DRY(不要重复自己)

宏通过提取函数和/或测试套件的共同部分，使我们能够编写符合 DRY（Don't Repeat Yourself）原则的代码。下面是一个在 Vec<T> 上实现并测试 +=、*= 和 -= 运算符的示例：

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$ rustc --test dry.rs && ./dry
running 3 tests
test test::mul_assign ... ok
test test::add_assign ... ok
test test::sub_assign ... ok

test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured