refactor: Simplify lifetimes for Commands and related types (#11445)

# Objective

It would be convenient to be able to call functions with `Commands` as a
parameter without having to move your own instance of `Commands`. Since
this struct is composed entirely of references, we can easily get an
owned instance of `Commands` by shortening the lifetime.

## Solution

Add `Commands::reborrow`, `EntiyCommands::reborrow`, and
`Deferred::reborrow`, which returns an owned version of themselves with
a shorter lifetime.

Remove unnecessary lifetimes from `EntityCommands`. The `'w` and `'s`
lifetimes only have to be separate for `Commands` because it's used as a
`SystemParam` -- this is not the case for `EntityCommands`.

---

## Changelog

Added `Commands::reborrow`. This is useful if you have `&mut Commands`
but need `Commands`. Also added `EntityCommands::reborrow` and
`Deferred:reborrow` which serve the same purpose.

## Migration Guide

The lifetimes for `EntityCommands` have been simplified.

```rust
// Before (Bevy 0.12)
struct MyStruct<'w, 's, 'a> {
     commands: EntityCommands<'w, 's, 'a>,
}

// After (Bevy 0.13)
struct MyStruct<'a> {
    commands: EntityCommands<'a>,
}
```

The method `EntityCommands::commands` now returns `Commands` rather than
`&mut Commands`.

```rust
// Before (Bevy 0.12)
let commands = entity_commands.commands();
commands.spawn(...);

// After (Bevy 0.13)
let mut commands = entity_commands.commands();
commands.spawn(...);
```

crates/bevy_ecs/src/reflect/entity_commands.rs

@@ -140,7 +140,7 @@ pub trait ReflectCommandExt {
     ) -> &mut Self;
 }
 
-impl<'w, 's, 'a> ReflectCommandExt for EntityCommands<'w, 's, 'a> {
+impl ReflectCommandExt for EntityCommands<'_> {
     fn insert_reflect(&mut self, component: Box<dyn Reflect>) -> &mut Self {
         self.commands.add(InsertReflect {
             entity: self.entity,

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

@@ -146,6 +146,31 @@ impl<'w, 's> Commands<'w, 's> {
         }
     }
 
+    /// Returns a [`Commands`] with a smaller lifetime.
+    /// This is useful if you have `&mut Commands` but need `Commands`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use bevy_ecs::prelude::*;
+    /// fn my_system(mut commands: Commands) {
+    ///     // We do our initialization in a separate function,
+    ///     // which expects an owned `Commands`.
+    ///     do_initialization(commands.reborrow());
+    ///
+    ///     // Since we only reborrowed the commands instead of moving them, we can still use them.
+    ///     commands.spawn_empty();
+    /// }
+    /// #
+    /// # fn do_initialization(_: Commands) {}
+    /// ```
+    pub fn reborrow(&mut self) -> Commands<'w, '_> {
+        Commands {
+            queue: self.queue.reborrow(),
+            entities: self.entities,
+        }
+    }
+
     /// Take all commands from `other` and append them to `self`, leaving `other` empty
     pub fn append(&mut self, other: &mut CommandQueue) {
         self.queue.append(other);
@@ -186,11 +211,11 @@ impl<'w, 's> Commands<'w, 's> {
     ///
     /// - [`spawn`](Self::spawn) to spawn an entity with a bundle.
     /// - [`spawn_batch`](Self::spawn_batch) to spawn entities with a bundle each.
-    pub fn spawn_empty<'a>(&'a mut self) -> EntityCommands<'w, 's, 'a> {
+    pub fn spawn_empty(&mut self) -> EntityCommands {
         let entity = self.entities.reserve_entity();
         EntityCommands {
             entity,
-            commands: self,
+            commands: self.reborrow(),
         }
     }
 
