Drop trait 只有一个方法：drop，它会在对象离开作用域时自动调用。Drop trait 的主要用途是释放实现该 trait 的实例所拥有的资源。
Box、Vec、String、File 和 Process 是一些实现了 Drop trait 以释放资源的类型示例。你也可以为任何自定义数据类型手动实现 Drop trait。
下面的例子在 drop 函数中添加了一个控制台打印，用于宣告它被调用的时机。
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For a more practical example, here's how the Drop trait can be used to automatically clean up temporary files when they're no longer needed:
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