error[E0596]: cannot borrow `v` as mutable, as it is not declared as mutable
  --> $DIR/issue-33819.rs:13:34
   |
LL |         Some(ref v) => { let a = &mut v; },
   |                                  ^^^^^^
   |                                  |
   |                                  cannot borrow as mutable
   |                                  try removing `&mut` here

error: aborting due to previous error

For more information about this error, try `rustc --explain E0596`.