Add more complex example

Author: varkor

src/test/ui/destructuring-assignment/warn-unused-duplication.rs

@@ -10,4 +10,14 @@ fn main() {
     // However, we emit warnings when this happens, so it is clear that this is happening.
     (a, a) = (0, 1); //~ WARN value assigned to `a` is never read
     assert_eq!(a, 1);
+
+    // We can't always tell when a variable is being assigned to twice, which is why we don't try
+    // to emit an error, which would be fallible.
+    let mut x = 1;
+    (*foo(&mut x), *foo(&mut x)) = (5, 6);
+    assert_eq!(x, 6);
+}
+
+fn foo<'a>(x: &'a mut u32) -> &'a mut u32 {
+    x
 }