error[E0596]: cannot borrow data in dereference of `F_a<Const, usize, X>` as mutable
  --> $DIR/autobad-deref-field-const-mut.rs:15:24
   |
15 |   let _f: &mut usize = &mut *d.a;
   |                        ^^^^^^^^^ cannot borrow as mutable
   |
   = help: trait `DerefMut` is required to modify through a dereference, but it is not implemented for `F_a<Const, usize, X>`