Down with the system! (#2496)

# Objective

- Remove all the `.system()` possible.
- Check for remaining missing cases.

## Solution

- Remove all `.system()`, fix compile errors
- 32 calls to `.system()` remains, mostly internals, the few others should be removed after #2446

Author: mockersf

benches/benches/bevy_ecs/stages.rs

@@ -1,7 +1,7 @@
 use bevy::ecs::{
-    world::World,
     schedule::{Stage, SystemStage},
     system::{IntoSystem, Query},
+    world::World,
 };
 use criterion::{criterion_group, criterion_main, Criterion};
 
@@ -29,7 +29,7 @@ fn empty_systems(criterion: &mut Criterion) {
     for amount in 0..5 {
         let mut stage = SystemStage::parallel();
         for _ in 0..amount {
-            stage.add_system(empty.system());
+            stage.add_system(empty);
         }
         run_stage(&mut stage, &mut world);
         group.bench_function(&format!("{:03}_systems", amount), |bencher| {
@@ -42,11 +42,11 @@ fn empty_systems(criterion: &mut Criterion) {
         let mut stage = SystemStage::parallel();
         for _ in 0..amount {
             stage
-                .add_system(empty.system())
-                .add_system(empty.system())
-                .add_system(empty.system())
-                .add_system(empty.system())
-                .add_system(empty.system());
+                .add_system(empty)
+                .add_system(empty)
+                .add_system(empty)
+                .add_system(empty)
+                .add_system(empty);
         }
         run_stage(&mut stage, &mut world);
         group.bench_function(&format!("{:03}_systems", 5 * amount), |bencher| {
@@ -85,15 +85,9 @@ fn busy_systems(criterion: &mut Criterion) {
         world.spawn_batch((0..ENTITY_BUNCH).map(|_| (A(0.0), B(0.0), C(0.0), E(0.0))));
         for system_amount in 0..5 {
             let mut stage = SystemStage::parallel();
-            stage
-                .add_system(ab.system())
-                .add_system(cd.system())
-                .add_system(ce.system());
+            stage.add_system(ab).add_system(cd).add_system(ce);
             for _ in 0..system_amount {
-                stage
-                    .add_system(ab.system())
-                    .add_system(cd.system())
-                    .add_system(ce.system());
+                stage.add_system(ab).add_system(cd).add_system(ce);
             }
             run_stage(&mut stage, &mut world);
             group.bench_function(
@@ -142,15 +136,9 @@ fn contrived(criterion: &mut Criterion) {
         world.spawn_batch((0..ENTITY_BUNCH).map(|_| (C(0.0), D(0.0))));
         for system_amount in 0..5 {
             let mut stage = SystemStage::parallel();
-            stage
-                .add_system(s_0.system())
-                .add_system(s_1.system())
-                .add_system(s_2.system());
+            stage.add_system(s_0).add_system(s_1).add_system(s_2);
             for _ in 0..system_amount {
-                stage
-                    .add_system(s_0.system())
-                    .add_system(s_1.system())
-                    .add_system(s_2.system());
+                stage.add_system(s_0).add_system(s_1).add_system(s_2);
             }
             run_stage(&mut stage, &mut world);
             group.bench_function(

crates/bevy_app/src/app.rs

@@ -1,7 +1,7 @@
 use crate::{CoreStage, Events, Plugin, PluginGroup, PluginGroupBuilder, StartupStage};
 use bevy_ecs::{
     component::{Component, ComponentDescriptor},
-    prelude::{FromWorld, IntoExclusiveSystem, IntoSystem},
+    prelude::{FromWorld, IntoExclusiveSystem},
     schedule::{
         IntoSystemDescriptor, RunOnce, Schedule, Stage, StageLabel, State, SystemSet, SystemStage,
     },
@@ -28,7 +28,7 @@ use bevy_utils::tracing::info_span;
 ///
 /// fn main() {
 ///    App::new()
-///        .add_system(hello_world_system.system())
+///        .add_system(hello_world_system)
 ///        .run();
 /// }
 ///
@@ -180,7 +180,7 @@ impl App {
     /// Adds a system that runs every time `app.update()` is called by the runner
     ///
     /// Systems are the main building block in the Bevy ECS app model. You can define
-    /// normal rust functions, and call `.system()` to make them be Bevy systems.
+    /// normal rust functions, and use them as a Bevy system.
     ///
     /// System functions can have parameters, through which one can query and
     /// mutate Bevy ECS states.
@@ -201,7 +201,7 @@ impl App {
     /// }
     ///
     /// App::new()
-    ///     .add_system(my_system.system());
+    ///     .add_system(my_system);
     /// ```
     pub fn add_system<Params>(&mut self, system: impl IntoSystemDescriptor<Params>) -> &mut Self {
         self.add_system_to_stage(CoreStage::Update, system)
@@ -248,7 +248,7 @@ 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,
@@ -335,7 +335,7 @@ impl App {
         T: Component,
     {
         self.insert_resource(Events::<T>::default())
-            .add_system_to_stage(CoreStage::First, Events::<T>::update_system.system())
+            .add_system_to_stage(CoreStage::First, Events::<T>::update_system)
     }
 
