more clear code

Co-authored-by: Oli Scherer <[email protected]>

Author: RalfJung

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -454,10 +454,10 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
                     // here since we cannot know if there really is an `UnsafeCell` inside
                     // `Option<UnsafeCell>` -- so we check that in the recursive descent behind this
                     // reference.
-                    if size == Size::ZERO || tam.mutbl == Mutability::Not {
+                    if size == Size::ZERO {
                         Mutability::Not
                     } else {
-                        Mutability::Mut
+                        tam.mutbl
                     }
                 }
             };