Is there any methods that will help to find offset for a field in serialized struct in a byte array?

Sometimes I need to access data parts, that was written on disk and I don't want to load full object to access it's using rkyv API. So I tried to find a way to calculate offset for some field, but it's very unstable and this logic differs from structure to structure. So I wonder if there is any method that can return field offset from byte array start (for existing structure)?

For example I have struct

struct Test {
  first: u32,
  second: u64,
  third: String,
}

Is it possible to find second's offset in byte array after rkyv::to_bytes?