add missing unsafe block

The // SAFETY comment already exists -- only the unsafe { .. } was
omitted.

rust/lib/src/traits.rs

@@ -314,7 +314,7 @@ where
     //  - list is properly aligned, dereferencable, and points to an initialized CL, since it is valid
     //  - the lifetime of the resulting reference is limited to this function, during which time
     //    nothing else refers to this memory.
-    let slice = list.as_mut().unwrap().slice();
+    let slice = unsafe { list.as_mut() }.unwrap().slice();
     if let Some(elt_ref) = slice.get_mut(index) {
         let mut rv = None;
         if let Some(elt) = elt_ref.as_mut() {