Match arms with comments exceed max width

E.g. from librustc_mir/borrow_check.rs:

Output

fn append_lvalue_to_string(lvalue: &Lvalue, buf: &mut String) {
    match *lvalue {
        Lvalue::Projection(ref proj) => {
            let (prefix, suffix, index_operand) = match proj.elem {
                ProjectionElem::Downcast(..) => ("", format!(""), None), // (dont emit downcast info)
                ProjectionElem::Field(field, _ty) => ("", format!(".{}", field.index()), None), // FIXME: report name of field
            };
        }
    }
}

Expected

fn append_lvalue_to_string(lvalue: &Lvalue, buf: &mut String) {
    match *lvalue {
        Lvalue::Projection(ref proj) => {
            let (prefix, suffix, index_operand) = match proj.elem {
                ProjectionElem::Downcast(..) => {
                    ("", format!(""), None) // (dont emit downcast info)
                }
                ProjectionElem::Field(field, _ty) => {
                    ("", format!(".{}", field.index()), None) // FIXME: report name of field
                }
            };
        }
    }
}

Start with the Rust example in librustc_mir/borrow_check.rs and reproduce how rustfmt handles the match arms and their trailing comments. Trace the formatter behavior for comments and line width, then verify that the output matches the issue's expected multiline form without exceeding the configured maximum width.