Remove States::variants and remove enum-only restriction its derive (#9945)

# Objective

The `States::variants` method was once used to construct `OnExit` and
`OnEnter` schedules for every possible value of a given `States` type.
[Since the switch to lazily initialized
schedules](https://github.com/bevyengine/bevy/pull/8028/files#diff-b2fba3a0c86e496085ce7f0e3f1de5960cb754c7d215ed0f087aa556e529f97f),
we no longer need to track every possible value.

This also opens the door to `States` types that aren't enums.

## Solution

- Remove the unused `States::variants` method and its associated type.
- Remove the enum-only restriction on derived States types.

---

## Changelog

- Removed `States::variants` and its associated type.
- Derived `States` can now be datatypes other than enums.

## Migration Guide

- `States::variants` no longer exists. If you relied on this function,
consider using a library that provides enum iterators.

crates/bevy_ecs/macros/src/states.rs

@@ -1,26 +1,11 @@
-use proc_macro::{Span, TokenStream};
+use proc_macro::TokenStream;
 use quote::{format_ident, quote};
-use syn::{parse_macro_input, Data::Enum, DeriveInput};
+use syn::{parse_macro_input, DeriveInput};
 
 use crate::bevy_ecs_path;
 
 pub fn derive_states(input: TokenStream) -> TokenStream {
     let ast = parse_macro_input!(input as DeriveInput);
-    let error = || {
-        syn::Error::new(
-            Span::call_site().into(),
-            "derive(States) only supports fieldless enums",
-        )
-        .into_compile_error()
-        .into()
-    };
-    let Enum(enumeration) = ast.data else {
-        return error();
-    };
-    if enumeration.variants.iter().any(|v| !v.fields.is_empty()) {
-        return error();
-    }
-
     let generics = ast.generics;
     let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
 
@@ -28,17 +13,9 @@ pub fn derive_states(input: TokenStream) -> TokenStream {
     trait_path.segments.push(format_ident!("schedule").into());
     trait_path.segments.push(format_ident!("States").into());
     let struct_name = &ast.ident;
-    let idents = enumeration.variants.iter().map(|v| &v.ident);
-    let len = idents.len();
 
     quote! {
-        impl #impl_generics #trait_path for #struct_name #ty_generics #where_clause {
-            type Iter = std::array::IntoIter<Self, #len>;
-
-            fn variants() -> Self::Iter {
-                [#(Self::#idents,)*].into_iter()
-            }
-        }
+        impl #impl_generics #trait_path for #struct_name #ty_generics #where_clause {}
     }
     .into()
 }

crates/bevy_ecs/src/schedule/condition.rs

@@ -1190,7 +1190,7 @@ mod tests {
                 .distributive_run_if(resource_changed_or_removed::<State<TestState>>())
                 .distributive_run_if(resource_removed::<State<TestState>>())
                 .distributive_run_if(state_exists::<TestState>())
-                .distributive_run_if(in_state(TestState::A))
+                .distributive_run_if(in_state(TestState::A).or_else(in_state(TestState::B)))
                 .distributive_run_if(state_changed::<TestState>())
                 .distributive_run_if(on_event::<TestEvent>())
                 .distributive_run_if(any_with_component::<TestComponent>())

crates/bevy_ecs/src/schedule/state.rs

@@ -40,13 +40,7 @@ pub use bevy_ecs_macros::States;
 /// }
 ///
 /// ```
-pub trait States: 'static + Send + Sync + Clone + PartialEq + Eq + Hash + Debug + Default {
-    /// The type returned when iterating over all [`variants`](States::variants) of this type.
-    type Iter: Iterator<Item = Self>;
-
-    /// Returns an iterator over all the state variants.
-    fn variants() -> Self::Iter;
-}
+pub trait States: 'static + Send + Sync + Clone + PartialEq + Eq + Hash + Debug + Default {}
 
 /// The label of a [`Schedule`](super::Schedule) that runs whenever [`State<S>`]
 /// enters this state.