This is unreachable code and it should never ever be reachable even in the future

src/librustc_mir/interpret/eval_context.rs

@@ -254,7 +254,7 @@ pub(super) fn const_to_value(&mut self, const_val: &ConstVal<'tcx>) -> EvalResul
             }
 
             Aggregate(..) |
-            Variant(_) => unimplemented!(),
+            Variant(_) => bug!("should not have aggregate or variant constants in MIR"),
             // function items are zero sized and thus have no readable value
             Function(..) => PrimVal::Undef,
         };