Fixes #5998

Background

(1) Comment without arrow =>

fn main() {
    match a {
        _ =>
        // comment with ==
                {
            println!("A")
        }
    }
}

(2) Comment without arrow =>

fn main() {
    match a {
        _ =>
        // comment with =>
                {
            println!("A")
        }
    }
}

Previously, second example fails to reformat unlike the first example. The cause was rustfmt returns original code snippet when comment is lost after rewrite. When rewrite_match_body, there's logic to check if comment exists between => and match body, but it tries to find another => in arrow_snippet considering the case with nested match. Since => in comment was also caught with rfind(), the comment was lost, which led to silent failure.

Solution

Add extra check whether found => was included in single line comment.

TODO

• add more test case and verify
• think for any other alternative approach