Add invalid doc comment help message

src/libsyntax/config.rs

@@ -9,7 +9,7 @@
 // except according to those terms.
 
 use attr::HasAttrs;
-use feature_gate::{emit_feature_err, EXPLAIN_STMT_ATTR_SYNTAX, Features, get_features, GateIssue};
+use feature_gate::{feature_err, EXPLAIN_STMT_ATTR_SYNTAX, Features, get_features, GateIssue};
 use {fold, attr};
 use ast;
 use codemap::{Spanned, respan};
@@ -157,11 +157,15 @@ impl<'a> StripUnconfigured<'a> {
         // flag the offending attributes
         for attr in attrs.iter() {
             if !self.features.map(|features| features.stmt_expr_attributes).unwrap_or(true) {
-                emit_feature_err(&self.sess,
+                let mut err = feature_err(&self.sess,
                                           "stmt_expr_attributes",
                                           attr.span,
                                           GateIssue::Language,
                                           EXPLAIN_STMT_ATTR_SYNTAX);
+                if attr.node.is_sugared_doc {
+                    err.help("`///` is for documentation comments. For a plain comment, use `//`.");
+                }
+                err.emit();
             }
         }
     }

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 =