Avoid using `concat!(fmt, "\n")` to improve the diagnostics being emitted when the first `println!()` argument isn't a formatting string literal.
- Point at format string position inside the formatting string - Explain that argument names can't start with an underscore
{
On cases of malformed format strings where a `{` hasn't been properly escaped, like `println!("{");`, present a note explaining how to escape the `{` char.