Move commands module into bevy::ecs::world (#12234)

# Objective

Fixes https://github.com/bevyengine/bevy/issues/11628

## Migration Guide

`Command` and `CommandQueue` have migrated from `bevy_ecs::system` to
`bevy_ecs::world`, so `use bevy_ecs::world::{Command, CommandQueue};`
when necessary.

benches/benches/bevy_ecs/world/commands.rs

@@ -1,8 +1,8 @@
 use bevy_ecs::{
     component::Component,
     entity::Entity,
-    system::{Command, CommandQueue, Commands},
-    world::World,
+    system::Commands,
+    world::{Command, CommandQueue, World},
 };
 use criterion::{black_box, Criterion};
 

crates/bevy_ecs/src/reflect/entity_commands.rs

@@ -1,6 +1,7 @@
 use crate::prelude::Mut;
 use crate::reflect::AppTypeRegistry;
-use crate::system::{Command, EntityCommands, Resource};
+use crate::system::{EntityCommands, Resource};
+use crate::world::Command;
 use crate::{entity::Entity, reflect::ReflectComponent, world::World};
 use bevy_reflect::{Reflect, TypeRegistry};
 use std::borrow::Cow;

crates/bevy_ecs/src/system/commands/mod.rs

@@ -1,57 +1,18 @@
-mod command_queue;
 mod parallel_scope;
 
+use super::{Deferred, Resource};
 use crate::{
     self as bevy_ecs,
     bundle::Bundle,
     entity::{Entities, Entity},
     system::{RunSystemWithInput, SystemId},
-    world::{EntityWorldMut, FromWorld, World},
+    world::{Command, CommandQueue, EntityWorldMut, FromWorld, World},
 };
 use bevy_ecs_macros::SystemParam;
 use bevy_utils::tracing::{error, info};
-pub use command_queue::CommandQueue;
 pub use parallel_scope::*;
 use std::marker::PhantomData;
 
-use super::{Deferred, Resource, SystemBuffer, SystemMeta};
-
-/// A [`World`] mutation.
-///
-/// Should be used with [`Commands::add`].
-///
-/// # Usage
-///
-/// ```
-/// # use bevy_ecs::prelude::*;
-/// # use bevy_ecs::system::Command;
-/// // Our world resource
-/// #[derive(Resource, Default)]
-/// struct Counter(u64);
-///
-/// // Our custom command
-/// struct AddToCounter(u64);
-///
-/// impl Command for AddToCounter {
-///     fn apply(self, world: &mut World) {
-///         let mut counter = world.get_resource_or_insert_with(Counter::default);
-///         counter.0 += self.0;
-///     }
-/// }
-///
-/// fn some_system(mut commands: Commands) {
-///     commands.add(AddToCounter(42));
-/// }
-/// ```
-pub trait Command: Send + 'static {
-    /// Applies this command, causing it to mutate the provided `world`.
-    ///
-    /// This method is used to define what a command "does" when it is ultimately applied.
-    /// Because this method takes `self`, you can store data or settings on the type that implements this trait.
-    /// This data is set by the system or other source of the command, and then ultimately read in this method.
-    fn apply(self, world: &mut World);
-}
-
 /// A [`Command`] queue to perform structural changes to the [`World`].
 ///
 /// Since each command requires exclusive access to the `World`,
@@ -115,15 +76,6 @@ pub struct Commands<'w, 's> {
     entities: &'w Entities,
 }
 
-impl SystemBuffer for CommandQueue {
-    #[inline]
-    fn apply(&mut self, _system_meta: &SystemMeta, world: &mut World) {
-        #[cfg(feature = "trace")]
-        let _span_guard = _system_meta.commands_span.enter();
-        self.apply(world);
-    }
-}
-
 impl<'w, 's> Commands<'w, 's> {
     /// Returns a new `Commands` instance from a [`CommandQueue`] and a [`World`].
     ///
@@ -581,7 +533,7 @@ impl<'w, 's> Commands<'w, 's> {
     /// # Example
     ///
     /// ```
-    /// # use bevy_ecs::{system::Command, prelude::*};
+    /// # use bevy_ecs::{world::Command, prelude::*};
     /// #[derive(Resource, Default)]
     /// struct Counter(u64);
     ///
@@ -1138,8 +1090,8 @@ mod tests {
     use crate::{
         self as bevy_ecs,
         component::Component,
-        system::{CommandQueue, Commands, Resource},
-        world::World,
+        system::{Commands, Resource},
+        world::{CommandQueue, World},
     };
     use std::sync::{
         atomic::{AtomicUsize, Ordering},

