impl<''de> Deserializer<'de> for MyDeserializer {
    // ...
    fn deserialize_struct<V: Visitor<'de>>(name: &'static str, fields: &'static [&'static str], visitor: V) -> Result<V::Value, Self::Error> {
        if serde_spanned::is_spanned(name, fields) {
            serde_spanned::emit_spanned(self, self.span)
        } else {
            // normal deserialization here
        }
    }
}