Point at unknown lang item attribute

src/test/ui/unknown-language-item.rs

@@ -0,0 +1,20 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![allow(unused)]
+#![feature(lang_items)]
+
+#[lang = "foo"]
+fn bar() -> ! {
+//~^^ ERROR definition of an unknown language item: `foo`
+    loop {}
+}
+
+fn main() {}

src/test/ui/unknown-language-item.stderr

@@ -0,0 +1,8 @@
+error[E0522]: definition of an unknown language item: `foo`
+  --> $DIR/unknown-language-item.rs:14:1
+   |
+14 | #[lang = "foo"]
+   | ^^^^^^^^^^^^^^^ definition of unknown language item `foo`
+
+error: aborting due to previous error
+