fn example2_down(x: &i32, f: impl FnOnce(&i32)) -> i32 {
  let val = *x;
  f(x);
  return val; // Can return `*x` instead.
}

fn main() {
  let mut local = 42;
  let raw_pointer = &mut local as *mut i32;
  let val = example2_down(
    unsafe { &*raw_pointer },
    |_x_inner| unsafe { *raw_pointer = 17; }, // Changes `*x`.
  );
  println!("{}", val); // Prints 42.
}