Auto merge of #31707 - GuillaumeGomez:macro_name, r=sfackler

I'm wondering if instead of a second help message, a note would be better. I let it up to reviewers.

src/libsyntax/ext/base.rs

@@ -760,7 +760,12 @@ pub fn suggest_macro_name(&mut self,
                               err: &mut DiagnosticBuilder<'a>) {
         let names = &self.syntax_env.names;
         if let Some(suggestion) = find_best_match_for_name(names.iter(), name, None) {
-            err.fileline_help(span, &format!("did you mean `{}!`?", suggestion));
+            if suggestion != name {
+                err.fileline_help(span, &format!("did you mean `{}!`?", suggestion));
+            } else {
+                err.fileline_help(span, &format!("have you added the `#[macro_use]` on the \
+                                                  module/import?"));
+            }
         }
     }
 }

src/test/compile-fail/macro_undefined.rs

@@ -0,0 +1,25 @@
+// Copyright 2014 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.
+
+// Test macro_undefined issue
+
+mod m {
+    #[macro_export]
+    macro_rules! kl {
+        () => ()
+    }
+}
+
+fn main() {
+    k!(); //~ ERROR macro undefined: 'k!'
+          //~^ HELP did you mean `kl!`?
+    kl!(); //~ ERROR macro undefined: 'kl!'
+           //~^ HELP have you added the `#[macro_use]` on the module/import?
+}