enum E<T> {
  A(T),
  B(T),
}

impl<T> E<T> {
    fn to_b(self) -> E<T> {
        match self {
            E::A(t) => E::B(t),
            _=> panic!(),
        }
    }
}

fn update<T>(e: &mut E<T>) {
  *e = match e {
    E::A(_t) => e.to_b(),
    _ => panic!(),
  }
}

fn main() {}