Tweak format string error to point at arguments always

Add secondary span labels with no text to make it clear when there's a
mismatch bewteen the positional arguments in a format string and the
arguments to the macro. This shouldn't affect experienced users, but it
should make it easier for newcomers to more clearly understand how
`format!()` and `println!()` are supposed to be used.

```
error: 2 positional arguments in format string, but there is 1 argument
 --> file8.rs:2:14
  |
2 |     format!("{} {}", 1);
  |              ^^ ^^   -
```

instead of

```
error: 2 positional arguments in format string, but there is 1 argument
 --> file8.rs:2:14
  |
2 |     format!("{} {}", 1);
  |              ^^ ^^
```

1 files changed, 4 insertions, 1 deletions

diff --git a/src/libsyntax_ext/format.rs b/src/libsyntax_ext/format.rs
index 45d9f79c28f..37310f46f7e 100644
--- a/src/libsyntax_ext/format.rs
+++ b/src/libsyntax_ext/format.rs
@@ -278,7 +278,7 @@ impl<'a, 'b> Context<'a, 'b> {
     /// format string.
     fn report_invalid_references(&self, numbered_position_args: bool) {
         let mut e;
-        let sp = if self.is_literal {
+        let sp = if self.is_literal { // Point at the formatting arguments.
             MultiSpan::from_spans(self.arg_spans.clone())
         } else {
             MultiSpan::from_span(self.fmtsp)
@@ -304,6 +304,9 @@ impl<'a, 'b> Context<'a, 'b> {
                     self.describe_num_args(),
                 ),
             );
+            for arg in &self.args { // Point at the arguments that will be formatted.
+                e.span_label(arg.span, "");
+            }
         } else {
             let (mut refs, spans): (Vec<_>, Vec<_>) = refs.unzip();
             // Avoid `invalid reference to positional arguments 7 and 7 (there is 1 argument)`