review comments: modify note wording and change println

- Don't print the newline on its own to avoid the possibility of
  printing it out of order due to `stdout` locking.
- Modify wording of `concat!()` with non-literals to not mislead into
  believing that only `&str` literals are accepted.
- Add test for `concat!()` with non-literals.

src/test/ui/macros/bad-concat.rs

@@ -0,0 +1,18 @@
+// 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() {
+    let x: u32 = 42;
+    let y: f64 = 3.14;
+    let z = "foo";
+    let _ = concat!(x, y, z, "bar");
+    //~^ ERROR expected a literal
+    //~| NOTE only literals
+}

src/test/ui/macros/bad-concat.stderr

@@ -0,0 +1,10 @@
+error: expected a literal
+  --> $DIR/bad-concat.rs:15:21
+   |
+LL |     let _ = concat!(x, y, z, "bar");
+   |                     ^  ^  ^
+   |
+   = note: only literals (like `"foo"`, `42` and `3.14`) can be passed to `concat!()`
+
+error: aborting due to previous error
+

src/test/ui/macros/trace-macro.stderr

@@ -5,9 +5,8 @@ LL |     println!("Hello, World!");
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: expanding `println! { "Hello, World!" }`
-   = note: to `{ print ! ( "Hello, World!" ) ; print ! ( "/n" ) ; }`
-   = note: expanding `print! { "Hello, World!" }`
-   = note: to `$crate :: io :: _print ( format_args ! ( "Hello, World!" ) )`
-   = note: expanding `print! { "/n" }`
-   = note: to `$crate :: io :: _print ( format_args ! ( "/n" ) )`
+   = note: to `print ! ( "{}/n" , format_args ! ( "Hello, World!" ) )`
+   = note: expanding `print! { "{}/n" , format_args ! ( "Hello, World!" ) }`
+   = note: to `$crate :: io :: _print (
+           format_args ! ( "{}/n" , format_args ! ( "Hello, World!" ) ) )`