Move some structs that impl Command to methods on World and EntityWorldMut (#16999)

## Objective

Commands were previously limited to structs that implemented `Command`.
Now there are blanket implementations for closures, which (in my
opinion) are generally preferable.

Internal commands within `commands/mod.rs` have been switched from
structs to closures, but there are a number of internal commands in
other areas of the engine that still use structs. I'd like to tidy these
up by moving their implementations to methods on
`World`/`EntityWorldMut` and changing `Commands` to use those methods
through closures.

This PR handles the following:
- `TriggerEvent` and `EmitDynamicTrigger` double as commands and helper
structs, and can just be moved to `World` methods.
- Four structs that enabled insertion/removal of components via
reflection. This functionality shouldn't be exclusive to commands, and
can be added to `EntityWorldMut`.
- Five structs that mostly just wrapped `World` methods, and can be
replaced with closures that do the same thing.

## Solution

- __Observer Triggers__ (`observer/trigger_event.rs` and
`observer/mod.rs`)
- Moved the internals of `TriggerEvent` to the `World` methods that used
it.
  - Replaced `EmitDynamicTrigger` with two `World` methods:
    - `trigger_targets_dynamic`
    - `trigger_targets_dynamic_ref`
- `TriggerTargets` was now the only thing in
`observer/trigger_event.rs`, so it's been moved to `observer/mod.rs` and
`trigger_event.rs` was deleted.
- __Reflection Insert/Remove__ (`reflect/entity_commands.rs`)
- Replaced the following `Command` impls with equivalent methods on
`EntityWorldMut`:
    - `InsertReflect` -> `insert_reflect`
    - `InsertReflectWithRegistry` -> `insert_reflect_with_registry`
    - `RemoveReflect` -> `remove_reflect`
    - `RemoveReflectWithRegistry` -> `remove_reflect_with_registry`
- __System Registration__ (`system/system_registry.rs`)
- The following `Command` impls just wrapped a `World` method and have
been replaced with closures:
    - `RunSystemWith`
    - `UnregisterSystem`
    - `RunSystemCachedWith`
    - `UnregisterSystemCached`
- `RegisterSystem` called a helper function that basically worked as a
constructor for `RegisteredSystem` and made sure it came with a marker
component. That helper function has been replaced with
`RegisteredSystem::new` and a `#[require]`.

## Possible Addition

The extension trait that adds the reflection commands,
`ReflectCommandExt`, isn't strictly necessary; we could just `impl
EntityCommands`. We could even move them to the same files as the main
impls and put it behind a `#[cfg]`.

The PR that added it [had a similar
conversation](#8895 (comment))
and decided to stick with the trait, but we could revisit it here if so
desired.

Author: JaySpruce

crates/bevy_ecs/src/observer/mod.rs

@@ -2,11 +2,9 @@
 
 mod entity_observer;
 mod runner;
-mod trigger_event;
 
 pub use entity_observer::CloneEntityWithObserversExt;
 pub use runner::*;
-pub use trigger_event::*;
 
 use crate::{
     archetype::ArchetypeFlags,
@@ -168,6 +166,98 @@ impl<'w, E, B: Bundle> DerefMut for Trigger<'w, E, B> {
     }
 }
 
