Please insert this text on page 215 between the last two paragraphs before the “Controlling how tests are run” section.

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Using Result<T, E> in Tests

So far, we’ve written tests that panic when they fail. We can also write tests that use Result<T, E>! Here’s the test from Listing 11-1, rewritten to use Result<T, E> and return an Err instead of panicking:

#[cfg(test)]
mod tests {
    #[test]
    fn it_works() -> Result<(), String> {
        if 2 + 2 == 4 {
            Ok(())
        } else {
            Err(String::from("two plus two does not equal four"))
        }
    }
}

The it_works function now has a return type, Result<(), String>. In the body of the function, rather than calling the assert_eq! macro, we return Ok(()) when the test passes and an Err with a String inside when the test fails.

Writing tests so they return a Result<T, E> enables you to use the question mark operator in the body of tests, which can be a convenient way to write tests that should fail if any operation within them returns an Err variant.

You can’t use the #[should_panic] annotation on tests that use Result<T, E>. Instead, you should return an Err value directly when the test should fail.