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/libstd/macros.rs

@@ -155,14 +155,7 @@ macro_rules! print {
 #[stable(feature = "rust1", since = "1.0.0")]
 macro_rules! println {
     () => (print!("\n"));
-    ($fmt:expr) => ({
-        print!($fmt);
-        print!("\n");
-    });
-    ($fmt:expr, $($arg:tt)*) => ({
-        print!($fmt, $($arg)*);
-        print!("\n");
-    });
+    ($($arg:tt)*) => (print!("{}\n", format_args!($($arg)*)));
 }
 
 /// Macro for printing to the standard error.

src/libsyntax_ext/concat.rs

@@ -58,7 +58,7 @@ pub fn expand_syntax_ext(
     }
     if missing_literal.len() > 0 {
         let mut err = cx.struct_span_err(missing_literal, "expected a literal");
-        err.note("only `&str` literals can be passed to `concat!()`");
+        err.note("only literals (like `\"foo\"`, `42` and `3.14`) can be passed to `concat!()`");
         err.emit();
     }
     let sp = sp.apply_mark(cx.current_expansion.mark);

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!" ) ) )`