Rust replace string

test.rs

    let path = Path::new("resources/graphql_queries_popular.txt");
    let path_new_file = Path::new("resources/graphql_generated_queries.txt");
    let file = File::create(path_new_file).unwrap();
    let mut file = LineWriter::new(file);

    let mut reader = my_reader::BufReader::open(&path).unwrap();
    let mut buffer = String::new();

    while let Some(line) = reader.read_line(&mut buffer) {
        use regex::{Regex,Captures};
        let re = Regex::new(r"first: \d+").unwrap();
        let mut text_for = &line;
        for n in 1..1000{
            let k = re.replace(text_for.as_ref().unwrap(), |caps: &Captures| {
                format!("first: {}", n) });
            file.write(k.as_ref().as_ref());
        }
    }

    file.flush().unwrap();