Issue rust-lang#50974: Adding tests

src/test/ui/struct-duplicate-comma.rs

@@ -0,0 +1,25 @@
+// 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.
+
+// compile-flags: -Z parse-only
+
+struct Foo {
+    a: u8,
+    b: u8
+}
+
+fn main() {
+    let bar = Foo {
+        a: 0,,
+          //~^ ERROR expected identifier
+        b: 42
+    };
+}
+

src/test/ui/struct-duplicate-comma.stderr

@@ -0,0 +1,13 @@
+error: expected identifier, found `,`
+  --> $DIR/struct-duplicate-comma.rs:20:14
+   |
+LL |     let bar = Foo {
+   |               --- while parsing this struct
+LL |         a: 0,,
+   |              ^
+   |              |
+   |              expected identifier
+   |              help: remove this comma: `,`
+
+error: aborting due to previous error
+