Improve suggestion for missing fmt str in println

Avoid using `concat!(fmt, "\n")` to improve the diagnostics being
emitted when the first `println!()` argument isn't a formatting string
literal.

src/test/ui/macros/bad_hello.rs

@@ -9,5 +9,8 @@
 // except according to those terms.
 
 fn main() {
-    println!(3 + 4); //~ ERROR expected a literal
+    println!(3 + 4);
+    //~^ ERROR format argument must be a string literal
+    println!(3, 4);
+    //~^ ERROR format argument must be a string literal
 }

src/test/ui/macros/bad_hello.stderr

@@ -1,12 +1,22 @@
-error: expected a literal
+error: format argument must be a string literal
   --> $DIR/bad_hello.rs:12:14
    |
-LL |     println!(3 + 4); //~ ERROR expected a literal
+LL |     println!(3 + 4);
    |              ^^^^^
 help: you might be missing a string literal to format with
    |
-LL |     println!("{}", 3 + 4); //~ ERROR expected a literal
-   |              ^^^^^^^^^^^
+LL |     println!("{}", 3 + 4);
+   |              ^^^^^
 
-error: aborting due to previous error
+error: format argument must be a string literal
+  --> $DIR/bad_hello.rs:14:14
+   |
+LL |     println!(3, 4);
+   |              ^
+help: you might be missing a string literal to format with
+   |
+LL |     println!("{}, {}", 3, 4);
+   |              ^^^^^^^^^
+
+error: aborting due to 2 previous errors
 

src/test/ui/macros/format-foreign.stderr

@@ -2,12 +2,13 @@ error: multiple unused formatting arguments
   --> $DIR/format-foreign.rs:12:30
    |
 LL |     println!("%.*3$s %s!/n", "Hello,", "World", 4); //~ ERROR multiple unused formatting arguments
-   |     -------------------------^^^^^^^^--^^^^^^^--^-- multiple unused arguments in this statement
+   |              --------------  ^^^^^^^^  ^^^^^^^  ^
+   |              |
+   |              multiple missing formatting arguments
    |
    = help: `%.*3$s` should be written as `{:.2$}`
    = help: `%s` should be written as `{}`
    = note: printf formatting not supported; see the documentation for `std::fmt`
-   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
 error: argument never used
   --> $DIR/format-foreign.rs:13:29

src/test/ui/macros/format-unused-lables.stderr

@@ -2,24 +2,21 @@ error: multiple unused formatting arguments
   --> $DIR/format-unused-lables.rs:12:22
    |
 LL |     println!("Test", 123, 456, 789);
-   |     -----------------^^^--^^^--^^^-- multiple unused arguments in this statement
-   |
-   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
+   |              ------  ^^^  ^^^  ^^^
+   |              |
+   |              multiple missing formatting arguments
 
 error: multiple unused formatting arguments
   --> $DIR/format-unused-lables.rs:16:9
    |
-LL | /     println!("Test2",
-LL | |         123,  //~ ERROR multiple unused formatting arguments
-   | |         ^^^
-LL | |         456,
-   | |         ^^^
-LL | |         789
-   | |         ^^^
-LL | |     );
-   | |______- multiple unused arguments in this statement
-   |
-   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
+LL |     println!("Test2",
+   |              ------- multiple missing formatting arguments
+LL |         123,  //~ ERROR multiple unused formatting arguments
+   |         ^^^
+LL |         456,
+   |         ^^^
+LL |         789
+   |         ^^^
 
 error: named argument never used
   --> $DIR/format-unused-lables.rs:21:35
@@ -30,18 +27,18 @@ LL |     println!("Some stuff", UNUSED="args"); //~ ERROR named argument never u
 error: multiple unused formatting arguments
   --> $DIR/format-unused-lables.rs:24:9
    |
-LL | /     println!("Some more $STUFF",
-LL | |         "woo!",  //~ ERROR multiple unused formatting arguments
-   | |         ^^^^^^
-LL | |             STUFF=
-LL | |        "things"
-   | |        ^^^^^^^^
-LL | |              , UNUSED="args");
-   | |_______________________^^^^^^_- multiple unused arguments in this statement
+LL |     println!("Some more $STUFF",
+   |              ------------------ multiple missing formatting arguments
+LL |         "woo!",  //~ ERROR multiple unused formatting arguments
+   |         ^^^^^^
+LL |             STUFF=
+LL |        "things"
+   |        ^^^^^^^^
+LL |              , UNUSED="args");
+   |                       ^^^^^^
    |
    = help: `$STUFF` should be written as `{STUFF}`
    = note: shell formatting not supported; see the documentation for `std::fmt`
-   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
 error: aborting due to 4 previous errors
 

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

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