Add span for bad doc comment

src/libsyntax/parse/parser.rs

@@ -4483,13 +4483,17 @@ impl<'a> Parser<'a> {
     }
 
     /// Emit an expected item after attributes error.
-    fn expected_item_err(&self, attrs: &[Attribute]) {
+    fn expected_item_err(&mut self, attrs: &[Attribute]) -> PResult<'a,  ()> {
         let message = match attrs.last() {
             Some(&Attribute { is_sugared_doc: true, .. }) => "expected item after doc comment",
             _ => "expected item after attributes",
         };
 
-        self.span_err(self.prev_span, message);
+        let mut err = self.diagnostic().struct_span_err(self.prev_span, message);
+        if attrs.last().unwrap().is_sugared_doc {
+            err.span_label(self.prev_span, "this doc comment doesn't document anything");
+        }
+        Err(err)
     }
 
     /// Parse a statement. This stops just before trailing semicolons on everything but items.
@@ -7636,7 +7640,7 @@ impl<'a> Parser<'a> {
             }
             None => {
                 if !attrs.is_empty()  {
-                    self.expected_item_err(&attrs);
+                    self.expected_item_err(&attrs)?;
                 }
 
                 self.unexpected()
@@ -7699,7 +7703,7 @@ impl<'a> Parser<'a> {
         }
 
         if !attributes_allowed && !attrs.is_empty() {
-            self.expected_item_err(&attrs);
+            self.expected_item_err(&attrs)?;
         }
         Ok(None)
     }

src/test/ui/parser/doc-before-eof.stderr

@@ -2,7 +2,7 @@ error: expected item after doc comment
   --> $DIR/doc-before-eof.rs:3:1
    |
 LL | /// hi //~ERROR expected item after doc comment
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this doc comment doesn't document anything
 
 error: aborting due to previous error
 

src/test/ui/parser/doc-before-extern-rbrace.stderr

@@ -2,7 +2,7 @@ error: expected item after doc comment
   --> $DIR/doc-before-extern-rbrace.rs:2:5
    |
 LL |     /// hi
-   |     ^^^^^^
+   |     ^^^^^^ this doc comment doesn't document anything
 
 error: aborting due to previous error
 

src/test/ui/parser/doc-before-mod-rbrace.stderr

@@ -2,7 +2,7 @@ error: expected item after doc comment
   --> $DIR/doc-before-mod-rbrace.rs:4:5
    |
 LL |     /// document
-   |     ^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^ this doc comment doesn't document anything
 
 error: aborting due to previous error