+/// Represents a collection of targets for a specific [`Trigger`] of an [`Event`]. Targets can be of type [`Entity`] or [`ComponentId`].
+///
+/// When a trigger occurs for a given event and [`TriggerTargets`], any [`Observer`] that watches for that specific event-target combination
+/// will run.
+pub trait TriggerTargets {
+    /// The components the trigger should target.
+    fn components(&self) -> &[ComponentId];
+
+    /// The entities the trigger should target.
+    fn entities(&self) -> &[Entity];
+}
+
+impl TriggerTargets for () {
+    fn components(&self) -> &[ComponentId] {
+        &[]
+    }
+
+    fn entities(&self) -> &[Entity] {
+        &[]
+    }
+}
+
+impl TriggerTargets for Entity {
+    fn components(&self) -> &[ComponentId] {
+        &[]
+    }
+
+    fn entities(&self) -> &[Entity] {
+        core::slice::from_ref(self)
+    }
+}
+
+impl TriggerTargets for Vec<Entity> {
+    fn components(&self) -> &[ComponentId] {
+        &[]
+    }
+
+    fn entities(&self) -> &[Entity] {
+        self.as_slice()
+    }
+}
+
+impl<const N: usize> TriggerTargets for [Entity; N] {
+    fn components(&self) -> &[ComponentId] {
+        &[]
+    }
+
+    fn entities(&self) -> &[Entity] {
+        self.as_slice()
+    }
+}
+
+impl TriggerTargets for ComponentId {
+    fn components(&self) -> &[ComponentId] {
+        core::slice::from_ref(self)
+    }
+
+    fn entities(&self) -> &[Entity] {
+        &[]
+    }
+}
+
+impl TriggerTargets for Vec<ComponentId> {
+    fn components(&self) -> &[ComponentId] {
+        self.as_slice()
+    }
+
+    fn entities(&self) -> &[Entity] {
+        &[]
+    }
+}
+
+impl<const N: usize> TriggerTargets for [ComponentId; N] {
+    fn components(&self) -> &[ComponentId] {
+        self.as_slice()
+    }
+
+    fn entities(&self) -> &[Entity] {
+        &[]
+    }
+}
+
+impl TriggerTargets for &Vec<Entity> {
+    fn components(&self) -> &[ComponentId] {
+        &[]
+    }
+
+    fn entities(&self) -> &[Entity] {
+        self.as_slice()
+    }
+}
+
 /// A description of what an [`Observer`] observes.
 #[derive(Default, Clone)]
 pub struct ObserverDescriptor {
@@ -431,34 +521,106 @@ impl World {
     /// While event types commonly implement [`Copy`],
     /// those that don't will be consumed and will no longer be accessible.
     /// If you need to use the event after triggering it, use [`World::trigger_ref`] instead.
-    pub fn trigger(&mut self, event: impl Event) {
-        TriggerEvent { event, targets: () }.trigger(self);
+    pub fn trigger<E: Event>(&mut self, mut event: E) {
+        let event_id = self.register_component::<E>();
+        // SAFETY: We just registered `event_id` with the type of `event`
+        unsafe { self.trigger_targets_dynamic_ref(event_id, &mut event, ()) };
     }
 
     /// Triggers the given [`Event`] as a mutable reference, which will run any [`Observer`]s watching for it.
     ///
     /// Compared to [`World::trigger`], this method is most useful when it's necessary to check
     /// or use the event after it has been modified by observers.
-    pub fn trigger_ref(&mut self, event: &mut impl Event) {
-        TriggerEvent { event, targets: () }.trigger_ref(self);
+    pub fn trigger_ref<E: Event>(&mut self, event: &mut E) {
+        let event_id = self.register_component::<E>();
+        // SAFETY: We just registered `event_id` with the type of `event`
+        unsafe { self.trigger_targets_dynamic_ref(event_id, event, ()) };
     }
 
     /// Triggers the given [`Event`] for the given `targets`, which will run any [`Observer`]s watching for it.
     ///
     /// While event types commonly implement [`Copy`],
     /// those that don't will be consumed and will no longer be accessible.
     /// If you need to use the event after triggering it, use [`World::trigger_targets_ref`] instead.
-    pub fn trigger_targets(&mut self, event: impl Event, targets: impl TriggerTargets) {
-        TriggerEvent { event, targets }.trigger(self);
+    pub fn trigger_targets<E: Event>(&mut self, mut event: E, targets: impl TriggerTargets) {
+        let event_id = self.register_component::<E>();
+        // SAFETY: We just registered `event_id` with the type of `event`
+        unsafe { self.trigger_targets_dynamic_ref(event_id, &mut event, targets) };
     }
 
