Exclusive Systems Now Implement System. Flexible Exclusive System Params (#6083)

# Objective

The [Stageless RFC](bevyengine/rfcs#45) involves allowing exclusive systems to be referenced and ordered relative to parallel systems. We've agreed that unifying systems under `System` is the right move.

This is an alternative to #4166 (see rationale in the comments I left there). Note that this builds on the learnings established there (and borrows some patterns).

## Solution

This unifies parallel and exclusive systems under the shared `System` trait, removing the old `ExclusiveSystem` trait / impls. This is accomplished by adding a new `ExclusiveFunctionSystem` impl similar to `FunctionSystem`. It is backed by `ExclusiveSystemParam`, which is similar to `SystemParam`. There is a new flattened out SystemContainer api (which cuts out a lot of trait and type complexity). 

This means you can remove all cases of `exclusive_system()`:

```rust
// before
commands.add_system(some_system.exclusive_system());
// after
commands.add_system(some_system);
```

I've also implemented `ExclusiveSystemParam` for `&mut QueryState` and `&mut SystemState`, which makes this possible in exclusive systems:

```rust
fn some_exclusive_system(
    world: &mut World,
    transforms: &mut QueryState<&Transform>,
    state: &mut SystemState<(Res<Time>, Query<&Player>)>,
) {
    for transform in transforms.iter(world) {
        println!("{transform:?}");
    }
    let (time, players) = state.get(world);
    for player in players.iter() {
        println!("{player:?}");
    }
}
```

Note that "exclusive function systems" assume `&mut World` is present (and the first param). I think this is a fair assumption, given that the presence of `&mut World` is what defines the need for an exclusive system.

I added some targeted SystemParam `static` constraints, which removed the need for this:
``` rust
fn some_exclusive_system(state: &mut SystemState<(Res<'static, Time>, Query<&'static Player>)>) {}
```

## Related

- #2923
- #3001
- #3946

## Changelog

- `ExclusiveSystem` trait (and implementations) has been removed in favor of sharing the `System` trait.
- `ExclusiveFunctionSystem` and `ExclusiveSystemParam` were added, enabling flexible exclusive function systems
- `&mut SystemState` and `&mut QueryState` now implement `ExclusiveSystemParam`
- Exclusive and parallel System configuration is now done via a unified `SystemDescriptor`, `IntoSystemDescriptor`, and `SystemContainer` api.

## Migration Guide

Calling `.exclusive_system()` is no longer required (or supported) for converting exclusive system functions to exclusive systems:

```rust
// Old (0.8)
app.add_system(some_exclusive_system.exclusive_system());
// New (0.9)
app.add_system(some_exclusive_system);
```

Converting "normal" parallel systems to exclusive systems is done by calling the exclusive ordering apis:

```rust
// Old (0.8)
app.add_system(some_system.exclusive_system().at_end());
// New (0.9)
app.add_system(some_system.at_end());
```

Query state in exclusive systems can now be cached via ExclusiveSystemParams, which should be preferred for clarity and performance reasons:
```rust
// Old (0.8)
fn some_system(world: &mut World) {
  let mut transforms = world.query::<&Transform>();
  for transform in transforms.iter(world) {
  }
}
// New (0.9)
fn some_system(world: &mut World, transforms: &mut QueryState<&Transform>) {
  for transform in transforms.iter(world) {
  }
}
```

Author: cart

benches/benches/bevy_ecs/scheduling/run_criteria.rs

@@ -1,10 +1,4 @@
-use bevy_ecs::{
-    component::Component,
-    prelude::{ParallelSystemDescriptorCoercion, Res, Resource, RunCriteriaDescriptorCoercion},
-    schedule::{RunCriteriaLabel, ShouldRun, Stage, SystemStage},
-    system::Query,
-    world::World,
-};
+use bevy_ecs::{prelude::*, schedule::ShouldRun};
 use criterion::Criterion;
 
 fn run_stage(stage: &mut SystemStage, world: &mut World) {

crates/bevy_animation/src/lib.rs

@@ -12,7 +12,7 @@ use bevy_ecs::{
     entity::Entity,
     prelude::Component,
     reflect::ReflectComponent,
-    schedule::ParallelSystemDescriptorCoercion,
+    schedule::IntoSystemDescriptor,
     system::{Query, Res},
 };
 use bevy_hierarchy::Children;

