Deprecate .system (#3302)

# Objective

- Using `.system()` is no longer idiomatic.

## Solution

- Give a warning when using it

crates/bevy_app/src/app.rs

@@ -430,7 +430,7 @@ impl App {
     /// }
     ///
     /// App::new()
-    ///     .add_startup_system(my_startup_system.system());
+    ///     .add_startup_system(my_startup_system);
     /// ```
     pub fn add_startup_system<Params>(
         &mut self,

crates/bevy_asset/src/asset_server.rs

@@ -558,6 +558,7 @@ pub fn free_unused_assets_system(asset_server: Res<AssetServer>) {
 mod test {
     use super::*;
     use crate::{loader::LoadedAsset, update_asset_storage_system};
+    use bevy_app::App;
     use bevy_ecs::prelude::*;
     use bevy_reflect::TypeUuid;
     use bevy_utils::BoxedFuture;
@@ -771,21 +772,13 @@ mod test {
         asset_server.add_loader(FakePngLoader);
         let assets = asset_server.register_asset_type::<PngAsset>();
 
-        let mut world = World::new();
-        world.insert_resource(assets);
-        world.insert_resource(asset_server);
-
-        let mut tick = {
-            let mut free_unused_assets_system = free_unused_assets_system.system();
-            free_unused_assets_system.initialize(&mut world);
-            let mut update_asset_storage_system = update_asset_storage_system::<PngAsset>.system();
-            update_asset_storage_system.initialize(&mut world);
-
-            move |world: &mut World| {
-                free_unused_assets_system.run((), world);
-                update_asset_storage_system.run((), world);
-            }
-        };
+        #[derive(SystemLabel, Clone, Hash, Debug, PartialEq, Eq)]
+        struct FreeUnusedAssets;
+        let mut app = App::new();
+        app.insert_resource(assets);
+        app.insert_resource(asset_server);
+        app.add_system(free_unused_assets_system.label(FreeUnusedAssets));
+        app.add_system(update_asset_storage_system::<PngAsset>.after(FreeUnusedAssets));
 
         fn load_asset(path: AssetPath, world: &World) -> HandleUntyped {
             let asset_server = world.get_resource::<AssetServer>().unwrap();
@@ -813,37 +806,43 @@ mod test {
         // ---
 
         let path: AssetPath = "fake.png".into();
-        assert_eq!(LoadState::NotLoaded, get_load_state(path.get_id(), &world));
+        assert_eq!(
+            LoadState::NotLoaded,
+            get_load_state(path.get_id(), &app.world)
+        );
 
         // load the asset
-        let handle = load_asset(path.clone(), &world);
+        let handle = load_asset(path.clone(), &app.world);
         let weak_handle = handle.clone_weak();
 
         // asset is loading
-        assert_eq!(LoadState::Loading, get_load_state(&handle, &world));
+        assert_eq!(LoadState::Loading, get_load_state(&handle, &app.world));
 
-        tick(&mut world);
+        app.update();
         // asset should exist and be loaded at this point
-        assert_eq!(LoadState::Loaded, get_load_state(&handle, &world));
-        assert!(get_asset(&handle, &world).is_some());
+        assert_eq!(LoadState::Loaded, get_load_state(&handle, &app.world));
+        assert!(get_asset(&handle, &app.world).is_some());
 
         // after dropping the handle, next call to `tick` will prepare the assets for removal.
         drop(handle);
-        tick(&mut world);
-        assert_eq!(LoadState::Loaded, get_load_state(&weak_handle, &world));
-        assert!(get_asset(&weak_handle, &world).is_some());
+        app.update();
+        assert_eq!(LoadState::Loaded, get_load_state(&weak_handle, &app.world));
+        assert!(get_asset(&weak_handle, &app.world).is_some());
 
         // second call to tick will actually remove the asset.
-        tick(&mut world);
-        assert_eq!(LoadState::Unloaded, get_load_state(&weak_handle, &world));
-        assert!(get_asset(&weak_handle, &world).is_none());
+        app.update();
+        assert_eq!(
+            LoadState::Unloaded,
+            get_load_state(&weak_handle, &app.world)
+        );
+        assert!(get_asset(&weak_handle, &app.world).is_none());
 
