enum 关键字允许创建一个从数个不同取值中选其一的枚举类型（enumeration）。任何一个在 struct 中合法的取值在 enum 中也合法。
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若使用类型别名，则可以通过其别名引用每个枚举变量。当枚举的名称太长或者太一般化，且你想要对其重命名，那么这对你会有所帮助。
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最常见的情况就是在 impl 块中使用 Self 别名。
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该功能已在 Rust 中稳定下来， 可以阅读 stabilization report 来了解更多有关枚举和类型别名的知识。
match, fn, 和 String, “类型别名枚举变量” 的 RFC