Suggest comma when writing println!("{}" a);

src/test/ui/macros/missing-comma.rs

@@ -0,0 +1,14 @@
+// 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() {
+    println!("{}" a);
+    //~^ ERROR no rules expected the token `a`
+}

src/test/ui/macros/missing-comma.stderr

@@ -0,0 +1,10 @@
+error: no rules expected the token `a`
+  --> $DIR/missing-comma.rs:12:19
+   |
+LL |     println!("{}" a);
+   |                  -^
+   |                  |
+   |                  help: missing comma here
+
+error: aborting due to previous error
+