Add invalid doc comment help message

src/libsyntax/feature_gate.rs

@@ -30,7 +30,7 @@ use ast::{self, NodeId, PatKind};
 use attr;
 use codemap::{CodeMap, Spanned};
 use syntax_pos::Span;
-use errors::Handler;
+use errors::{DiagnosticBuilder, Handler};
 use visit::{self, FnKind, Visitor};
 use parse::ParseSess;
 use parse::token::InternedString;
@@ -792,6 +792,11 @@ pub enum GateIssue {
 
 pub fn emit_feature_err(sess: &ParseSess, feature: &str, span: Span, issue: GateIssue,
                         explain: &str) {
+    feature_err(sess, feature, span, issue, explain).emit();
+}
+
+pub fn feature_err<'a>(sess: &'a ParseSess, feature: &str, span: Span, issue: GateIssue,
+                   explain: &str) -> DiagnosticBuilder<'a> {
     let diag = &sess.span_diagnostic;
 
     let issue = match issue {
@@ -812,7 +817,7 @@ pub fn emit_feature_err(sess: &ParseSess, feature: &str, span: Span, issue: Gate
                           feature));
     }
 
-    err.emit();
+    err
 }
 
 const EXPLAIN_BOX_SYNTAX: &'static str =