Unclear how to impl Deserializer::deserialize_struct for formats that are not self-describing

[lolbinarycat]

Imagine a simple binary format that only handles integers and structs with integers in them. It deserializes based on field order and does not care about names. how does such a format handle deserialize_struct?

postcard uses visit_seq, but there's no indication from the docs that such an operation is guaranteed to work, what if the struct has a manually implemented Visitor and they didn't know to implement visit_seq?