crates/bevy_ecs/src/system/system_registry.rs

@@ -1,6 +1,6 @@
 use crate::entity::Entity;
-use crate::system::{BoxedSystem, Command, IntoSystem};
-use crate::world::World;
+use crate::system::{BoxedSystem, IntoSystem};
+use crate::world::{Command, World};
 use crate::{self as bevy_ecs};
 use bevy_ecs_macros::Component;
 use thiserror::Error;

crates/bevy_ecs/src/world/command_queue.rs

@@ -1,10 +1,11 @@
+use crate::system::{SystemBuffer, SystemMeta};
+
 use std::{fmt::Debug, mem::MaybeUninit};
 
 use bevy_ptr::{OwningPtr, Unaligned};
 use bevy_utils::tracing::warn;
 
-use super::Command;
-use crate::world::World;
+use crate::world::{Command, World};
 
 struct CommandMeta {
     /// SAFETY: The `value` must point to a value of type `T: Command`,
@@ -234,6 +235,15 @@ impl Drop for CommandQueue {
     }
 }
 
+impl SystemBuffer for CommandQueue {
+    #[inline]
+    fn apply(&mut self, _system_meta: &SystemMeta, world: &mut World) {
+        #[cfg(feature = "trace")]
+        let _span_guard = _system_meta.commands_span.enter();
+        self.apply(world);
+    }
+}
+
 #[cfg(test)]
 mod test {
     use super::*;

crates/bevy_ecs/src/world/mod.rs

@@ -1,5 +1,6 @@
 //! Defines the [`World`] and APIs for accessing it directly.
 
+mod command_queue;
 mod deferred_world;
 mod entity_ref;
 pub mod error;
@@ -8,6 +9,7 @@ pub mod unsafe_world_cell;
 mod world_cell;
 
 pub use crate::change_detection::{Mut, Ref, CHECK_TICK_THRESHOLD};
+pub use crate::world::command_queue::CommandQueue;
 pub use deferred_world::DeferredWorld;
 pub use entity_ref::{
     EntityMut, EntityRef, EntityWorldMut, Entry, FilteredEntityMut, FilteredEntityRef,
@@ -30,7 +32,7 @@ use crate::{
     removal_detection::RemovedComponentEvents,
     schedule::{Schedule, ScheduleLabel, Schedules},
     storage::{ResourceData, Storages},
-    system::{CommandQueue, Commands, Res, Resource},
+    system::{Commands, Res, Resource},
     world::error::TryRunScheduleError,
 };
 use bevy_ptr::{OwningPtr, Ptr};
@@ -43,9 +45,44 @@ use std::{
 };
 mod identifier;
 
+use self::unsafe_world_cell::{UnsafeEntityCell, UnsafeWorldCell};
 pub use identifier::WorldId;
 
-use self::unsafe_world_cell::{UnsafeEntityCell, UnsafeWorldCell};
+/// A [`World`] mutation.
+///
+/// Should be used with [`Commands::add`].
+///
+/// # Usage
+///
+/// ```
+/// # use bevy_ecs::prelude::*;
+/// # use bevy_ecs::world::Command;
+/// // Our world resource
+/// #[derive(Resource, Default)]
+/// struct Counter(u64);
+///
+/// // Our custom command
+/// struct AddToCounter(u64);
+///
+/// impl Command for AddToCounter {
+///     fn apply(self, world: &mut World) {
+///         let mut counter = world.get_resource_or_insert_with(Counter::default);
+///         counter.0 += self.0;
+///     }
+/// }
+///
+/// fn some_system(mut commands: Commands) {
+///     commands.add(AddToCounter(42));
+/// }
+/// ```
+pub trait Command: Send + 'static {
+    /// Applies this command, causing it to mutate the provided `world`.
+    ///
+    /// This method is used to define what a command "does" when it is ultimately applied.
+    /// Because this method takes `self`, you can store data or settings on the type that implements this trait.
+    /// This data is set by the system or other source of the command, and then ultimately read in this method.
+    fn apply(self, world: &mut World);
+}
 
 /// Stores and exposes operations on [entities](Entity), [components](Component), resources,
 /// and their associated metadata.

crates/bevy_ecs/src/world/unsafe_world_cell.rs

@@ -2,7 +2,7 @@
 
 #![warn(unsafe_op_in_unsafe_fn)]
 
