From 93417fa71ee73ddbb2e5c531e79ba39ec45beb5c Mon Sep 17 00:00:00 2001
From: Guillaume Gomez <guillaume1.gomez@gmail.com>
Date: Tue, 4 Oct 2016 19:10:33 +0200
Subject: [PATCH] Add invalid doc comment help message

---
 src/libsyntax/config.rs       | 16 ++++++++++------
 src/libsyntax/feature_gate.rs |  9 +++++++--
 2 files changed, 17 insertions(+), 8 deletions(-)

diff --git a/src/libsyntax/config.rs b/src/libsyntax/config.rs
index 3b81ea4917f2b..01f81e5e2de17 100644
--- a/src/libsyntax/config.rs
+++ b/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,
-                                 "stmt_expr_attributes",
-                                 attr.span,
-                                 GateIssue::Language,
-                                 EXPLAIN_STMT_ATTR_SYNTAX);
+                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();
             }
         }
     }
diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 62b88888fc878..2e7a706a583f1 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/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 =