refactor(arena): use slice::as_ptr_range in Arena::get_node_id

Also changed up the control flow to reduce nesting, which is purely a style preference.

src/arena.rs

@@ -79,18 +79,19 @@ impl<T> Arena<T> {
     /// assert_eq!(*arena[node_id].get(), "foo");
     /// ```
     pub fn get_node_id(&self, node: &Node<T>) -> Option<NodeId> {
-        // self.nodes.as_ptr_range() is not stable until rust 1.48
-        let start = self.nodes.as_ptr() as usize;
-        let end = start + self.nodes.len() * core::mem::size_of::<Node<T>>();
-        let p = node as *const Node<T> as usize;
-        if (start..end).contains(&p) {
-            let node_id = (p - start) / core::mem::size_of::<Node<T>>();
-            NonZeroUsize::new(node_id.wrapping_add(1)).map(|node_id_non_zero| {
-                NodeId::from_non_zero_usize(node_id_non_zero, self.nodes[node_id].stamp)
-            })
-        } else {
-            None
+        let nodes_range = self.nodes.as_ptr_range();
+        let p = node as *const Node<T>;
+
+        if !nodes_range.contains(&p) {
+            return None;
         }
+
+        let node_index = (p as usize - nodes_range.start as usize) / core::mem::size_of::<Node<T>>();
+        let Some(node_id) = NonZeroUsize::new(node_index.wrapping_add(1)) else {
+            return None;
+        };
+
+        Some(NodeId::from_non_zero_usize(node_id, self.nodes[node_index].stamp))
     }
 
     /// Retrieves the `NodeId` corresponding to the `Node` at `index` in the `Arena`, if it exists.