rust/tests/compile-fail/stacked_borrows/illegal_read3.rs

#![feature(untagged_unions)]
// A callee may not read the destination of our `&mut` without
// us noticing.
// Thise code got carefully checked to not introduce any reborrows
// that are not explicit in the source.  Let's hope the compiler does not break this later!

use std::mem;

fn main() {
    let mut x: i32 = 15;
    let xref1 = &mut x;
    let xref1_sneaky: usize = unsafe { mem::transmute_copy(&xref1) };
    let xref2 = &mut *xref1; // derived from xref1, so using raw is still okay...
    callee(xref1_sneaky);
    let _val = *xref2; // ...but any use of it will invalidate our ref.
    //~^ ERROR: does not exist on the borrow stack
}

fn callee(xref1: usize) {
    // Transmuting through a union to avoid retagging
    union UsizeToRef {
        from: usize,
        to: &'static mut i32,
    }
    let xref1 = UsizeToRef { from: xref1 };
    // Doing the deref and the transmute (through the union) in the same place expression
    // should avoid retagging.
    let _val = unsafe { *xref1.to };
}