Add a help message for deprecated #[plugin] extern crate

src/librustc/plugin/load.rs

@@ -139,6 +139,9 @@ impl<'a, 'v> Visitor<'v> for PluginLoader<'a> {
                 }
                 "plugin" => {
                     self.sess.span_err(attr.span, "#[plugin] on `extern crate` is deprecated");
+                    self.sess.span_help(attr.span, &format!("use a crate attribute instead, \
+                                                            i.e. #![plugin({})]",
+                                                            item.ident.as_str())[]);
                 }
                 "macro_use" => {
                     let names = attr.meta_item_list();

src/test/compile-fail/plugin-extern-crate-attr-deprecated.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #[plugin]  //~ ERROR #[plugin] on `extern crate` is deprecated
+//~^ HELP use a crate attribute instead, i.e. #![plugin(std)]
 extern crate std;
 
 fn main() {}