Provide missing comma in match arm suggestion

When finding:

```rust
match &Some(3) {
    &None => 1
    &Some(2) => { 3 }
    _ => 2
}
```

provide the following diagnostic:

```
error: expected one of `,`, `.`, `?`, `}`, or an operator, found `=>`
 --> $DIR/missing-comma-in-match.rs:15:18
  |
X |         &None => 1
  |               --  - help: missing comma
  |               |
  |               while parsing the match arm starting here
X |         &Some(2) => { 3 }
  |                  ^^ expected one of `,`, `.`, `?`, `}`, or an operator here
```

Author: estebank

src/libsyntax/parse/parser.rs

@@ -3404,14 +3404,48 @@ impl<'a> Parser<'a> {
         } else {
             None
         };
+        let arrow_span = self.span;
         self.expect(&token::FatArrow)?;
-        let expr = self.parse_expr_res(Restrictions::STMT_EXPR, None)?;
+        let arm_start_span = self.span;
+
+        let expr = self.parse_expr_res(Restrictions::STMT_EXPR, None)
+            .map_err(|mut err| {
+                err.span_label(arrow_span, "while parsing the match arm starting here");
+                err
+            })?;
 
         let require_comma = classify::expr_requires_semi_to_be_stmt(&expr)
             && self.token != token::CloseDelim(token::Brace);
 
         if require_comma {
-            self.expect_one_of(&[token::Comma], &[token::CloseDelim(token::Brace)])?;
+            let cm = self.sess.codemap();
+            self.expect_one_of(&[token::Comma], &[token::CloseDelim(token::Brace)])
+                .map_err(|mut err| {
+                    err.span_label(arrow_span, "while parsing the match arm starting here");
+                    match (cm.span_to_lines(expr.span), cm.span_to_lines(arm_start_span)) {
+                        (Ok(ref expr_lines), Ok(ref arm_start_lines))
+                        if arm_start_lines.lines[0].end_col == expr_lines.lines[0].end_col
+                            && expr_lines.lines.len() == 2
+                            && self.token == token::FatArrow => {
+                            // We check wether there's any trailing code in the parse span, if there
+                            // isn't, we very likely have the following:
+                            //
+                            // X |     &Y => "y"
+                            //   |        --    - missing comma
+                            //   |        |
+                            //   |        arrow_span
+                            // X |     &X => "x"
+                            //   |      - ^^ self.span
+                            //   |      |
+                            //   |      parsed until here as `"y" & X`
+                            err.span_suggestion_short(cm.next_point(arm_start_span),
+                                                      "missing a comma here to end this match arm",
+                                                      ",".to_owned());
+                        }
+                        _ => {}
+                    }
+                    err
+                })?;
         } else {
             self.eat(&token::Comma);
         }

src/test/ui/suggestions/missing-comma-in-match.rs

@@ -0,0 +1,20 @@
+// 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.
+
+fn main() {
+    match &Some(3) {
+        &None => 1
+        &Some(2) => { 3 }
+        //~^ ERROR expected one of `,`, `.`, `?`, `}`, or an operator, found `=>`
+        //~| NOTE expected one of `,`, `.`, `?`, `}`, or an operator here
+        //~^^^^ NOTE while parsing the match arm starting here
+        _ => 2
+    };
+}

src/test/ui/suggestions/missing-comma-in-match.stderr

@@ -0,0 +1,12 @@
+error: expected one of `,`, `.`, `?`, `}`, or an operator, found `=>`
+  --> $DIR/missing-comma-in-match.rs:14:18
+   |
+13 |         &None => 1
+   |               --  - help: missing a comma here to end this match arm
+   |               |
+   |               while parsing the match arm starting here
+14 |         &Some(2) => { 3 }
+   |                  ^^ expected one of `,`, `.`, `?`, `}`, or an operator here
+
+error: aborting due to previous error
+