small optimization

Author: RalfJung

src/helpers.rs

@@ -232,10 +232,13 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                             // we cannot determine the variant we are in. Reading from
                             // memory would be subject to Stacked Borrows rules, leading
                             // to all sorts of "funny" recursion.
-                            self.visit_union(v)
+                            // We only end up here if the type is *not* freeze, so we just call the
+                            // `UnsafeCell` action.
+                            (self.unsafe_cell_action)(v)
                         }
                         layout::Variants::Single { .. } => {
-                            // Proceed further
+                            // Proceed further, try to find where exactly that `UnsafeCell`
+                            // is hiding.
                             self.walk_value(v)
                         }
                     }