Function deserialize 

pub fn deserialize<'de, D, T>(
    deserializer: D,
) -> Result<Option<T>, <D as Deserializer<'de>>::Error>
where
    D: Deserializer<'de>,
    T: FromStr,
    <T as FromStr>::Err: Display,

Deserializes a string into a T. T must implement std::str::FromStr.