Test

If code does not work correctly, it is a problem that can cause all sorts of errors. With the test macro in Rust, we can run tests to ensure our code works correctly.

With this macro, we specify that a function is a test method. Then when the program is evaluated with "cargo test," the tests are executed and cargo ensures no panics occur.

Example

We have a Rust program with 3 functions—the first 2 functions are decorated with the test macro. These are test functions.

// Part 1: use test macro with assert_eq check for panics.
#[test]
fn multiply() {
    let x = 10;
    if let Ok(y) = "2".parse::<usize>() {
        let result = x * y;
        assert_eq!(result, 20);
    }
}

// Part 2: this test fails as assert_eq! panics.
#[test]
fn fail() {
    let x = 10;
    if let Ok(y) = "2".parse::<usize>() {
        let result = x * y;
        assert_eq!(result, 30);
    }
}

fn main() {
    // Part 3: if executed normally, this message is printed, and no tests run.
    println!("Done");
}
' % cargo test
running 2 tests
test multiply ... ok
test fail ... FAILED

failures:

---- fail stdout ----

thread 'fail' panicked at src/main.rs:17:9:
assertion `left == right` failed
  left: 20
 right: 30
' % cargo run
Done

Ensuring code quality is paramount in Rust programs (as it is in programs from all other languages). With the built-in test macro we can add command-line tests to ensure our logic remains correct.