch as SystemParamState>::Config),
+    ) -> Self::System;
+}
+
+impl<In, Out, Param: SystemParam, Marker, F> ConfigurableSystem<In, Out, Param, Marker> for F
+where
+    In: 'static,
+    Out: 'static,
+    Param: SystemParam + 'static,
+    Marker: 'static,
+    F: SystemParamFunction<In, Out, Param, Marker>
+        + IntoSystem<
+            In,
+            Out,
+            (IsFunctionSystem, Param, Marker),
+            System = FunctionSystem<In, Out, Param, Marker, F>,
+        > + Send
+        + Sync
+        + 'static,
+{
+    fn config(
+        self,
+        f: impl FnOnce(&mut <<Param as SystemParam>::Fetch as SystemParamState>::Config),
+    ) -> Self::System {
+        self.system().config(f)
+    }
+}
+
 pub struct IsFunctionSystem;
 
 impl<In, Out, Param, Marker, F> IntoSystem<In, Out, (IsFunctionSystem, Param, Marker)> for F

crates/bevy_ecs/src/system/mod.rs

@@ -27,8 +27,8 @@ mod tests {
         query::{Added, Changed, Or, With, Without},
         schedule::{Schedule, Stage, SystemStage},
         system::{
-            IntoExclusiveSystem, IntoSystem, Local, Query, QuerySet, RemovedComponents, Res,
-            ResMut, System, SystemState,
+            ConfigurableSystem, IntoExclusiveSystem, IntoSystem, Local, Query, QuerySet,
+            RemovedComponents, Res, ResMut, System, SystemState,
         },
         world::{FromWorld, World},
     };
@@ -372,7 +372,13 @@ mod tests {
 
         // ensure the system actually ran
         assert!(*world.get_resource::<bool>().unwrap());
+
+        // Now do the same with omitted `.system()`.
+        world.insert_resource(false);
+        run_system(&mut world, sys.config(|config| config.0 = Some(42)));
+        assert!(*world.get_resource::<bool>().unwrap());
     }
+
     #[test]
     fn world_collections_system() {
         let mut world = World::default();