add //~ ERROR line to test for privacy respecting FRU (RFC 736).

src/test/compile-fail/functional-struct-update-respects-privacy.rs

@@ -36,7 +36,7 @@ mod foo {
 fn main() {
     let s_1 = foo::make_secrets(3, format!("ess one"));
     let s_2 = foo::S { b: format!("ess two"), ..s_1 }; // FRU ...
-
+    //~^ ERROR field `secret_uid` of struct `foo::S` is private
     println!("main forged an S named: {}", s_2.b);
     // at end of scope, ... both s_1 *and* s_2 get dropped.  Boom!
 }