         // finally, reload the asset
-        let handle = load_asset(path.clone(), &world);
-        assert_eq!(LoadState::Loading, get_load_state(&handle, &world));
-        tick(&mut world);
-        assert_eq!(LoadState::Loaded, get_load_state(&handle, &world));
-        assert!(get_asset(&handle, &world).is_some());
+        let handle = load_asset(path.clone(), &app.world);
+        assert_eq!(LoadState::Loading, get_load_state(&handle, &app.world));
+        app.update();
+        assert_eq!(LoadState::Loaded, get_load_state(&handle, &app.world));
+        assert!(get_asset(&handle, &app.world).is_some());
     }
 
     #[test]

crates/bevy_core/src/time/fixed_timestep.rs

@@ -79,7 +79,7 @@ impl Default for FixedTimestep {
     fn default() -> Self {
         Self {
             state: LocalFixedTimestepState::default(),
-            internal_system: Box::new(Self::prepare_system.system()),
+            internal_system: Box::new(IntoSystem::into_system(Self::prepare_system)),
         }
     }
 }

crates/bevy_ecs/src/query/fetch.rs

@@ -806,7 +806,7 @@ impl<'w, 's, T: Fetch<'w, 's>> Fetch<'w, 's> for OptionFetch<T> {
 ///         }
 ///     }
 /// }
-/// # print_moving_objects_system.system();
+/// # bevy_ecs::system::assert_is_system(print_moving_objects_system);
 /// ```
 #[derive(Clone)]
 pub struct ChangeTrackers<T: Component> {

crates/bevy_ecs/src/query/filter.rs

@@ -65,7 +65,7 @@ where
 ///         println!("{} is looking lovely today!", name.name);
 ///     }
 /// }
-/// # compliment_entity_system.system();
+/// # bevy_ecs::system::assert_is_system(compliment_entity_system);
 /// ```
 pub struct With<T>(PhantomData<T>);
 
@@ -188,7 +188,7 @@ unsafe impl<T> ReadOnlyFetch for WithFetch<T> {}
 ///         println!("{} has no permit!", name.name);
 ///     }
 /// }
-/// # no_permit_system.system();
+/// # bevy_ecs::system::assert_is_system(no_permit_system);
 /// ```
 pub struct Without<T>(PhantomData<T>);
 
@@ -317,7 +317,7 @@ unsafe impl<T> ReadOnlyFetch for WithoutFetch<T> {}
 ///         println!("Entity {:?} got a new style or color", entity);
 ///     }
 /// }
-/// # print_cool_entity_system.system();
+/// # bevy_ecs::system::assert_is_system(print_cool_entity_system);
 /// ```
 pub struct Or<T>(pub T);
 
@@ -619,7 +619,7 @@ impl_tick_filter!(
     ///     }
     /// }
     ///
-    /// # print_add_name_component.system();
+    /// # bevy_ecs::system::assert_is_system(print_add_name_component);
     /// ```
     Added,
     /// The [`FetchState`] of [`Added`].
@@ -662,7 +662,7 @@ impl_tick_filter!(
     ///     }
     /// }
     ///
-    /// # print_moving_objects_system.system();
+    /// # bevy_ecs::system::assert_is_system(print_moving_objects_system);
     /// ```
     Changed,
     /// The [`FetchState`] of [`Changed`].

crates/bevy_ecs/src/schedule/run_criteria.rs

@@ -235,7 +235,7 @@ where
 {
     fn into(self) -> RunCriteriaDescriptorOrLabel {
         RunCriteriaDescriptorOrLabel::Descriptor(new_run_criteria_descriptor(Box::new(
-            self.system(),
+            IntoSystem::into_system(self),
         )))
     }
 }
@@ -333,19 +333,20 @@ where
     S: IntoSystem<(), ShouldRun, Param>,
 {
     fn label(self, label: impl RunCriteriaLabel) -> RunCriteriaDescriptor {
-        new_run_criteria_descriptor(Box::new(self.system())).label(label)
+        new_run_criteria_descriptor(Box::new(IntoSystem::into_system(self))).label(label)
     }
 
     fn label_discard_if_duplicate(self, label: impl RunCriteriaLabel) -> RunCriteriaDescriptor {
-        new_run_criteria_descriptor(Box::new(self.system())).label_discard_if_duplicate(label)
+        new_run_criteria_descriptor(Box::new(IntoSystem::into_system(self)))
+            .label_discard_if_duplicate(label)
     }
 
     fn before(self, label: impl RunCriteriaLabel) -> RunCriteriaDescriptor {
-        new_run_criteria_descriptor(Box::new(self.system())).before(label)
+        new_run_criteria_descriptor(Box::new(IntoSystem::into_system(self))).before(label)
     }
 
     fn after(self, label: impl RunCriteriaLabel) -> RunCriteriaDescriptor {
-        new_run_criteria_descriptor(Box::new(self.system())).after(label)
+        new_run_criteria_descriptor(Box::new(IntoSystem::into_system(self))).after(label)
     }
 }
 
