Implement SystemParam for WorldId (bevyengine#7741)

# Objective

Fixes bevyengine#7736

## Solution

Implement the `SystemParam` trait for `WorldId`

## Changelog

- `WorldId` can now be taken as a system parameter and will return the id of the world the system is running in

crates/bevy_ecs/src/world/identifier.rs

@@ -1,4 +1,7 @@
-use crate::storage::SparseSetIndex;
+use crate::{
+ storage::SparseSetIndex,
+ system::{ReadOnlySystemParam, SystemParam},
+};
 use std::sync::atomic::{AtomicUsize, Ordering};
 
 #[derive(Copy, Clone, PartialEq, Eq, Debug, Hash)]
@@ -27,6 +30,27 @@ impl WorldId {
  }
 }
 
+// SAFETY: Has read-only access to shared World metadata
+unsafe impl ReadOnlySystemParam for WorldId {}
+
+// SAFETY: A World's ID is immutable and fetching it from the World does not borrow anything
+unsafe impl SystemParam for WorldId {
+ type State = ();
+
+ type Item<'world, 'state> = WorldId;
+
+ fn init_state(_: &mut super::World, _: &mut crate::system::SystemMeta) -> Self::State {}
+
+ unsafe fn get_param<'world, 'state>(
+ _: &'state mut Self::State,
+ _: &crate::system::SystemMeta,
+ world: &'world super::World,
+ _: u32,
+ ) -> Self::Item<'world, 'state> {
+ world.id
+ }
+}
+
 impl SparseSetIndex for WorldId {
  #[inline]
  fn sparse_set_index(&self) -> usize {