Auto merge of #48946 - PramodBisht:issues/48636, r=estebank

Doc comments present after a particular syntax error cause an unhelpful error message to be output.

fixed: #48636

r? @estebank

src/libsyntax/parse/parser.rs

@@ -5759,18 +5759,33 @@ impl<'a> Parser<'a> {
                                      vis: Visibility,
                                      attrs: Vec<Attribute> )
                                      -> PResult<'a, StructField> {
+        let mut seen_comma: bool = false;
         let a_var = self.parse_name_and_ty(lo, vis, attrs)?;
+        if self.token == token::Comma {
+            seen_comma = true;
+        }
         match self.token {
             token::Comma => {
                 self.bump();
             }
             token::CloseDelim(token::Brace) => {}
             token::DocComment(_) => {
+                let previous_span = self.prev_span;
                 let mut err = self.span_fatal_err(self.span, Error::UselessDocComment);
                 self.bump(); // consume the doc comment
-                if self.eat(&token::Comma) || self.token == token::CloseDelim(token::Brace) {
+                let comma_after_doc_seen = self.eat(&token::Comma);
+                // `seen_comma` is always false, because we are inside doc block
+                // condition is here to make code more readable
+                if seen_comma == false && comma_after_doc_seen == true {
+                    seen_comma = true;
+                }
+                if comma_after_doc_seen || self.token == token::CloseDelim(token::Brace) {
                     err.emit();
                 } else {
+                    if seen_comma == false {
+                        let sp = self.sess.codemap().next_point(previous_span);
+                        err.span_suggestion(sp, "missing comma here", ",".into());
+                    }
                     return Err(err);
                 }
             }

src/test/ui/issue-48636.rs

@@ -0,0 +1,17 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+struct S {
+    x: u8
+    /// The id of the parent core
+    y: u8,
+}
+//~^^^ ERROR found a documentation comment that doesn't document anything
+fn main() {}

src/test/ui/issue-48636.stderr

@@ -0,0 +1,13 @@
+error[E0585]: found a documentation comment that doesn't document anything
+  --> $DIR/issue-48636.rs:13:5
+   |
+LL |     x: u8
+   |          - help: missing comma here: `,`
+LL |     /// The id of the parent core
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: doc comments must come before what they document, maybe a comment was intended with `//`?
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0585`.