diff --git a/src/librustc/diagnostics.rs b/src/librustc/diagnostics.rs
index 92db527ef9803..f474f7d4585f9 100644
--- a/src/librustc/diagnostics.rs
+++ b/src/librustc/diagnostics.rs
@@ -1962,6 +1962,23 @@ each method; it is not possible to annotate the entire impl with an `#[inline]`
 attribute.
 "##,
 
+E0522: r##"
+The lang attribute is intended for marking special items that are built-in to
+Rust itself. This includes special traits (like `Copy` and `Sized`) that affect
+how the compiler behaves, as well as special functions that may be automatically
+invoked (such as the handler for out-of-bounds accesses when indexing a slice).
+Erroneous code example:
+
+```compile_fail
+#![feature(lang_items)]
+
+#[lang = "cookie"]
+fn cookie() -> ! { // error: definition of an unknown language item: `cookie`
+    loop {}
+}
+```
+"##,
+
 }
 
 
diff --git a/src/librustc/middle/lang_items.rs b/src/librustc/middle/lang_items.rs
index c432095ff0697..b08c2d6054c89 100644
--- a/src/librustc/middle/lang_items.rs
+++ b/src/librustc/middle/lang_items.rs
@@ -158,6 +158,11 @@ impl<'a, 'v, 'tcx> Visitor<'v> for LanguageItemCollector<'a, 'tcx> {
 
             if let Some(item_index) = item_index {
                 self.collect_item(item_index, self.ast_map.local_def_id(item.id))
+            } else {
+                let span = self.ast_map.span(item.id);
+                span_err!(self.session, span, E0522,
+                          "definition of an unknown language item: `{}`.",
+                          &value[..]);
             }
         }
     }