11.4 用should_panic检查恐慌

11.4.1. 验证错误处理的情况

除了验证代码是否返回正确的值，测试还需要验证代码是否如预期处理了错误情况。例如，可以编写一个测试来验证代码是否在特定条件下发生恐慌。

这类测试需要额外的should_panic属性。用它标记的函数，如果函数内发生了恐慌，测试就通过；否则就失败。

看个例子：

#![allow(unused)]
fn main() {
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有一个类型为i32的字段value。它提供关联函数new用于创建Guess实例，但前提是传给new的参数在1到100之间；否则就会恐慌。
• greater_than_100测试函数给new传入大于100的值。这时应该发生恐慌，所以给这个测试函数加了should_panic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 0.12s
     Running unittests src/lib.rs (target/debug/deps/guessing_game-bdc6b9a45c563cc0)

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

现在故意引入一个bug，把new里的value > 100检查去掉：

#![allow(unused)]
fn main() {
pub struct Guess {
    value: i32,
}

impl Guess {
    pub fn new(value: i32) -> Guess {
        if value < 1 {
            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);
    }
}
}

这时测试函数中的Guess::new(200);不会恐慌。但因为函数加了should_panic标记，本应恐慌却没有恐慌的测试就会失败：

$ cargo test
   Compiling guessing_game v0.1.0 (file:///projects/guessing_game)
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.09s
     Running unittests src/lib.rs (target/debug/deps/guessing_game-bdc6b9a45c563cc0)

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

failures:

---- tests::greater_than_100 stdout ----
note: test did not panic as expected at src/lib.rs:21:8

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

有时使用should_panic的测试会有点含糊，因为它们只能说明代码是否发生了恐慌，即使这个恐慌并不是程序员预期的那个。

为了让测试更精确，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指定的文字。

看个例子：

#![allow(unused)]
fn main() {
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);
    }
}
}

• 上面的结构体稍作了修改：new里value < 1和value > 100的情况现在使用两条不同的恐慌信息。
• 给should_panic添加了expected参数，=后面的文字就是期待的错误信息。只有函数发生了恐慌，并且恐慌信息包含期待的文字，测试才通过；否则就失败。

这个程序肯定能通过。

用同样的套路手动引入错误。例如，把new里小于1和大于100时的恐慌信息交换一下：

#![allow(unused)]
fn main() {
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 0.08s
     Running unittests src/lib.rs (target/debug/deps/guessing_game-bdc6b9a45c563cc0)

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

failures:

---- tests::greater_than_100 stdout ----

thread 'tests::greater_than_100' (457156) 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`

失败信息表明测试确实发生了恐慌，但恐慌消息没有包含期待的字符串less than or equal to 100。在这种情况下，我们实际收到的恐慌信息是Guess value must be greater than or equal to 1, got 200.。根据这些信息就足以修复这个bug。