crates/bevy_app/src/app.rs

@@ -3,7 +3,7 @@ pub use bevy_derive::AppLabel;
 use bevy_derive::{Deref, DerefMut};
 use bevy_ecs::{
     event::{Event, Events},
-    prelude::{FromWorld, IntoExclusiveSystem},
+    prelude::FromWorld,
     schedule::{
         IntoSystemDescriptor, Schedule, ShouldRun, Stage, StageLabel, State, StateData, SystemSet,
         SystemStage,
@@ -84,7 +84,7 @@ impl Default for App {
 
         app.add_default_stages()
             .add_event::<AppExit>()
-            .add_system_to_stage(CoreStage::Last, World::clear_trackers.exclusive_system());
+            .add_system_to_stage(CoreStage::Last, World::clear_trackers);
 
         #[cfg(feature = "bevy_ci_testing")]
         {

crates/bevy_ecs/src/entity/mod.rs

@@ -79,7 +79,7 @@ type IdCursor = isize;
 /// }
 /// #
 /// # bevy_ecs::system::assert_is_system(setup);
-/// # bevy_ecs::system::IntoExclusiveSystem::exclusive_system(exclusive_system);
+/// # bevy_ecs::system::assert_is_system(exclusive_system);
 /// ```
 ///
 /// It can be used to refer to a specific entity to apply [`EntityCommands`], or to call [`Query::get`] (or similar methods) to access its components.

crates/bevy_ecs/src/lib.rs

@@ -34,14 +34,13 @@ pub mod prelude {
         event::{EventReader, EventWriter, Events},
         query::{Added, AnyOf, ChangeTrackers, Changed, Or, QueryState, With, Without},
         schedule::{
-            ExclusiveSystemDescriptorCoercion, ParallelSystemDescriptorCoercion, RunCriteria,
-            RunCriteriaDescriptorCoercion, RunCriteriaLabel, Schedule, Stage, StageLabel, State,
-            SystemLabel, SystemSet, SystemStage,
+            IntoSystemDescriptor, RunCriteria, RunCriteriaDescriptorCoercion, RunCriteriaLabel,
+            Schedule, Stage, StageLabel, State, SystemLabel, SystemSet, SystemStage,
         },
         system::{
-            adapter as system_adapter, Commands, In, IntoChainSystem, IntoExclusiveSystem,
-            IntoSystem, Local, NonSend, NonSendMut, ParallelCommands, ParamSet, Query,
-            RemovedComponents, Res, ResMut, Resource, System, SystemParamFunction,
+            adapter as system_adapter, Commands, In, IntoChainSystem, IntoSystem, Local, NonSend,
+            NonSendMut, ParallelCommands, ParamSet, Query, RemovedComponents, Res, ResMut,
+            Resource, System, SystemParamFunction,
         },
         world::{FromWorld, Mut, World},
     };

crates/bevy_ecs/src/schedule/ambiguity_detection.rs

@@ -45,7 +45,6 @@ impl SystemOrderAmbiguity {
         stage: &SystemStage,
         world: &World,
     ) -> Self {
-        use crate::schedule::graph_utils::GraphNode;
         use SystemStageSegment::*;
 
         // TODO: blocked on https://github.com/bevyengine/bevy/pull/4166
@@ -220,7 +219,7 @@ impl SystemStage {
 /// Returns vector containing all pairs of indices of systems with ambiguous execution order,
 /// along with specific components that have triggered the warning.
 /// Systems must be topologically sorted beforehand.
-fn find_ambiguities(systems: &[impl SystemContainer]) -> Vec<(usize, usize, Vec<ComponentId>)> {
+fn find_ambiguities(systems: &[SystemContainer]) -> Vec<(usize, usize, Vec<ComponentId>)> {
     let mut all_dependencies = Vec::<FixedBitSet>::with_capacity(systems.len());
     let mut all_dependants = Vec::<FixedBitSet>::with_capacity(systems.len());
     for (index, container) in systems.iter().enumerate() {
@@ -263,15 +262,17 @@ fn find_ambiguities(systems: &[impl SystemContainer]) -> Vec<(usize, usize, Vec<
         // .take(index_a)
         {
             if !processed.contains(index_b) {
-                let a_access = systems[index_a].component_access();
-                let b_access = systems[index_b].component_access();
-                if let (Some(a), Some(b)) = (a_access, b_access) {
-                    let conflicts = a.get_conflicts(b);
+                let system_a = &systems[index_a];
+                let system_b = &systems[index_b];
+                if system_a.is_exclusive() || system_b.is_exclusive() {
+                    ambiguities.push((index_a, index_b, Vec::new()));
+                } else {
+                    let a_access = systems[index_a].component_access();
+                    let b_access = systems[index_b].component_access();
+                    let conflicts = a_access.get_conflicts(b_access);
                     if !conflicts.is_empty() {
                         ambiguities.push((index_a, index_b, conflicts));
                     }
-                } else {
-                    ambiguities.push((index_a, index_b, Vec::new()));
                 }
             }
         }
@@ -467,12 +468,12 @@ mod tests {
         let mut test_stage = SystemStage::parallel();
         test_stage
             // All 3 of these conflict with each other
-            .add_system(write_world_system.exclusive_system())
-            .add_system(write_world_system.exclusive_system().at_end())
-            .add_system(res_system.exclusive_system())
+            .add_system(write_world_system)
+            .add_system(write_world_system.at_end())
+            .add_system(res_system.at_start())
             // These do not, as they're in different segments of the stage
-            .add_system(write_world_system.exclusive_system().at_start())
-            .add_system(write_world_system.exclusive_system().before_commands());
+            .add_system(write_world_system.at_start())
+            .add_system(write_world_system.before_commands());
 
