struct Sessionv1 {
    pub some_field: bool,
    pub some_field2: bool
}

enum Session<T: SessionTrait> {
    V1(Sessionv1),
    Whatever(T),
}

impl<NotFinSession> Session<NotFinSession>
where NotFinSession: SessionTrait {
    pub fn set_some_field(&self, _val: bool) {

    }

    pub fn finalize(self) -> Result<Session::<FinSession>, ()> {
        match self {
            Session::V1(v) => Ok(Session::<FinSession>::V1(v)),
            _ => Err(()),
        }

    }
}

impl<FinSession> Session<FinSession>
where FinSession: SessionTrait {
    pub fn set_some_field2(&self, _val: bool) {

    }
}

impl<FinSession> Session<FinSession>
where FinSession: SessionTrait {
}

trait SessionTrait {
}

struct FinSession {
}

impl SessionTrait for FinSession {
}

struct NotFinSession {
}

impl SessionTrait for NotFinSession {
}

pub fn main() {
    let session = Session::<NotFinSession>::V1(Sessionv1{some_field: false, some_field2: false});
    session.set_some_field(false);
    let fin_session: Session<FinSession> = session.finalize().unwrap();
    fin_session.set_some_field2(true);
}