     /// Triggers the given [`Event`] as a mutable reference for the given `targets`,
     /// which will run any [`Observer`]s watching for it.
     ///
     /// Compared to [`World::trigger_targets`], this method is most useful when it's necessary to check
     /// or use the event after it has been modified by observers.
-    pub fn trigger_targets_ref(&mut self, event: &mut impl Event, targets: impl TriggerTargets) {
-        TriggerEvent { event, targets }.trigger_ref(self);
+    pub fn trigger_targets_ref<E: Event>(&mut self, event: &mut E, targets: impl TriggerTargets) {
+        let event_id = self.register_component::<E>();
+        // SAFETY: We just registered `event_id` with the type of `event`
+        unsafe { self.trigger_targets_dynamic_ref(event_id, event, targets) };
+    }
+
+    /// Triggers the given [`Event`] for the given `targets`, which will run any [`Observer`]s watching for it.
+    ///
+    /// While event types commonly implement [`Copy`],
+    /// those that don't will be consumed and will no longer be accessible.
+    /// If you need to use the event after triggering it, use [`World::trigger_targets_dynamic_ref`] instead.
+    ///
+    /// # Safety
+    ///
+    /// Caller must ensure that `event_data` is accessible as the type represented by `event_id`.
+    pub unsafe fn trigger_targets_dynamic<E: Event, Targets: TriggerTargets>(
+        &mut self,
+        event_id: ComponentId,
+        mut event_data: E,
+        targets: Targets,
+    ) {
+        // SAFETY: `event_data` is accessible as the type represented by `event_id`
+        unsafe {
+            self.trigger_targets_dynamic_ref(event_id, &mut event_data, targets);
+        };
+    }
+
+    /// Triggers the given [`Event`] as a mutable reference for the given `targets`,
+    /// which will run any [`Observer`]s watching for it.
+    ///
+    /// Compared to [`World::trigger_targets_dynamic`], this method is most useful when it's necessary to check
+    /// or use the event after it has been modified by observers.
+    ///
+    /// # Safety
+    ///
+    /// Caller must ensure that `event_data` is accessible as the type represented by `event_id`.
+    pub unsafe fn trigger_targets_dynamic_ref<E: Event, Targets: TriggerTargets>(
+        &mut self,
+        event_id: ComponentId,
+        event_data: &mut E,
+        targets: Targets,
+    ) {
+        let mut world = DeferredWorld::from(self);
+        if targets.entities().is_empty() {
+            // SAFETY: `event_data` is accessible as the type represented by `event_id`
+            unsafe {
+                world.trigger_observers_with_data::<_, E::Traversal>(
+                    event_id,
+                    Entity::PLACEHOLDER,
+                    targets.components(),
+                    event_data,
+                    false,
+                );
+            };
+        } else {
+            for target in targets.entities() {
+                // SAFETY: `event_data` is accessible as the type represented by `event_id`
+                unsafe {
+                    world.trigger_observers_with_data::<_, E::Traversal>(
+                        event_id,
+                        *target,
+                        targets.components(),
+                        event_data,
+                        E::AUTO_PROPAGATE,
+                    );
+                };
+            }
+        }
     }
 
     /// Register an observer to the cache, called when an observer is created
@@ -590,7 +752,7 @@ mod tests {
     use crate as bevy_ecs;
     use crate::component::ComponentId;
     use crate::{
-        observer::{EmitDynamicTrigger, Observer, ObserverDescriptor, ObserverState, OnReplace},
+        observer::{Observer, ObserverDescriptor, ObserverState, OnReplace},
         prelude::*,
         traversal::Traversal,
     };
@@ -996,18 +1158,18 @@ mod tests {
         let event_a = world.register_component::<EventA>();
 
         world.spawn(ObserverState {
-            // SAFETY: we registered `event_a` above and it matches the type of TriggerA
+            // SAFETY: we registered `event_a` above and it matches the type of EventA
             descriptor: unsafe { ObserverDescriptor::default().with_events(vec![event_a]) },
             runner: |mut world, _trigger, _ptr, _propagate| {
                 world.resource_mut::<Order>().observed("event_a");
             },
             ..Default::default()
         });
 
-        world.commands().queue(
-            // SAFETY: we registered `event_a` above and it matches the type of TriggerA
-            unsafe { EmitDynamicTrigger::new_with_id(event_a, EventA, ()) },
-        );
+        world.commands().queue(move |world: &mut World| {
+            // SAFETY: we registered `event_a` above and it matches the type of EventA
+            unsafe { world.trigger_targets_dynamic(event_a, EventA, ()) };
+        });
         world.flush();
         assert_eq!(vec!["event_a"], world.resource::<Order>().0);
     }