Today I learned how to write and run tests in the Rust programming language.

Writing a test in Rust

I'm attending a Rust tutorial at EuroPython 2023 and I just learned how to write and run tests in Rust. Turns out it is quite straightforward!

Suppose you have a function that determines whether or not a given age corresponds to an adult age in Europe (or, at least, most European countries):

fn is_adult(age: u8) -> bool {
    age >= 18
}

In order to test this function, you create test functions (like you do in pytest, for example). In pytest, a function is a test function whenever its name starts with test_. In Rust, if you add a #[test] at the top of the function definition, that function is now a test function.

Then, similarly to how the keyword assert works in Python, you can use the macro assert! to check for the test result in Rust.

Here are a couple of tests for the function is_adult, above:

#[test]
fn is_adult_is_true_for_adults() {
    assert!(is_adult(18));
    assert!(is_adult(19));
    assert!(is_adult(123));
}

#[test]
fn is_adult_is_false_for_kids() {
    assert!( !is_adult(1) );
    assert!( !is_adult(17) );
}

The macro assert! expects a Boolean value and it will fail if the Boolean is false.

I know that there is, at least, one other assertion macro in Rust. assert_eq! takes two values, compares them, and if they differ, the test fails.

Your whole file could look like this:

fn is_adult(age: u8) -> bool {
    age >= 18
}

fn main() {
    println!("What's up?");
}

#[test]
fn is_adult_is_true_for_adults() {
    assert!(is_adult(18));
    assert!(is_adult(19));
    assert!(is_adult(123));
}

#[test]
fn is_adult_is_false_for_kids() {
    assert!( !is_adult(1) );
    assert!( !is_adult(17) );
}

How to run your Rust tests

To run your Rust tests, you just use the command cargo test. That's it! If you do, you should get output like this:

That's it for now! Stay tuned and I'll see you around!

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