enable_state_scoped_entities() as a derive attribute (#16180)

# Objective

- I got tired of calling `enable_state_scoped_entities`, and though it
would make more sense to define that at the place where the state is
defined

## Solution

- add a derive attribute `#[states(scoped_entities)]` when derive
`States` or `SubStates` that enables it automatically when adding the
state

## Testing

- Ran the examples using it, they still work

crates/bevy_state/macros/src/lib.rs

@@ -9,12 +9,12 @@ mod states;
 use bevy_macro_utils::BevyManifest;
 use proc_macro::TokenStream;
 
-#[proc_macro_derive(States)]
+#[proc_macro_derive(States, attributes(states))]
 pub fn derive_states(input: TokenStream) -> TokenStream {
     states::derive_states(input)
 }
 
-#[proc_macro_derive(SubStates, attributes(source))]
+#[proc_macro_derive(SubStates, attributes(states, source))]
 pub fn derive_substates(input: TokenStream) -> TokenStream {
     states::derive_substates(input)
 }

crates/bevy_state/macros/src/states.rs

@@ -4,9 +4,42 @@ use syn::{parse_macro_input, spanned::Spanned, DeriveInput, Pat, Path, Result};
 
 use crate::bevy_state_path;
 
+pub const STATES: &str = "states";
+pub const SCOPED_ENTITIES: &str = "scoped_entities";
+
+struct StatesAttrs {
+    scoped_entities_enabled: bool,
+}
+
+fn parse_states_attr(ast: &DeriveInput) -> Result<StatesAttrs> {
+    let mut attrs = StatesAttrs {
+        scoped_entities_enabled: false,
+    };
+
+    for attr in ast.attrs.iter() {
+        if attr.path().is_ident(STATES) {
+            attr.parse_nested_meta(|nested| {
+                if nested.path.is_ident(SCOPED_ENTITIES) {
+                    attrs.scoped_entities_enabled = true;
+                    Ok(())
+                } else {
+                    Err(nested.error("Unsupported attribute"))
+                }
+            })?;
+        }
+    }
+
+    Ok(attrs)
+}
+
 pub fn derive_states(input: TokenStream) -> TokenStream {
     let ast = parse_macro_input!(input as DeriveInput);
 
+    let attrs = match parse_states_attr(&ast) {
+        Ok(attrs) => attrs,
+        Err(e) => return e.into_compile_error().into(),
+    };
+
     let generics = ast.generics;
     let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
 
@@ -23,8 +56,12 @@ pub fn derive_states(input: TokenStream) -> TokenStream {
 
     let struct_name = &ast.ident;
 
+    let scoped_entities_enabled = attrs.scoped_entities_enabled;
+
     quote! {
-        impl #impl_generics #trait_path for #struct_name #ty_generics #where_clause {}
+        impl #impl_generics #trait_path for #struct_name #ty_generics #where_clause {
+            const SCOPED_ENTITIES_ENABLED: bool = #scoped_entities_enabled;
+        }
 
         impl #impl_generics #state_mutation_trait_path for #struct_name #ty_generics #where_clause {
         }
@@ -37,7 +74,7 @@ struct Source {
     source_value: Pat,
 }
 
-fn parse_sources_attr(ast: &DeriveInput) -> Result<Source> {
+fn parse_sources_attr(ast: &DeriveInput) -> Result<(StatesAttrs, Source)> {
     let mut result = ast
         .attrs
         .iter()
@@ -73,16 +110,19 @@ fn parse_sources_attr(ast: &DeriveInput) -> Result<Source> {
         ));
     }
 
+    let states_attrs = parse_states_attr(ast)?;
+
     let Some(result) = result.pop() else {
         return Err(syn::Error::new(ast.span(), "SubStates require a source"));
     };
 
-    Ok(result)
+    Ok((states_attrs, result))
 }
 
 pub fn derive_substates(input: TokenStream) -> TokenStream {
     let ast = parse_macro_input!(input as DeriveInput);
-    let sources = parse_sources_attr(&ast).expect("Failed to parse substate sources");
+    let (states_attrs, sources) =
+        parse_sources_attr(&ast).expect("Failed to parse substate sources");
 
     let generics = ast.generics;
     let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
@@ -113,6 +153,8 @@ pub fn derive_substates(input: TokenStream) -> TokenStream {
     let source_state_type = sources.source_type;
     let source_state_value = sources.source_value;
 
+    let scoped_entities_enabled = states_attrs.scoped_entities_enabled;
+
     let result = quote! {
         impl #impl_generics #trait_path for #struct_name #ty_generics #where_clause {
             type SourceStates = #source_state_type;
@@ -124,6 +166,8 @@ pub fn derive_substates(input: TokenStream) -> TokenStream {
 
         impl #impl_generics #state_trait_path for #struct_name #ty_generics #where_clause {
             const DEPENDENCY_DEPTH : usize = <Self as #trait_path>::SourceStates::SET_DEPENDENCY_DEPTH + 1;
+
+            const SCOPED_ENTITIES_ENABLED: bool = #scoped_entities_enabled;
         }
 
         impl #impl_generics #state_mutation_trait_path for #struct_name #ty_generics #where_clause {

crates/bevy_state/src/app.rs

@@ -58,6 +58,9 @@ pub trait AppExtStates {
 
     /// Enable state-scoped entity clearing for state `S`.
     ///
+    /// If the [`States`] trait was derived with the `#[states(scoped_entities)]` attribute, it
+    /// will be called automatically.
+    ///
     /// For more information refer to [`StateScoped`](crate::state_scoped::StateScoped).
     fn enable_state_scoped_entities<S: States>(&mut self) -> &mut Self;
 
