关于struct中String字段的问题不理解的地方，请明白人给讲解一下，万分感谢！！！

struct User {
active: bool,
username: String,
email: String,
sign_in_count: u64,
}

fn main() {
// –snip–

let user1 = User {
    email: String::from("someone@example.com"),
    username: String::from("someusername123"),
    active: true,
    sign_in_count: 1,
};

let user2 = User {
    active: user1.active,
    username: user1.username,
    email: String::from("another@example.com"),
    sign_in_count: user1.sign_in_count,
};

}
其中user2.username使用user1.username，致使user1.username变的不可用，但user1是不可变的，这是否违背了不可变变量的准则？