Function deserialize_parsed 

pub fn deserialize_parsed<'de, V, M: Display, D: Deserializer<'de>>(
    deserializer: D,
    expected: &'static str,
    parse: fn(&str) -> Result<V, M>,
) -> Result<V, D::Error>

Through the parser, so a request cannot build the value without the check; a refusal is written in its alternate form, as the field being read already names the value.