error[E0596]: cannot borrow data in dereference of `X__Partial<Const>` as mutable
  --> $DIR/autobad-deref-whole-const-mut.rs:15:20
   |
15 |   let _o: &mut X = &mut **d;
   |                    ^^^^^^^^ cannot borrow as mutable
   |
   = help: trait `DerefMut` is required to modify through a dereference, but it is not implemented for `X__Partial<Const>`