@@ -208,13 +233,13 @@ impl<'w, 's> Commands<'w, 's> {
     /// [`Commands::spawn`]. This method should generally only be used for sharing entities across
     /// apps, and only when they have a scheme worked out to share an ID space (which doesn't happen
     /// by default).
-    pub fn get_or_spawn<'a>(&'a mut self, entity: Entity) -> EntityCommands<'w, 's, 'a> {
+    pub fn get_or_spawn(&mut self, entity: Entity) -> EntityCommands {
         self.add(move |world: &mut World| {
             world.get_or_spawn(entity);
         });
         EntityCommands {
             entity,
-            commands: self,
+            commands: self.reborrow(),
         }
     }
 
@@ -268,7 +293,7 @@ impl<'w, 's> Commands<'w, 's> {
     ///
     /// - [`spawn_empty`](Self::spawn_empty) to spawn an entity without any components.
     /// - [`spawn_batch`](Self::spawn_batch) to spawn entities with a bundle each.
-    pub fn spawn<'a, T: Bundle>(&'a mut self, bundle: T) -> EntityCommands<'w, 's, 'a> {
+    pub fn spawn<T: Bundle>(&mut self, bundle: T) -> EntityCommands {
         let mut e = self.spawn_empty();
         e.insert(bundle);
         e
@@ -310,7 +335,7 @@ impl<'w, 's> Commands<'w, 's> {
     /// - [`get_entity`](Self::get_entity) for the fallible version.
     #[inline]
     #[track_caller]
-    pub fn entity<'a>(&'a mut self, entity: Entity) -> EntityCommands<'w, 's, 'a> {
+    pub fn entity(&mut self, entity: Entity) -> EntityCommands {
         #[inline(never)]
         #[cold]
         #[track_caller]
@@ -359,10 +384,10 @@ impl<'w, 's> Commands<'w, 's> {
     /// - [`entity`](Self::entity) for the panicking version.
     #[inline]
     #[track_caller]
-    pub fn get_entity<'a>(&'a mut self, entity: Entity) -> Option<EntityCommands<'w, 's, 'a>> {
+    pub fn get_entity(&mut self, entity: Entity) -> Option<EntityCommands> {
         self.entities.contains(entity).then_some(EntityCommands {
             entity,
-            commands: self,
+            commands: self.reborrow(),
         })
     }
 
@@ -674,12 +699,12 @@ where
 }
 
 /// A list of commands that will be run to modify an [entity](crate::entity).
-pub struct EntityCommands<'w, 's, 'a> {
+pub struct EntityCommands<'a> {
     pub(crate) entity: Entity,
-    pub(crate) commands: &'a mut Commands<'w, 's>,
+    pub(crate) commands: Commands<'a, 'a>,
 }
 
-impl<'w, 's, 'a> EntityCommands<'w, 's, 'a> {
+impl EntityCommands<'_> {
     /// Returns the [`Entity`] id of the entity.
     ///
     /// # Example
@@ -698,6 +723,15 @@ impl<'w, 's, 'a> EntityCommands<'w, 's, 'a> {
         self.entity
     }
 
+    /// Returns an [`EntityCommands`] with a smaller lifetime.
+    /// This is useful if you have `&mut EntityCommands` but you need `EntityCommands`.
+    pub fn reborrow(&mut self) -> EntityCommands {
+        EntityCommands {
+            entity: self.entity,
+            commands: self.commands.reborrow(),
+        }
+    }
+
     /// Adds a [`Bundle`] of components to the entity.
     ///
     /// This will overwrite any previous value(s) of the same component type.
@@ -956,8 +990,8 @@ impl<'w, 's, 'a> EntityCommands<'w, 's, 'a> {
     }
 
     /// Returns the underlying [`Commands`].
-    pub fn commands(&mut self) -> &mut Commands<'w, 's> {
+    pub fn commands(&mut self) -> Commands {
-        self.commands
+        self.commands.reborrow()
     }
 }
 

crates/bevy_ecs/src/system/system_param.rs

@@ -919,6 +919,14 @@ impl<'a, T: SystemBuffer> DerefMut for Deferred<'a, T> {
     }
 }
 