     /// Inserts a resource to the current [App] and overwrites any resource previously added of the same type.

crates/bevy_app/src/ci_testing.rs

@@ -1,7 +1,6 @@
 use serde::Deserialize;
 
 use crate::{app::AppExit, App};
-use bevy_ecs::system::IntoSystem;
 
 /// Configuration for automated testing on CI
 #[derive(Deserialize)]
@@ -32,7 +31,7 @@ pub(crate) fn setup_app(app_builder: &mut App) -> &mut App {
     .expect("error deserializing CI testing configuration file");
     app_builder
         .insert_resource(config)
-        .add_system(ci_testing_exit_after.system());
+        .add_system(ci_testing_exit_after);
 
     app_builder
 }

crates/bevy_asset/src/assets.rs

@@ -3,10 +3,7 @@ use crate::{
     RefChange,
 };
 use bevy_app::{App, EventWriter, Events};
-use bevy_ecs::{
-    system::{IntoSystem, ResMut},
-    world::FromWorld,
-};
+use bevy_ecs::{system::ResMut, world::FromWorld};
 use bevy_utils::HashMap;
 use crossbeam_channel::Sender;
 use std::fmt::Debug;
@@ -217,14 +214,8 @@ impl AddAsset for App {
         };
 
         self.insert_resource(assets)
-            .add_system_to_stage(
-                AssetStage::AssetEvents,
-                Assets::<T>::asset_event_system.system(),
-            )
-            .add_system_to_stage(
-                AssetStage::LoadAssets,
-                update_asset_storage_system::<T>.system(),
-            )
+            .add_system_to_stage(AssetStage::AssetEvents, Assets::<T>::asset_event_system)
+            .add_system_to_stage(AssetStage::LoadAssets, update_asset_storage_system::<T>)
             .register_type::<Handle<T>>()
             .add_event::<AssetEvent<T>>()
     }

crates/bevy_asset/src/diagnostic/asset_count_diagnostics_plugin.rs

@@ -1,7 +1,7 @@
 use crate::{Asset, Assets};
 use bevy_app::prelude::*;
 use bevy_diagnostic::{Diagnostic, DiagnosticId, Diagnostics, MAX_DIAGNOSTIC_NAME_WIDTH};
-use bevy_ecs::system::{IntoSystem, Res, ResMut};
+use bevy_ecs::system::{Res, ResMut};
 
 /// Adds "asset count" diagnostic to an App
 pub struct AssetCountDiagnosticsPlugin<T: Asset> {
@@ -18,8 +18,8 @@ impl<T: Asset> Default for AssetCountDiagnosticsPlugin<T> {
 
 impl<T: Asset> Plugin for AssetCountDiagnosticsPlugin<T> {
     fn build(&self, app: &mut App) {
-        app.add_startup_system(Self::setup_system.system())
-            .add_system(Self::diagnostic_system.system());
+        app.add_startup_system(Self::setup_system)
+            .add_system(Self::diagnostic_system);
     }
 }
 

crates/bevy_asset/src/lib.rs

@@ -27,10 +27,7 @@ pub use loader::*;
 pub use path::*;
 
 use bevy_app::{prelude::Plugin, App};
-use bevy_ecs::{
-    schedule::{StageLabel, SystemStage},
-    system::IntoSystem,
-};
+use bevy_ecs::schedule::{StageLabel, SystemStage};
 use bevy_tasks::IoTaskPool;
 
 /// The names of asset stages in an App Schedule
@@ -106,16 +103,13 @@ impl Plugin for AssetPlugin {
         .register_type::<HandleId>()
         .add_system_to_stage(
             bevy_app::CoreStage::PreUpdate,
-            asset_server::free_unused_assets_system.system(),
+            asset_server::free_unused_assets_system,
         );
 
