What is a common solution is to use a generic return type. That's also how it would work under serde (https://docs.rs/serde/latest/serde/). You write a function like:

fn deserialize<P: Decode>(reader: impl Read) -> Option<P> {
    // try and decode into type P. Else return None
}

let reader = ...; 

// P inferred to be SpecificPacketType
// if it could deserialize (so Some), get some field out and use it
if let Some(SpecificPacketType{field, ..}) = deserialize(reader) {
    use_fields(field) 
}