+impl<T: SystemBuffer> Deferred<'_, T> {
+    /// Returns a [`Deferred<T>`] with a smaller lifetime.
+    /// This is useful if you have `&mut Deferred<T>` but need `Deferred<T>`.
+    pub fn reborrow(&mut self) -> Deferred<T> {
+        Deferred(self.0)
+    }
+}
+
 // SAFETY: Only local state is accessed.
 unsafe impl<T: SystemBuffer> ReadOnlySystemParam for Deferred<'_, T> {}
 

crates/bevy_hierarchy/src/child_builder.rs

@@ -274,15 +274,15 @@ impl Command for RemoveParent {
 /// });
 /// # }
 /// ```
-pub struct ChildBuilder<'w, 's, 'a> {
+pub struct ChildBuilder<'a> {
-    commands: &'a mut Commands<'w, 's>,
+    commands: Commands<'a, 'a>,
     push_children: PushChildren,
 }
 
-impl<'w, 's, 'a> ChildBuilder<'w, 's, 'a> {
+impl ChildBuilder<'_> {
     /// Spawns an entity with the given bundle and inserts it into the parent entity's [`Children`].
     /// Also adds [`Parent`] component to the created entity.
-    pub fn spawn(&mut self, bundle: impl Bundle) -> EntityCommands<'w, 's, '_> {
+    pub fn spawn(&mut self, bundle: impl Bundle) -> EntityCommands {
         let e = self.commands.spawn(bundle);
         self.push_children.children.push(e.id());
         e
@@ -290,7 +290,7 @@ impl<'w, 's, 'a> ChildBuilder<'w, 's, 'a> {
 
     /// Spawns an [`Entity`] with no components and inserts it into the parent entity's [`Children`].
     /// Also adds [`Parent`] component to the created entity.
-    pub fn spawn_empty(&mut self) -> EntityCommands<'w, 's, '_> {
+    pub fn spawn_empty(&mut self) -> EntityCommands {
         let e = self.commands.spawn_empty();
         self.push_children.children.push(e.id());
         e
@@ -302,7 +302,7 @@ impl<'w, 's, 'a> ChildBuilder<'w, 's, 'a> {
     }
 
     /// Adds a command to be executed, like [`Commands::add`].
-    pub fn add_command<C: Command + 'static>(&mut self, command: C) -> &mut Self {
+    pub fn add_command<C: Command>(&mut self, command: C) -> &mut Self {
         self.commands.add(command);
         self
     }
@@ -374,7 +374,7 @@ pub trait BuildChildren {
     fn remove_parent(&mut self) -> &mut Self;
 }
 
-impl<'w, 's, 'a> BuildChildren for EntityCommands<'w, 's, 'a> {
+impl BuildChildren for EntityCommands<'_> {
     fn with_children(&mut self, spawn_children: impl FnOnce(&mut ChildBuilder)) -> &mut Self {
         let parent = self.id();
         let mut builder = ChildBuilder {

crates/bevy_hierarchy/src/hierarchy.rs

@@ -89,7 +89,7 @@ pub trait DespawnRecursiveExt {
     fn despawn_descendants(&mut self) -> &mut Self;
 }
 
-impl<'w, 's, 'a> DespawnRecursiveExt for EntityCommands<'w, 's, 'a> {
+impl DespawnRecursiveExt for EntityCommands<'_> {
     /// Despawns the provided entity and its children.
     fn despawn_recursive(mut self) {
         let entity = self.id();

crates/bevy_transform/src/commands.rs

@@ -85,7 +85,7 @@ pub trait BuildChildrenTransformExt {
     /// (during [`apply_deferred`](bevy_ecs::schedule::apply_deferred)).
     fn remove_parent_in_place(&mut self) -> &mut Self;
 }
-impl<'w, 's, 'a> BuildChildrenTransformExt for EntityCommands<'w, 's, 'a> {
+impl BuildChildrenTransformExt for EntityCommands<'_> {
     fn set_parent_in_place(&mut self, parent: Entity) -> &mut Self {
         let child = self.id();
         self.commands().add(PushChildInPlace { child, parent });