Make BroadcastEvent and EntityEvent mutually exclusive

crates/bevy_ecs/macros/src/component.rs

@@ -30,8 +30,9 @@ pub fn derive_broadcast_event(input: TokenStream) -> TokenStream {
     let (impl_generics, type_generics, where_clause) = &ast.generics.split_for_impl();
 
     TokenStream::from(quote! {
-        impl #impl_generics #bevy_ecs_path::event::Event for #struct_name #type_generics #where_clause {}
-        impl #impl_generics #bevy_ecs_path::event::BroadcastEvent for #struct_name #type_generics #where_clause {}
+        impl #impl_generics #bevy_ecs_path::event::Event for #struct_name #type_generics #where_clause {
+            type Kind = #bevy_ecs_path::event::BroadcastEventKind;
+        }
     })
 }
 
@@ -74,10 +75,8 @@ pub fn derive_entity_event(input: TokenStream) -> TokenStream {
     let (impl_generics, type_generics, where_clause) = &ast.generics.split_for_impl();
 
     TokenStream::from(quote! {
-        impl #impl_generics #bevy_ecs_path::event::Event for #struct_name #type_generics #where_clause {}
-        impl #impl_generics #bevy_ecs_path::event::EntityEvent for #struct_name #type_generics #where_clause {
-            type Traversal = #traversal;
-            const AUTO_PROPAGATE: bool = #auto_propagate;
+        impl #impl_generics #bevy_ecs_path::event::Event for #struct_name #type_generics #where_clause {
+            type Kind = #bevy_ecs_path::event::EntityEventKind<Self, #traversal, #auto_propagate>;
         }
     })
 }

crates/bevy_ecs/src/event/base.rs

@@ -11,7 +11,7 @@ use core::{
     marker::PhantomData,
 };
 
-// TODO: Adjust these docs
+// TODO: These docs need to be moved around and adjusted
 /// Something that "happens" and can be processed by app logic.
 ///
 /// Events can be triggered on a [`World`] using a method like [`trigger`](World::trigger),
@@ -36,7 +36,7 @@ use core::{
 /// ```
 /// # use bevy_ecs::prelude::*;
 /// #
-/// #[derive(Event)]
+/// #[derive(BroadcastEvent)]
 /// struct Speak {
 ///     message: String,
 /// }
@@ -47,7 +47,7 @@ use core::{
 /// ```
 /// # use bevy_ecs::prelude::*;
 /// #
-/// # #[derive(Event)]
+/// # #[derive(BroadcastEvent)]
 /// # struct Speak {
 /// #     message: String,
 /// # }
@@ -64,7 +64,7 @@ use core::{
 /// ```
 /// # use bevy_ecs::prelude::*;
 /// #
-/// # #[derive(Event)]
+/// # #[derive(BroadcastEvent)]
 /// # struct Speak {
 /// #     message: String,
 /// # }
@@ -90,11 +90,14 @@ use core::{
 /// [`EventReader`]: super::EventReader
 /// [`EventWriter`]: super::EventWriter
 #[diagnostic::on_unimplemented(
-    message = "`{Self}` is not an `ObserverEvent`",
-    label = "invalid `ObserverEvent`",
+    message = "`{Self}` is not an `Event`",
+    label = "invalid `Event`",
     note = "consider annotating `{Self}` with `#[derive(BroadcastEvent)]` or `#[derive(EntityEvent)]`"
 )]
 pub trait Event: Send + Sync + 'static {
+    ///
+    type Kind;
+
     /// Generates the [`EventKey`] for this event type.
     ///
     /// If this type has already been registered,
@@ -130,7 +133,32 @@ pub trait Event: Send + Sync + 'static {
 }
 
 /// A global [`Event`] without an entity target.
-pub trait BroadcastEvent: Event {}
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` is not an `BroadcastEvent`",
+    label = "invalid `BroadcastEvent`",
+    note = "consider annotating `{Self}` with `#[derive(BroadcastEvent)]`"
+)]
+pub trait BroadcastEvent: Event + sealed_a::SealedA {}
+
+#[doc(hidden)]
+pub struct BroadcastEventKind;
+
+#[diagnostic::do_not_recommend]
+impl<T> BroadcastEvent for T where T: Event<Kind = BroadcastEventKind> {}
+
+pub(crate) mod sealed_a {
+    use super::*;
+
+    /// Seal for the [`BroadcastEvent`] trait.
+    #[diagnostic::on_unimplemented(
+        message = "manual implementations of `BroadcastEvent` are disallowed",
+        note = "consider annotating `{Self}` with `#[derive(BroadcastEvent)]` instead"
+    )]
+    pub trait SealedA {}
+
+    #[diagnostic::do_not_recommend]
+    impl<T> SealedA for T where T: Event<Kind = BroadcastEventKind> {}
+}
 
 /// An [`Event`] that can be targeted at specific entities.
 ///
@@ -248,7 +276,7 @@ pub trait BroadcastEvent: Event {}
     label = "invalid `EntityEvent`",
     note = "consider annotating `{Self}` with `#[derive(EntityEvent)]`"
 )]
-pub trait EntityEvent: Event {
+pub trait EntityEvent: Event + sealed_b::SealedB {
     /// The component that describes which [`Entity`] to propagate this event to next, when [propagation] is enabled.
     ///
     /// [`Entity`]: crate::entity::Entity
@@ -263,6 +291,37 @@ pub trait EntityEvent: Event {
     const AUTO_PROPAGATE: bool = false;
 }
 
+#[doc(hidden)]
+pub struct EntityEventKind<T: ?Sized, R: Traversal<T>, const A: bool> {
+    _t: PhantomData<T>,
+    _r: PhantomData<R>,
+}
+
+// Blanket impl for EntityEvent
+#[diagnostic::do_not_recommend]
+impl<T, R: Traversal<T>, const A: bool> EntityEvent for T
+where
+    T: Event<Kind = EntityEventKind<T, R, A>>,
+{
+    type Traversal = R;
+    const AUTO_PROPAGATE: bool = A;
+}
+
+pub(crate) mod sealed_b {
+    use super::*;
+
+    /// Seal for the [`EntityEvent`] trait.
+    #[diagnostic::on_unimplemented(
+        message = "manual implementations of `EntityEvent` are disallowed",
+        note = "consider annotating `{Self}` with `#[derive(EntityEvent)]` instead"
+    )]
+    pub trait SealedB {}
+
+    #[diagnostic::do_not_recommend]
+    impl<T, R: Traversal<T>, const A: bool> SealedB for T where T: Event<Kind = EntityEventKind<T, R, A>>
+    {}
+}
+
 /// A buffered event for pull-based event handling.
 ///
 /// Buffered events can be written with [`EventWriter`] and read using the [`EventReader`] system parameter.

crates/bevy_ecs/src/event/mod.rs

@@ -11,7 +11,10 @@ mod update;
 mod writer;
 
 pub(crate) use base::EventInstance;
-pub use base::{BroadcastEvent, BufferedEvent, EntityEvent, Event, EventId, EventKey};
+pub use base::{
+    BroadcastEvent, BroadcastEventKind, BufferedEvent, EntityEvent, EntityEventKind, Event,
+    EventId, EventKey,
+};
 pub use bevy_ecs_macros::{BroadcastEvent, BufferedEvent, EntityEvent};
 #[expect(deprecated, reason = "`SendBatchIds` was renamed to `WriteBatchIds`.")]
 pub use collections::{Events, SendBatchIds, WriteBatchIds};