-use super::{Mut, Ref, World, WorldId};
+use super::{command_queue::CommandQueue, Mut, Ref, World, WorldId};
 use crate::{
     archetype::{Archetype, ArchetypeComponentId, Archetypes},
     bundle::Bundles,
@@ -14,7 +14,7 @@ use crate::{
     prelude::Component,
     removal_detection::RemovedComponentEvents,
     storage::{Column, ComponentSparseSet, Storages},
-    system::{CommandQueue, Res, Resource},
+    system::{Res, Resource},
 };
 use bevy_ptr::Ptr;
 use std::{any::TypeId, cell::UnsafeCell, fmt::Debug, marker::PhantomData, ptr, ptr::addr_of_mut};

crates/bevy_hierarchy/src/child_builder.rs

@@ -3,8 +3,8 @@ use bevy_ecs::{
     bundle::Bundle,
     entity::Entity,
     prelude::Events,
-    system::{Command, Commands, EntityCommands},
-    world::{EntityWorldMut, World},
+    system::{Commands, EntityCommands},
+    world::{Command, EntityWorldMut, World},
 };
 use bevy_utils::smallvec::{smallvec, SmallVec};
 
@@ -702,8 +702,8 @@ mod tests {
         component::Component,
         entity::Entity,
         event::Events,
-        system::{CommandQueue, Commands},
-        world::World,
+        system::Commands,
+        world::{CommandQueue, World},
     };
 
     /// Assert the (non)existence and state of the child's [`Parent`] component.

crates/bevy_hierarchy/src/hierarchy.rs

@@ -1,8 +1,8 @@
 use crate::components::{Children, Parent};
 use bevy_ecs::{
     entity::Entity,
-    system::{Command, EntityCommands},
-    world::{EntityWorldMut, World},
+    system::EntityCommands,
+    world::{Command, EntityWorldMut, World},
 };
 use bevy_utils::tracing::debug;
 
@@ -139,8 +139,8 @@ impl<'w> DespawnRecursiveExt for EntityWorldMut<'w> {
 mod tests {
     use bevy_ecs::{
         component::Component,
-        system::{CommandQueue, Commands},
-        world::World,
+        system::Commands,
+        world::{CommandQueue, World},
     };
 
     use super::DespawnRecursiveExt;

crates/bevy_scene/src/dynamic_scene.rs

@@ -193,7 +193,7 @@ where
 #[cfg(test)]
 mod tests {
     use bevy_ecs::entity::EntityHashMap;
-    use bevy_ecs::{reflect::AppTypeRegistry, system::Command, world::World};
+    use bevy_ecs::{reflect::AppTypeRegistry, world::Command, world::World};
     use bevy_hierarchy::{Parent, PushChild};
 
     use crate::dynamic_scene_builder::DynamicSceneBuilder;

crates/bevy_scene/src/scene_spawner.rs

@@ -5,8 +5,8 @@ use bevy_ecs::{
     entity::Entity,
     event::{Event, Events, ManualEventReader},
     reflect::AppTypeRegistry,
-    system::{Command, Resource},
-    world::{Mut, World},
+    system::Resource,
+    world::{Command, Mut, World},
 };
 use bevy_hierarchy::{Parent, PushChild};
 use bevy_utils::{tracing::error, HashMap, HashSet};

crates/bevy_transform/src/commands.rs

@@ -1,7 +1,7 @@
 //! Extension to [`EntityCommands`] to modify `bevy_hierarchy` hierarchies
 //! while preserving [`GlobalTransform`].
 
-use bevy_ecs::{prelude::Entity, system::Command, system::EntityCommands, world::World};
+use bevy_ecs::{prelude::Entity, system::EntityCommands, world::Command, world::World};
 use bevy_hierarchy::{PushChild, RemoveParent};
 
 use crate::{GlobalTransform, Transform};

crates/bevy_transform/src/systems.rs

@@ -182,7 +182,7 @@ unsafe fn propagate_recursive(
 mod test {
     use bevy_app::prelude::*;
     use bevy_ecs::prelude::*;
-    use bevy_ecs::system::CommandQueue;
+    use bevy_ecs::world::CommandQueue;
     use bevy_math::{vec3, Vec3};
     use bevy_tasks::{ComputeTaskPool, TaskPool};
 

crates/bevy_ui/src/stack.rs

@@ -126,8 +126,8 @@ mod tests {
     use bevy_ecs::{
         component::Component,
         schedule::Schedule,
-        system::{CommandQueue, Commands},
-        world::World,
+        system::Commands,
+        world::{CommandQueue, World},
     };
     use bevy_hierarchy::BuildChildren;
 

examples/async_tasks/async_compute.rs

@@ -2,7 +2,8 @@
 //! to spawn, poll, and complete tasks across systems and system ticks.
 
 use bevy::{
-    ecs::system::{CommandQueue, SystemState},
+    ecs::system::SystemState,
+    ecs::world::CommandQueue,
     prelude::*,
     tasks::{block_on, futures_lite::future, AsyncComputeTaskPool, Task},
 };