Function reject_if_some 

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

Deserializes an explicit null as None and rejects a non-null value.

Use with Serde’s deserialize_with. Add #[serde(default)] to accept a missing field as None; Serde handles that case without calling this function.