类型会实现 From 和 Into 以加快类型转换：
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实现 From 后，系统会自动实现 Into：
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Speaker Notes
This slide should take about 10 minutes.
- 这就是为什么通常只需实现
From，因为您的类型也会实现 Into。
- 若要声明某个函数实参输入类型（例如“任何可转换成
String 的类型”），规则便会相反，此时应使用 Into。 您的函数会接受可实现 From 的类型，以及那些仅实现 Into 的类型。