         test_stage.run(&mut world);
 

crates/bevy_ecs/src/schedule/executor.rs

@@ -1,11 +1,11 @@
-use crate::{schedule::ParallelSystemContainer, world::World};
+use crate::{schedule::SystemContainer, world::World};
 use downcast_rs::{impl_downcast, Downcast};
 
 pub trait ParallelSystemExecutor: Downcast + Send + Sync {
     /// Called by `SystemStage` whenever `systems` have been changed.
-    fn rebuild_cached_data(&mut self, systems: &[ParallelSystemContainer]);
+    fn rebuild_cached_data(&mut self, systems: &[SystemContainer]);
 
-    fn run_systems(&mut self, systems: &mut [ParallelSystemContainer], world: &mut World);
+    fn run_systems(&mut self, systems: &mut [SystemContainer], world: &mut World);
 }
 
 impl_downcast!(ParallelSystemExecutor);
@@ -14,9 +14,9 @@ impl_downcast!(ParallelSystemExecutor);
 pub struct SingleThreadedExecutor;
 
 impl ParallelSystemExecutor for SingleThreadedExecutor {
-    fn rebuild_cached_data(&mut self, _: &[ParallelSystemContainer]) {}
+    fn rebuild_cached_data(&mut self, _: &[SystemContainer]) {}
 
-    fn run_systems(&mut self, systems: &mut [ParallelSystemContainer], world: &mut World) {
+    fn run_systems(&mut self, systems: &mut [SystemContainer], world: &mut World) {
         for system in systems {
             if system.should_run() {
                 #[cfg(feature = "trace")]

crates/bevy_ecs/src/schedule/executor_parallel.rs

@@ -1,7 +1,7 @@
 use crate::{
     archetype::ArchetypeComponentId,
     query::Access,
-    schedule::{ParallelSystemContainer, ParallelSystemExecutor},
+    schedule::{ParallelSystemExecutor, SystemContainer},
     world::World,
 };
 use async_channel::{Receiver, Sender};
@@ -77,7 +77,7 @@ impl Default for ParallelExecutor {
 }
 
 impl ParallelSystemExecutor for ParallelExecutor {
-    fn rebuild_cached_data(&mut self, systems: &[ParallelSystemContainer]) {
+    fn rebuild_cached_data(&mut self, systems: &[SystemContainer]) {
         self.system_metadata.clear();
         self.queued.grow(systems.len());
         self.running.grow(systems.len());
@@ -104,7 +104,7 @@ impl ParallelSystemExecutor for ParallelExecutor {
         }
     }
 
-    fn run_systems(&mut self, systems: &mut [ParallelSystemContainer], world: &mut World) {
+    fn run_systems(&mut self, systems: &mut [SystemContainer], world: &mut World) {
         #[cfg(test)]
         if self.events_sender.is_none() {
             let (sender, receiver) = async_channel::unbounded::<SchedulingEvent>();
@@ -167,7 +167,7 @@ impl ParallelExecutor {
     fn prepare_systems<'scope>(
         &mut self,
         scope: &mut Scope<'scope, ()>,
-        systems: &'scope mut [ParallelSystemContainer],
+        systems: &'scope mut [SystemContainer],
         world: &'scope World,
     ) {
         // These are used as a part of a unit test.