error[E0596]: cannot borrow `x` as mutable, as it is not declared as mutable
  --> tests/unwrap_variant/fail/unwrap_mut_on_immutable.rs:14:5
   |
14 |     x.unwrap_a_mut().push('r');
   |     ^ cannot borrow as mutable
   |
help: consider changing this to be mutable
   |
13 |     let mut x = Test::A(String::from("x"));
   |         +++