add another mean test case

Author: RalfJung

tests/compile-fail/stacked_borrows/illegal_read3.rs

@@ -0,0 +1,29 @@
+#![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 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 };
+}