Function deserialize_double_option 

pub fn deserialize_double_option<'de, D, T>(
    deserializer: D,
) -> Result<Option<Option<T>>, <D as Deserializer<'de>>::Error>
where
    D: Deserializer<'de>,
    T: Deserialize<'de>,

Deserializes Option<Option<T>> distinguishing “key absent” from “key present with value null”. Pair with #[serde(default, ...)] — see the module doc for why default is required once a field opts into a custom deserialize_with.