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);
| ^^ ^^
```
- Point at opening mismatched formatting brace
- Account for differences between raw and regular strings
- Account for differences between the code snippet and `InternedString`
- Add more tests
When encountering format string errors in a raw string, or regular
string literal with embedded newlines, account for the positional
change to use correct spans.
:drive by fix: 🚗
On cases of malformed format strings where a `{` hasn't been properly
escaped, like `println!("{");`, present a note explaining how to escape
the `{` char.
