Function plain_coded_error_with 

pub fn plain_coded_error_with(
    class: &str,
    code: &str,
    msg: &str,
    fields: &[(&str, &str)],
) -> String

plain_coded_error plus extra enumerable string own properties, set after code in the order given — new URL('x', 'nope') throws with Object.keys(e) reading ["code","input","base"].

Each field is key\u{3}<byte length>\u{3}value, so a value (a URL input is arbitrary user text) may carry any character, the separators included.