#![allow(dead_code, deprecated, unused_variables, unused_mut)]
fn stringify(x: u32) -> String {
    format!("error code: {x}")
}
fn main() {
    let x: Result<u32, u32> = Ok(2);
    assert_eq!(x.map_err(stringify), Ok(2));
    let x: Result<u32, u32> = Err(13);
    assert_eq!(x.map_err(stringify), Err("error code: 13".to_string()));
}