error[E0596]: cannot borrow `v` as mutable, as it is not declared as mutable
  --> $DIR/issue-33819.rs:4:34
   |
LL |         Some(ref v) => { let a = &mut v; },
   |                                  ^^^^^^ cannot borrow as mutable
   |
help: try removing `&mut` here
   |
LL -         Some(ref v) => { let a = &mut v; },
LL +         Some(ref v) => { let a = v; },
   |

error: aborting due to 1 previous error

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