Serde: I got lost between MapAccess, DeserializeSeed and PhantomData

I try to implement my own Serde deserializer for my non-default parsing problem. It is an educational project to sharpen my Rust skill, which means that I have real world use case, but my focus is on really understanding what's going on. I'll try to point out where I get lost in the Serde code: I have an example struct that derives Deserialize. I use cargo expand to look at the generated code and find the following call: __field0 = _serde::__private228::Some( _serde:🇩🇪:MapAccess::next_v...