【Rust自学】11.4. 用should_panic检查恐慌

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11.4.1. 验证错误处理的情况

测试函数出了验证代码的返回值是否正确，还需要验证代码是否如预期的去处理了发生错误的情况。比如说可以编写一个测试来验证代码是否在特定情况下发生了panic!。

这种测试需要为函数额外增加should_panic属性。使用它标记的函数，如果在函数内发生了恐慌，则代表通过测试；反之就失败。

看个例子：

pub struct Guess {
    value: i32,
}

impl Guess {
    pub fn new(value: i32) -> Guess {
        if value < 1 || value > 100 {
            panic!("Guess value must be between 1 and 100, got {value}.");
        }

        Guess { value }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    #[should_panic]
    fn greater_than_100() {
        Guess::new(200);
    }
}

• 结构体Guess有一个存储u32类型数据的字段value，它有一个关联函数new用于创建一个Guess实例，但前提是传进new的参数大于1小于100，否则就要恐慌。
• greater_than_100这个测试函数测试给new函数传入大于100的值，这时候应该发生恐慌，所以为这个测试函数添加了一个should_panic的attribute(属性)，也就是写#[should_panic]。

测试结果：

$ cargo test
   Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.58s
     Running unittests src/lib.rs (target/debug/deps/guessing_game-57d70c3acb738f4d)

running 1 test
test tests::greater_than_100 - should panic ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

   Doc-tests guessing_game

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

下面来人为引入bug，把new函数里的value > 100的判断去掉：

pub struct Guess {
    value: i32,
}

impl Guess {
    pub fn new(value: i32) -> Guess {
        if value < 1 || value > 100 {
            panic!("Guess value must be between 1 and 100, got {value}.");
        }

        Guess { value }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    #[should_panic]
    fn greater_than_100() {
        Guess::new(200);
    }
}

这时候测试函数中的Guess::new(200);就不会恐慌，但是因为它添加了should_panic这个attribute，所以本应该恐慌的函数没有恐慌就会导致测试失败：

$ cargo test
   Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.62s
     Running unittests src/lib.rs (target/debug/deps/guessing_game-57d70c3acb738f4d)

running 1 test
test tests::greater_than_100 - should panic ... FAILED

failures:

---- tests::greater_than_100 stdout ----
note: test did not panic as expected

failures:
    tests::greater_than_100

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

error: test failed, to rerun pass `--lib`

11.4.2. 让should_panic更精确

有的时候使用should_panic进行的测试会有点含糊不清，因为它仅仅能够说明被检查的代码是否发生了恐慌，即使这个恐慌和程序员预期的恐慌不一样。

为了使测试更精确，可以为should_panic添加一个可选的expected参数。这样程序就会检查失败消息中是否包含所指定的文字。

看个例子：

pub struct Guess {
    value: i32,
}

impl Guess {
    pub fn new(value: i32) -> Guess {
        if value < 1 {
            panic!(
                "Guess value must be greater than or equal to 1, got {value}."
            );
        } else if value > 100 {
            panic!(
                "Guess value must be less than or equal to 100, got {value}."
            );
        }

        Guess { value }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    #[should_panic(expected = "less than or equal to 100")]
    fn greater_than_100() {
        Guess::new(200);
    }
}

• 在刚才的结构体上稍微进行了修改，把new函数里value < 1和value > 100的情况分开写了两个不同的恐慌信息。
• 给should_panic属性添加了expected参数，=后面跟的就是期待的报错信息。只有测试函数恐慌并且恐慌信息包括期待的报错信息才算测试成功，否则就算失败。

这个程序肯定能成功。

一样的套路，我们来手动引入错误，比如我们把new函数里小于1和大于100的恐慌信息交换一下：

pub struct Guess {
    value: i32,
}

impl Guess {
    pub fn new(value: i32) -> Guess {
        if value < 1 {
            panic!(
                "Guess value must be less than or equal to 100, got {value}."
            );
        } else if value > 100 {
            panic!(
                "Guess value must be greater than or equal to 1, got {value}."
            );
        }

        Guess { value }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    #[should_panic(expected = "less than or equal to 100")]
    fn greater_than_100() {
        Guess::new(200);
    }
}

测试结果：

$ cargo test
   Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.66s
     Running unittests src/lib.rs (target/debug/deps/guessing_game-57d70c3acb738f4d)

running 1 test
test tests::greater_than_100 - should panic ... FAILED

failures:

---- tests::greater_than_100 stdout ----
thread 'tests::greater_than_100' panicked at src/lib.rs:12:13:
Guess value must be greater than or equal to 1, got 200.
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
note: panic did not contain expected string
      panic message: `"Guess value must be greater than or equal to 1, got 200."`,
 expected substring: `"less than or equal to 100"`

failures:
    tests::greater_than_100

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

error: test failed, to rerun pass `--lib`

失败消息表明此测试确实发生了恐慌，但是恐慌消息不包含less than or equal to 100预期字符串。在这种情况下我们确实收到的恐慌信息是 Guess value must be greater than or equal to 1, got 200.。根据这个就可以纠错。