Unescaped empty braces in format string should suggest escaping them

[kpreid]

Code

fn main() {
    println!(
        "fn main() {\n\
            \n\
        }"
    );
}

Current output

error: 1 positional argument in format string, but no arguments were given
 --> src/main.rs:3:20
  |
3 |           "fn main() {\n\
  |  ____________________^
4 | |             \n\
5 | |         }"
  | |_________^

Desired output

The above, followed by

= note: if you intended to print `{`, you can escape it using `{{`

Rationale and extra context

It’s not necessarily obvious that the problem is unescaped braces, in this situation. The proposed note (which already exists for other cases) should, in my opinion, be included in the error whenever there is any kind of whitespace within the braces, which is unusual in correct code.

Other cases

println!(
        "fn main() {\n\
            println!();\n\
        }"
    );

    println!(
        "fn none() -> Option<()> {\n\
            None\n\
        }"
    );

Rust Version

1.95.0

Anything else?

@rustbot label A-fmt

[Kokoro2336]

@rustbot claim

[Kokoro2336]

@kpreid I've opened a PR for this issue. But for the 2 cases metioned in extra context, there is already another invalid format string error for them, where a note suggesting escaping { is attached, so I didn't handle them.