Split BufferedEvent from Event (#20101)

# Objective

> I think we should axe the shared `Event` trait entirely
It doesn't serve any functional purpose, and I don't think it's useful
pedagogically
@alice-i-cecile on discord

## Solution

- Remove `Event` as a supertrait of `BufferedEvent`
- Remove any `Event` derives that were made unnecessary
- Update release notes

---------

Co-authored-by: SpecificProtagonist <[email protected]>

Author: tim-blackbird

benches/benches/bevy_ecs/events/iter.rs

@@ -1,6 +1,6 @@
 use bevy_ecs::prelude::*;
 
-#[derive(Event, BufferedEvent)]
+#[derive(BufferedEvent)]
 struct BenchEvent<const SIZE: usize>([u8; SIZE]);
 
 pub struct Benchmark<const SIZE: usize>(Events<BenchEvent<SIZE>>);

benches/benches/bevy_ecs/events/write.rs

@@ -1,6 +1,6 @@
 use bevy_ecs::prelude::*;
 
-#[derive(Event, BufferedEvent)]
+#[derive(BufferedEvent)]
 struct BenchEvent<const SIZE: usize>([u8; SIZE]);
 
 impl<const SIZE: usize> Default for BenchEvent<SIZE> {

crates/bevy_a11y/src/lib.rs

@@ -26,10 +26,7 @@ use accesskit::Node;
 use bevy_app::Plugin;
 use bevy_derive::{Deref, DerefMut};
 use bevy_ecs::{
-    component::Component,
-    event::{BufferedEvent, Event},
-    resource::Resource,
-    schedule::SystemSet,
+    component::Component, event::BufferedEvent, resource::Resource, schedule::SystemSet,
 };
 
 #[cfg(feature = "bevy_reflect")]
@@ -45,7 +42,7 @@ use serde::{Deserialize, Serialize};
 use bevy_reflect::{ReflectDeserialize, ReflectSerialize};
 
 /// Wrapper struct for [`accesskit::ActionRequest`]. Required to allow it to be used as an `Event`.
-#[derive(Event, BufferedEvent, Deref, DerefMut)]
+#[derive(BufferedEvent, Deref, DerefMut)]
 #[cfg_attr(feature = "serialize", derive(Serialize, Deserialize))]
 pub struct ActionRequest(pub accesskit::ActionRequest);
 

crates/bevy_app/src/app.rs

@@ -355,7 +355,7 @@ impl App {
     /// # use bevy_app::prelude::*;
     /// # use bevy_ecs::prelude::*;
     /// #
-    /// # #[derive(Event, BufferedEvent)]
+    /// # #[derive(BufferedEvent)]
     /// # struct MyEvent;
     /// # let mut app = App::new();
     /// #
@@ -1417,7 +1417,7 @@ fn run_once(mut app: App) -> AppExit {
 /// This type is roughly meant to map to a standard definition of a process exit code (0 means success, not 0 means error). Due to portability concerns
 /// (see [`ExitCode`](https://doc.rust-lang.org/std/process/struct.ExitCode.html) and [`process::exit`](https://doc.rust-lang.org/std/process/fn.exit.html#))
 /// we only allow error codes between 1 and [255](u8::MAX).
-#[derive(Event, BufferedEvent, Debug, Clone, Default, PartialEq, Eq)]
+#[derive(BufferedEvent, Debug, Clone, Default, PartialEq, Eq)]
 pub enum AppExit {
     /// [`App`] exited without any problems.
     #[default]
@@ -1485,7 +1485,7 @@ mod tests {
         change_detection::{DetectChanges, ResMut},
         component::Component,
         entity::Entity,
-        event::{BufferedEvent, Event, EventWriter, Events},
+        event::{BufferedEvent, EventWriter, Events},
         lifecycle::RemovedComponents,
         query::With,
         resource::Resource,
@@ -1851,7 +1851,7 @@ mod tests {
     }
     #[test]
     fn events_should_be_updated_once_per_update() {
-        #[derive(Event, BufferedEvent, Clone)]
+        #[derive(BufferedEvent, Clone)]
         struct TestEvent;
 
         let mut app = App::new();

crates/bevy_asset/src/event.rs

@@ -1,12 +1,12 @@
 use crate::{Asset, AssetId, AssetLoadError, AssetPath, UntypedAssetId};
-use bevy_ecs::event::{BufferedEvent, Event};
+use bevy_ecs::event::BufferedEvent;
 use bevy_reflect::Reflect;
 use core::fmt::Debug;
 
 /// A [`BufferedEvent`] emitted when a specific [`Asset`] fails to load.
 ///
 /// For an untyped equivalent, see [`UntypedAssetLoadFailedEvent`].
-#[derive(Event, BufferedEvent, Clone, Debug)]
+#[derive(BufferedEvent, Clone, Debug)]
 pub struct AssetLoadFailedEvent<A: Asset> {
     /// The stable identifier of the asset that failed to load.
     pub id: AssetId<A>,
@@ -24,7 +24,7 @@ impl<A: Asset> AssetLoadFailedEvent<A> {
 }
 
