error[E0502]: cannot borrow `*self` as mutable because it is also borrowed as immutable

error[E0502]: cannot borrow `*self` as mutable because it is also borrowed as immutable
  --> src/main.rs:15:9
   |
14 |     let x = &self.data;
   |             ---------- immutable borrow occurs here
15 |     self.update();
   |     ^^^^^^^^^^^^^ mutable borrow occurs here
16 |     println!("{}", x);
   |                    - immutable borrow later used here