Add Required system param

crates/bevy_ecs/src/system/system_param.rs

@@ -596,6 +596,71 @@ impl<'a, T: Component + FromWorld> SystemParamFetch<'a> for LocalState<T> {
     }
 }
 
+/// A version of [`Local`] which requires manual initialization
+pub struct Required<'a, T: Component>(&'a mut T);
+
+// SAFE: Required only accesses internal state
+unsafe impl<T: Component> ReadOnlySystemParamFetch for RequiredState<T> {}
+
+impl<'a, T: Component> Debug for Required<'a, T>
+where
+    T: Debug,
+{
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_tuple("Required").field(&self.0).finish()
+    }
+}
+
+impl<'a, T: Component> Deref for Required<'a, T> {
+    type Target = T;
+
+    #[inline]
+    fn deref(&self) -> &Self::Target {
+        self.0
+    }
+}
+
+impl<'a, T: Component> DerefMut for Required<'a, T> {
+    #[inline]
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        self.0
+    }
+}
+
+/// The [`SystemParamState`] of [`Required`].
+pub struct RequiredState<T: Component>(T);
+
+impl<'a, T: Component> SystemParam for Required<'a, T> {
+    type Fetch = RequiredState<T>;
+}
+
+// SAFE: only local state is accessed
+unsafe impl<T: Component> SystemParamState for RequiredState<T> {
+    type Config = Option<T>;
+
+    fn init(_world: &mut World, _system_meta: &mut SystemMeta, config: Self::Config) -> Self {
+        Self(config.expect("Required must be initialized using config!"))
+    }
+
+    fn default_config() -> Option<T> {
+        None
+    }
+}
+
+impl<'a, T: Component> SystemParamFetch<'a> for RequiredState<T> {
+    type Item = Required<'a, T>;
+
+    #[inline]
+    unsafe fn get_param(
+        state: &'a mut Self,
+        _system_meta: &SystemMeta,
+        _world: &'a World,
+        _change_tick: u32,
+    ) -> Self::Item {
+        Required(&mut state.0)
+    }
+}
+
 /// A [`SystemParam`] that grants access to the entities that had their `T` [`Component`] removed.
 ///
 /// # Examples