add an interesting demo for &mut being unique

Author: RalfJung

tests/compile-fail/stacked_borrows/mut_exclusive_violation1.rs

@@ -0,0 +1,29 @@
+fn demo_mut_advanced_unique(our: &mut i32) -> i32 {
+  unknown_code_1(&*our);
+
+  // This "re-asserts" uniqueness of the reference: After writing, we know
+  // our tag is at the top of the stack.
+  *our = 5;
+
+  unknown_code_2();
+
+  // We know this will return 5
+  *our
+}
+
+// Now comes the evil context
+use std::ptr;
+
+static mut LEAK: *mut i32 = ptr::null_mut();
+
+fn unknown_code_1(x: &i32) { unsafe {
+    LEAK = x as *const _ as *mut _;
+} }
+
+fn unknown_code_2() { unsafe {
+    *LEAK = 7; //~ ERROR does not exist on the stack
+} }
+
+fn main() {
+    assert_eq!(demo_mut_advanced_unique(&mut 0), 5);
+}