 /// An untyped version of [`AssetLoadFailedEvent`].
-#[derive(Event, BufferedEvent, Clone, Debug)]
+#[derive(BufferedEvent, Clone, Debug)]
 pub struct UntypedAssetLoadFailedEvent {
     /// The stable identifier of the asset that failed to load.
     pub id: UntypedAssetId,
@@ -46,7 +46,7 @@ impl<A: Asset> From<&AssetLoadFailedEvent<A>> for UntypedAssetLoadFailedEvent {
 
 /// [`BufferedEvent`]s that occur for a specific loaded [`Asset`], such as "value changed" events and "dependency" events.
 #[expect(missing_docs, reason = "Documenting the id fields is unhelpful.")]
-#[derive(Event, BufferedEvent, Reflect)]
+#[derive(BufferedEvent, Reflect)]
 pub enum AssetEvent<A: Asset> {
     /// Emitted whenever an [`Asset`] is added.
     Added { id: AssetId<A> },

crates/bevy_dev_tools/src/ci_testing/config.rs

@@ -52,7 +52,7 @@ pub enum CiTestingEvent {
 }
 
 /// A custom event that can be configured from a configuration file for CI testing.
-#[derive(Event, BufferedEvent)]
+#[derive(BufferedEvent)]
 pub struct CiTestingCustomEvent(pub String);
 
 #[cfg(test)]

crates/bevy_ecs/README.md

@@ -285,7 +285,7 @@ They can be sent using the `EventWriter` system parameter and received with `Eve
 ```rust
 use bevy_ecs::prelude::*;
 
-#[derive(Event, BufferedEvent)]
+#[derive(BufferedEvent)]
 struct Message(String);
 
 fn writer(mut writer: EventWriter<Message>) {

crates/bevy_ecs/examples/events.rs

@@ -37,7 +37,7 @@ fn main() {
 }
 
 // This is our event that we will send and receive in systems
-#[derive(Event, BufferedEvent)]
+#[derive(BufferedEvent)]
 struct MyEvent {
     pub message: String,
     pub random_value: f32,

crates/bevy_ecs/src/change_detection.rs

@@ -230,7 +230,7 @@ pub trait DetectChangesMut: DetectChanges {
     /// #[derive(Resource, PartialEq, Eq)]
     /// pub struct Score(u32);
     ///
-    /// #[derive(Event, BufferedEvent, PartialEq, Eq)]
+    /// #[derive(BufferedEvent, PartialEq, Eq)]
     /// pub struct ScoreChanged {
     ///     current: u32,
     ///     previous: u32,

crates/bevy_ecs/src/event/base.rs

@@ -259,7 +259,7 @@ pub trait EntityEvent: Event {
     const AUTO_PROPAGATE: bool = false;
 }
 
-/// A buffered [`Event`] for pull-based event handling.
+/// A buffered event for pull-based event handling.
 ///
 /// Buffered events can be written with [`EventWriter`] and read using the [`EventReader`] system parameter.
 /// These events are stored in the [`Events<E>`] resource, and require periodically polling the world for new events,
@@ -286,7 +286,7 @@ pub trait EntityEvent: Event {
 /// ```
 /// # use bevy_ecs::prelude::*;
 /// #
-/// #[derive(Event, BufferedEvent)]
+/// #[derive(BufferedEvent)]
 /// struct Message(String);
 /// ```
 ///
@@ -295,7 +295,7 @@ pub trait EntityEvent: Event {
 /// ```
 /// # use bevy_ecs::prelude::*;
 /// #
-/// # #[derive(Event, BufferedEvent)]
+/// # #[derive(BufferedEvent)]
 /// # struct Message(String);
 /// #
 /// fn write_hello(mut writer: EventWriter<Message>) {
@@ -308,7 +308,7 @@ pub trait EntityEvent: Event {
 /// ```
 /// # use bevy_ecs::prelude::*;
 /// #
-/// # #[derive(Event, BufferedEvent)]
+/// # #[derive(BufferedEvent)]
 /// # struct Message(String);
 /// #
 /// fn read_messages(mut reader: EventReader<Message>) {
@@ -327,9 +327,9 @@ pub trait EntityEvent: Event {
 #[diagnostic::on_unimplemented(
     message = "`{Self}` is not an `BufferedEvent`",
     label = "invalid `BufferedEvent`",
-    note = "consider annotating `{Self}` with `#[derive(Event, BufferedEvent)]`"
+    note = "consider annotating `{Self}` with `#[derive(BufferedEvent)]`"
 )]
-pub trait BufferedEvent: Event {}
+pub trait BufferedEvent: Send + Sync + 'static {}
 
 /// An internal type that implements [`Component`] for a given [`Event`] type.
 ///