Writing Automated Tests in Rust

How to write tests

In Rust, tests are functions that verify the expected behavior of non-test code. To create a test function, add the #[test] attribute before the fn declaration.

#[cfg(test)]
mod tests {
    // ...
}

The anatomy of a test function

A typical test function performs three actions:

1. Set up: Initialize any necessary data or state.
2. Run the code under test: Execute the code being tested.
3. Assert the results: Verify that the expected outcome occurs.

Checking Results with assert!

The assert! macro is used to verify that a condition is true. If the condition is false, the test will fail and print an error message.

#[test]
fn greeting_contains_name() {
    let result = greeting("Carol");
    assert!(result.contains("Carol"), "Greeting did not contain name, value was `{}`", result);
}

Testing equality with assert_eq! and assert_ne!

The assert_eq! macro checks that two values are equal. The assert_ne! macro checks that two values are not equal.

#[test]
fn adder_test() {
    let result = add(2, 3);
    assert_eq!(result, 5);
}

Adding custom failure messages

When a test fails, it's helpful to provide a descriptive error message. You can create custom failure messages using the assert! macro.

#[test]
fn greeting_contains_name() {
    let result = greeting("Carol");
    assert!(result.contains("Carol"), "Greeting did not contain name, value was `{}`", result);
}

Checking for panics with should_panic

The should_panic attribute is used to test that a function panics under certain conditions.

#[test]
#[should_panic]
fn create_guess_with_invalid_value() {
    Guess::new(101);
}

Example code and output

Here's an example of testing the greeting function:

pub fn greeting(name: &str) -> String {
    format!("Hello, {}!", name)
}

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

    #[test]
    fn greeting_contains_name() {
        let result = greeting("Carol");
        assert!(result.contains("Carol"), "Greeting did not contain name, value was `{}`", result);
    }
}

Output

When you run the test with cargo test, you'll see the following output:

running 1 test

test tests::greeting_contains_name ... FAILED

failures:
    tests::greeting_contains_name

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

This output indicates that the test has failed, and provides a descriptive error message.

I hope this helps! Let me know if you have any questions or need further clarification.