Rust Unit Test Example

tech
,

起因

一个标准的单元测试写法为:

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pub fn add(a: i32, b: i32) -> i32 {
    a + b
}

// This is a really bad adding function, its purpose is to fail in this
// example.
#[allow(dead_code)]
fn bad_add(a: i32, b: i32) -> i32 {
    a - b
}

#[cfg(test)]
mod tests {
    // Note this useful idiom: importing names from outer (for mod tests) scope.
    use super::*;

    #[test]
    fn test_add() {
        assert_eq!(add(1, 2), 3);
    }

    #[test]
    fn test_bad_add() {
        // This assert would fire and test will fail.
        // Please note, that private functions can be tested too!
        assert_eq!(bad_add(1, 2), 3);
    }
}

这里让我产生了一个疑问: 什么要在 mod test 前加 #[cfg(test)].

过程

在 How to Write Tests - The Rust Programming Language 和 Unit testing - Rust By Example 章节中都可以看到这种写法的一致性, 但是都没有说明为什么要这么写.

但是至少 Rust By Example 中提到了这样一句:

Most unit tests go into a tests mod with the #[cfg(test)] attribute . Test functions are marked with the #[test] attribute.

我们就去标准库中查看一下 std::cfg - Rust 的用法.

Evaluates boolean combinations of configuration flags at compile-time.
In addition to the #[cfg] attribute, this macro is provided to allow boolean expression evaluation of configuration flags. This frequently leads to less duplicated code.
The syntax given to this macro is the same syntax as the cfg attribute.

通过描述可以看到, #[cfg] 是在编译时用来判断是否加载其下包含代码的. 那 #[cfg(test)] 的意义也就很明显了, 即其下包含的代码只有在测试时会加载.

在 Conditional compilation - The Rust Reference 中我们可以找到答案.

结果

编写测试的时候, 应该将测试用例使用 #[cfg(test)] mod test{} 的方式包裹起来. 可以保证相关代码只有在测试时会被编译.

#tech/rust