一个 Rust 程序（主要）由一系列语句组成
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Rust 中有几种语句。最常见的两种是声明变量绑定，以及在表达式中使用 ;
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代码块也是表达式，因此它们可以作为赋值中的值。代码块中最后一个表达式的值将被赋给诸如局部变量之类的位置表达式。然而，如果代码块的最后一个表达式以分号结尾，返回值将是 ()。
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