fn foo(x: &mut i32) -> i32 {
  *x = 5;
  unknown_code(&*x);
  *x // must return 5
}

fn main() {
    println!("{}", foo(&mut 0));
}

fn unknown_code(x: &i32) {
    unsafe { *(x as *const i32 as *mut i32) = 7; } //~ ERROR borrow stack
}