         #[cfg(all(
             feature = "filesystem_watcher",
             all(not(target_arch = "wasm32"), not(target_os = "android"))
         ))]
-        app.add_system_to_stage(
-            AssetStage::LoadAssets,
-            io::filesystem_watcher_system.system(),
-        );
+        app.add_system_to_stage(AssetStage::LoadAssets, io::filesystem_watcher_system);
     }
 }

crates/bevy_core/src/label.rs

@@ -123,7 +123,6 @@ pub(crate) fn entity_labels_system(
 mod tests {
     use bevy_ecs::{
         schedule::{Schedule, Stage, SystemStage},
-        system::IntoSystem,
         world::World,
     };
 
@@ -134,7 +133,7 @@ mod tests {
         world.insert_resource(EntityLabels::default());
         let mut schedule = Schedule::default();
         schedule.add_stage("test", SystemStage::single_threaded());
-        schedule.add_system_to_stage("test", entity_labels_system.system());
+        schedule.add_system_to_stage("test", entity_labels_system);
         (world, schedule)
     }
 

crates/bevy_core/src/lib.rs

@@ -21,7 +21,7 @@ use bevy_app::prelude::*;
 use bevy_ecs::{
     entity::Entity,
     schedule::{ExclusiveSystemDescriptorCoercion, SystemLabel},
-    system::{IntoExclusiveSystem, IntoSystem},
+    system::IntoExclusiveSystem,
 };
 use bevy_utils::HashSet;
 use std::ops::Range;
@@ -62,8 +62,8 @@ impl Plugin for CorePlugin {
                 CoreStage::First,
                 time_system.exclusive_system().label(CoreSystem::Time),
             )
-            .add_startup_system_to_stage(StartupStage::PostStartup, entity_labels_system.system())
-            .add_system_to_stage(CoreStage::PostUpdate, entity_labels_system.system());
+            .add_startup_system_to_stage(StartupStage::PostStartup, entity_labels_system)
+            .add_system_to_stage(CoreStage::PostUpdate, entity_labels_system);
 
         register_rust_types(app);
         register_math_types(app);

crates/bevy_core/src/time/fixed_timestep.rs

@@ -4,7 +4,7 @@ use bevy_ecs::{
     component::ComponentId,
     query::Access,
     schedule::ShouldRun,
-    system::{IntoSystem, Local, Res, ResMut, System, SystemId},
+    system::{ConfigurableSystem, IntoSystem, Local, Res, ResMut, System, SystemId},
     world::World,
 };
 use bevy_utils::HashMap;
@@ -179,11 +179,8 @@ impl System for FixedTimestep {
     }
 
     fn initialize(&mut self, world: &mut World) {
-        self.internal_system = Box::new(
-            Self::prepare_system
-                .system()
-                .config(|c| c.0 = Some(self.state.clone())),
-        );
+        self.internal_system =
+            Box::new(Self::prepare_system.config(|c| c.0 = Some(self.state.clone())));
         self.internal_system.initialize(world);
         if let Some(ref label) = self.state.label {
             let mut fixed_timesteps = world.get_resource_mut::<FixedTimesteps>().unwrap();

crates/bevy_diagnostic/src/entity_count_diagnostics_plugin.rs

@@ -1,6 +1,6 @@
 use bevy_app::{App, Plugin};
 use bevy_ecs::{
-    system::{IntoExclusiveSystem, IntoSystem, ResMut},
+    system::{IntoExclusiveSystem, ResMut},
     world::World,
 };
 
@@ -12,7 +12,7 @@ pub struct EntityCountDiagnosticsPlugin;
 
 impl Plugin for EntityCountDiagnosticsPlugin {
     fn build(&self, app: &mut App) {
-        app.add_startup_system(Self::setup_system.system())
+        app.add_startup_system(Self::setup_system)
             .add_system(Self::diagnostic_system.exclusive_system());
     }
 }

crates/bevy_diagnostic/src/frame_time_diagnostics_plugin.rs

@@ -1,7 +1,7 @@
 use crate::{Diagnostic, DiagnosticId, Diagnostics};
 use bevy_app::prelude::*;
 use bevy_core::Time;
-use bevy_ecs::system::{IntoSystem, Res, ResMut};
+use bevy_ecs::system::{Res, ResMut};
 
