Testing

Unit tests with #[test]

Tests are functions annotated with #[test]. They live in the same file as the code or in a tests module (often behind #[cfg(test)]).

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

    #[test]
    fn it_works() {
        assert_eq!(2 + 2, 4);
    }
}

In C#: Like xUnit/NUnit: a method with [Fact] or [Test] that runs as a test. Rust has no separate test runner process by default; cargo test compiles and runs tests.

Running tests

cargo test
cargo test -p course_15_testing
cargo test it_works   # run tests whose name contains "it_works"

In C#: Like dotnet test; cargo test <name> is like running a single test or filter.

assert! and assert_eq!

• assert!(condition) – panics if the condition is false.
• assert_eq!(left, right) – panics if left != right, and prints both values.
• assert_ne!(left, right) – panics if left == right.

In C#: Like Assert.True(condition) and Assert.Equal(expected, actual).

Tests in same file vs tests/ directory

• Same file: Put tests in a mod tests { ... } with #[cfg(test)]. Good for unit testing private helpers.
• tests/ directory: Each .rs file under tests/ is a separate crate that links your library. Good for integration tests.

In C#: Unit tests next to code vs integration test projects. Rust’s tests/ is like a separate test project that depends on your lib.

Key takeaway

Use #[test] for test functions; run them with cargo test. Use assert!, assert_eq!, and assert_ne! for checks. Use #[cfg(test)] for unit tests in the same crate and tests/ for integration tests.