implBlog6.rs

// user_model.rs
struct UserModel { /* ... */ }
impl UserModel {
    pub fn fullname() -> String;
    // ...
}

// happy_birthday_service.rs
pub fn send_message(session_context: &SessionContext, /* ... */) {
    let message = session_context.user.happy_birthday_message();
    send_email(sesson_context.user.email, message);
}

impl UserModel {
    pub fn happy_birthday_message(&self) -> String {
        format!("Happy birthday, {} 🎉", self.fullname());
    }
}