Rust 语言学习手册

Builder 模式用于构建复杂对象，尤其当构造参数较多或可选时。 ```rust #[derive(Debug)] struct User { username: String, email: String, active: bool, sign_in_count: u64, } struct UserBuilder { username: Option, email: Option, active: bool, sign_in_count: u64, } impl UserBuilder { fn new() -> Self { UserBuilder { username: None, email: None, active: true, sign_in_count: 1 } } fn username(mut self, username: &str) -> Self { self.username = Some(username.to_string()); self } fn email(mut self, email: &str) -> Self { self.email = Some(email.to_string()); self } fn build(self) -> Result