@@ -366,6 +367,8 @@ impl RunCriteria {
 
 pub trait RunCriteriaPiping {
     /// See [`RunCriteria::pipe()`].
+    // TODO: Support `IntoSystem` here instead, and stop using
+    // `IntoSystem::into_system` in the call sites
     fn pipe(self, system: impl System<In = ShouldRun, Out = ShouldRun>) -> RunCriteriaDescriptor;
 }
 

crates/bevy_ecs/src/schedule/stage.rs

@@ -312,15 +312,16 @@ impl SystemStage {
         mut self,
         system: S,
     ) -> Self {
-        self.set_run_criteria(system.system());
+        self.set_run_criteria(system);
         self
     }
 
     pub fn set_run_criteria<Param, S: IntoSystem<(), ShouldRun, Param>>(
         &mut self,
         system: S,
     ) -> &mut Self {
-        self.stage_run_criteria.set(Box::new(system.system()));
+        self.stage_run_criteria
+            .set(Box::new(IntoSystem::into_system(system)));
         self
     }
 
@@ -1513,17 +1514,15 @@ mod tests {
                     .after("0")
                     .with_run_criteria(every_other_time.label("every other time")),
             )
+            .with_system(make_parallel(2).label("2").after("1").with_run_criteria(
+                RunCriteria::pipe("every other time", IntoSystem::into_system(eot_piped)),
+            ))
             .with_system(
-                make_parallel(2)
-                    .label("2")
-                    .after("1")
-                    .with_run_criteria(RunCriteria::pipe("every other time", eot_piped.system())),
-            )
-            .with_system(
-                make_parallel(3)
-                    .label("3")
-                    .after("2")
-                    .with_run_criteria("every other time".pipe(eot_piped.system()).label("piped")),
+                make_parallel(3).label("3").after("2").with_run_criteria(
+                    "every other time"
+                        .pipe(IntoSystem::into_system(eot_piped))
+                        .label("piped"),
+                ),
             )
             .with_system(make_parallel(4).after("3").with_run_criteria("piped"));
         for _ in 0..4 {

crates/bevy_ecs/src/schedule/system_descriptor.rs

@@ -56,7 +56,7 @@ where
     S: IntoSystem<(), (), Params>,
 {
     fn into_descriptor(self) -> SystemDescriptor {
-        new_parallel_descriptor(Box::new(self.system())).into_descriptor()
+        new_parallel_descriptor(Box::new(IntoSystem::into_system(self))).into_descriptor()
     }
 }
 
@@ -174,23 +174,24 @@ where
         self,
         run_criteria: impl IntoRunCriteria<Marker>,
     ) -> ParallelSystemDescriptor {
-        new_parallel_descriptor(Box::new(self.system())).with_run_criteria(run_criteria)
+        new_parallel_descriptor(Box::new(IntoSystem::into_system(self)))
+            .with_run_criteria(run_criteria)
     }
 
     fn label(self, label: impl SystemLabel) -> ParallelSystemDescriptor {
-        new_parallel_descriptor(Box::new(self.system())).label(label)
+        new_parallel_descriptor(Box::new(IntoSystem::into_system(self))).label(label)
     }
 
     fn before(self, label: impl SystemLabel) -> ParallelSystemDescriptor {
-        new_parallel_descriptor(Box::new(self.system())).before(label)
+        new_parallel_descriptor(Box::new(IntoSystem::into_system(self))).before(label)
     }
 
     fn after(self, label: impl SystemLabel) -> ParallelSystemDescriptor {
-        new_parallel_descriptor(Box::new(self.system())).after(label)
+        new_parallel_descriptor(Box::new(IntoSystem::into_system(self))).after(label)
     }
 
     fn in_ambiguity_set(self, set: impl AmbiguitySetLabel) -> ParallelSystemDescriptor {
-        new_parallel_descriptor(Box::new(self.system())).in_ambiguity_set(set)
+        new_parallel_descriptor(Box::new(IntoSystem::into_system(self))).in_ambiguity_set(set)
     }
 }