syntax: improve message for misused inner attributes

Author: liigo

src/libsyntax/parse/attr.rs

@@ -75,6 +75,8 @@ impl<'a> ParserAttr for Parser<'a> {
                         self.span_err(span,
                                       "an inner attribute is not permitted in \
                                        this context");
+                        self.span_help(span,
+                                       "place inner attribute at the top of the module or block");
                     }
                     ast::AttrInner
                 } else {