@@ -104,6 +107,9 @@ impl AppExtStates for SubApp {
                 exited: None,
                 entered: Some(state),
             });
+            if S::SCOPED_ENTITIES_ENABLED {
+                self.enable_state_scoped_entities::<S>();
+            }
         } else {
             let name = core::any::type_name::<S>();
             warn!("State {} is already initialized.", name);
@@ -126,6 +132,9 @@ impl AppExtStates for SubApp {
                 exited: None,
                 entered: Some(state),
             });
+            if S::SCOPED_ENTITIES_ENABLED {
+                self.enable_state_scoped_entities::<S>();
+            }
         } else {
             // Overwrite previous state and initial event
             self.insert_resource::<State<S>>(State::new(state.clone()));
@@ -160,6 +169,9 @@ impl AppExtStates for SubApp {
                 exited: None,
                 entered: state,
             });
+            if S::SCOPED_ENTITIES_ENABLED {
+                self.enable_state_scoped_entities::<S>();
+            }
         } else {
             let name = core::any::type_name::<S>();
             warn!("Computed state {} is already initialized.", name);
@@ -188,6 +200,9 @@ impl AppExtStates for SubApp {
                 exited: None,
                 entered: state,
             });
+            if S::SCOPED_ENTITIES_ENABLED {
+                self.enable_state_scoped_entities::<S>();
+            }
         } else {
             let name = core::any::type_name::<S>();
             warn!("Sub state {} is already initialized.", name);

crates/bevy_state/src/state/states.rs

@@ -64,4 +64,8 @@ pub trait States: 'static + Send + Sync + Clone + PartialEq + Eq + Hash + Debug
     /// Used to help order transitions and de-duplicate [`ComputedStates`](crate::state::ComputedStates), as well as prevent cyclical
     /// `ComputedState` dependencies.
     const DEPENDENCY_DEPTH: usize = 1;
+
+    /// Should [`StateScoped`](crate::state_scoped::StateScoped) be enabled for this state? If set to `true`,
+    /// the `StateScoped` component will be used to remove entities when changing state.
+    const SCOPED_ENTITIES_ENABLED: bool = false;
 }

crates/bevy_state/src/state_scoped.rs

@@ -16,8 +16,8 @@ use crate::state::{StateTransitionEvent, States};
 /// Entities marked with this component will be removed
 /// when the world's state of the matching type no longer matches the supplied value.
 ///
-/// To enable this feature remember to configure your application
+/// To enable this feature remember to add the attribute `#[states(scoped_entities)]` when deriving [`States`].
-/// with [`enable_state_scoped_entities`](crate::app::AppExtStates::enable_state_scoped_entities) on your state(s) of choice.
+/// It's also possible to enable it when adding the state to an app with [`enable_state_scoped_entities`](crate::app::AppExtStates::enable_state_scoped_entities).
 ///
 /// If `bevy_hierarchy` feature is enabled, which it is by default, the despawn will be recursive.
 ///
@@ -26,6 +26,7 @@ use crate::state::{StateTransitionEvent, States};
 /// use bevy_ecs::prelude::*;
 ///
 /// #[derive(Clone, Copy, PartialEq, Eq, Hash, Debug, Default, States)]
+/// #[states(scoped_entities)]
 /// enum GameState {
 ///     #[default]
 ///     MainMenu,
@@ -53,7 +54,6 @@ use crate::state::{StateTransitionEvent, States};
 /// # let mut app = AppMock;
 ///
 /// app.init_state::<GameState>();
-/// app.enable_state_scoped_entities::<GameState>();
 /// app.add_systems(OnEnter(GameState::InGame), spawn_player);
 /// ```
 #[derive(Component, Clone)]

examples/state/sub_states.rs

@@ -25,6 +25,7 @@ enum AppState {
 // in [`AppState::InGame`], the [`IsPaused`] state resource
 // will not exist.
 #[source(AppState = AppState::InGame)]
+#[states(scoped_entities)]
 enum IsPaused {
     #[default]
     Running,
@@ -43,7 +44,6 @@ fn main() {
         .add_systems(OnExit(AppState::Menu), cleanup_menu)
         .add_systems(OnEnter(AppState::InGame), setup_game)
         .add_systems(OnEnter(IsPaused::Paused), setup_paused_screen)
-        .enable_state_scoped_entities::<IsPaused>()
         .add_systems(
             Update,
             (

examples/testbed/2d.rs

@@ -10,7 +10,6 @@ fn main() {
     let mut app = App::new();
     app.add_plugins((DefaultPlugins,))
         .init_state::<Scene>()
-        .enable_state_scoped_entities::<Scene>()
         .add_systems(OnEnter(Scene::Shapes), shapes::setup)
         .add_systems(OnEnter(Scene::Bloom), bloom::setup)
         .add_systems(OnEnter(Scene::Text), text::setup)
@@ -20,6 +19,7 @@ fn main() {
 }
 
 #[derive(Debug, Clone, Eq, PartialEq, Hash, States, Default)]
+#[states(scoped_entities)]
 enum Scene {
     #[default]
     Shapes,

examples/testbed/3d.rs

@@ -10,7 +10,6 @@ fn main() {
     let mut app = App::new();
     app.add_plugins((DefaultPlugins,))
         .init_state::<Scene>()
-        .enable_state_scoped_entities::<Scene>()
         .add_systems(OnEnter(Scene::Light), light::setup)
         .add_systems(OnEnter(Scene::Animation), animation::setup)
         .add_systems(Update, switch_scene);
@@ -24,6 +23,7 @@ fn main() {
 }
 
 #[derive(Debug, Clone, Eq, PartialEq, Hash, States, Default)]
+#[states(scoped_entities)]
 enum Scene {
     #[default]
     Light,