// A callee may not write to the destination of our `&mut` without us noticing. fn main() { let mut x = 15; let xraw = &mut x as *mut _; // Derived from raw value, so using raw value is still ok ... let xref = unsafe { &mut *xraw }; callee(xraw); // ... though any use of raw value will invalidate our ref. let _val = *xref; //~^ ERROR: does not exist on the borrow stack } fn callee(xraw: *mut i32) { unsafe { *xraw = 15 }; }