 /// Adds "frame time" diagnostic to an App, specifically "frame time", "fps" and "frame count"
 #[derive(Default)]
@@ -13,9 +13,9 @@ pub struct FrameTimeDiagnosticsState {
 
 impl Plugin for FrameTimeDiagnosticsPlugin {
     fn build(&self, app: &mut bevy_app::App) {
-        app.add_startup_system(Self::setup_system.system())
+        app.add_startup_system(Self::setup_system)
             .insert_resource(FrameTimeDiagnosticsState { frame_count: 0.0 })
-            .add_system(Self::diagnostic_system.system());
+            .add_system(Self::diagnostic_system);
     }
 }
 

crates/bevy_diagnostic/src/log_diagnostics_plugin.rs

@@ -1,7 +1,7 @@
 use super::{Diagnostic, DiagnosticId, Diagnostics};
 use bevy_app::prelude::*;
 use bevy_core::{Time, Timer};
-use bevy_ecs::system::{IntoSystem, Res, ResMut};
+use bevy_ecs::system::{Res, ResMut};
 use bevy_log::{debug, info};
 use bevy_utils::Duration;
 
@@ -36,12 +36,9 @@ impl Plugin for LogDiagnosticsPlugin {
         });
 
         if self.debug {
-            app.add_system_to_stage(
-                CoreStage::PostUpdate,
-                Self::log_diagnostics_debug_system.system(),
-            );
+            app.add_system_to_stage(CoreStage::PostUpdate, Self::log_diagnostics_debug_system);
         } else {
-            app.add_system_to_stage(CoreStage::PostUpdate, Self::log_diagnostics_system.system());
+            app.add_system_to_stage(CoreStage::PostUpdate, Self::log_diagnostics_system);
         }
     }
 }

crates/bevy_ecs/README.md

@@ -131,7 +131,7 @@ fn main() {
     // Add a Stage to our schedule. Each Stage in a schedule runs all of its systems
     // before moving on to the next Stage
     schedule.add_stage("update", SystemStage::parallel()
-        .with_system(movement.system())
+        .with_system(movement)
     );
 
     // Run the schedule once. If your app has a "loop", you would run this once per loop

crates/bevy_ecs/examples/change_detection.rs

@@ -24,15 +24,11 @@ fn main() {
 
     // Add systems to the Stage to execute our app logic
     // We can label our systems to force a specific run-order between some of them
-    update.add_system(spawn_entities.system().label(SimulationSystem::Spawn));
-    update.add_system(
-        print_counter_when_changed
-            .system()
-            .after(SimulationSystem::Spawn),
-    );
-    update.add_system(age_all_entities.system().label(SimulationSystem::Age));
-    update.add_system(remove_old_entities.system().after(SimulationSystem::Age));
-    update.add_system(print_changed_entities.system().after(SimulationSystem::Age));
+    update.add_system(spawn_entities.label(SimulationSystem::Spawn));
+    update.add_system(print_counter_when_changed.after(SimulationSystem::Spawn));
+    update.add_system(age_all_entities.label(SimulationSystem::Age));
+    update.add_system(remove_old_entities.after(SimulationSystem::Age));
+    update.add_system(print_changed_entities.after(SimulationSystem::Age));
     // Add the Stage with our systems to the Schedule
     schedule.add_stage("update", update);
 

crates/bevy_ecs/examples/component_storage.rs

@@ -22,7 +22,7 @@ fn main() {
     // Setup a schedule and stage to add a system querying for the just spawned entities
     let mut schedule = Schedule::default();
     let mut update = SystemStage::parallel();
-    update.add_system(query_entities.system());
+    update.add_system(query_entities);
     schedule.add_stage("update", update);
 
     schedule.run(&mut world);

crates/bevy_ecs/examples/events.rs

@@ -16,13 +16,13 @@ fn main() {
     // called "second". In "first" we update the events and in "second" we run our systems
     // sending and receiving events.
     let mut first = SystemStage::parallel();
-    first.add_system(Events::<MyEvent>::update_system.system());
+    first.add_system(Events::<MyEvent>::update_system);
     schedule.add_stage("first", first);
 
     // Add systems sending and receiving events to a "second" Stage
     let mut second = SystemStage::parallel();
-    second.add_system(sending_system.system().label(EventSystem::Sending));
-    second.add_system(receiving_system.system().after(EventSystem::Sending));
+    second.add_system(sending_system.label(EventSystem::Sending));
+    second.add_system(receiving_system.after(EventSystem::Sending));
 
     // Run the "second" Stage after the "first" Stage, so our Events always get updated before we use them
     schedule.add_stage_after("first", "second", second);