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fn main() {
    let immutable_box = Box::new(5u32);

    println!("immutable_box contains {}", immutable_box);

    // 可变性错误
    //*immutable_box = 4;

    // *移动* box，改变所有权（和可变性）
    let mut mutable_box = immutable_box;

    println!("mutable_box contains {}", mutable_box);

    // 修改 box 的内容
    *mutable_box = 4;

    println!("mutable_box now contains {}", mutable_box);
}