rust/doc/tutorial/test.md

Testing

The Rust language has a facility for testing built into the language. Tests can be interspersed with other code, and annotated with the #[test] attribute.

use std;

fn twice(x: int) -> int { x + x }

#[test]
fn test_twice() {
    let i = -100;
    while i < 100 {
        assert twice(i) == 2 * i;
        i += 1;
    }
}

When you compile the program normally, the test_twice function will not be included. To compile and run such tests, compile with the --test flag, and then run the result:

## notrust
> rustc --test twice.rs
> ./twice
running 1 tests
test test_twice ... ok
result: ok. 1 passed; 0 failed; 0 ignored

Or, if we change the file to fail, for example by replacing x + x with x + 1:

## notrust
running 1 tests
test test_twice ... FAILED
failures:
    test_twice
result: FAILED. 0 passed; 1 failed; 0 ignored

You can pass a command-line argument to a program compiled with --test to run only the tests whose name matches the given string. If we had, for example, test functions test_twice, test_once_1, and test_once_2, running our program with ./twice test_once would run the latter two, and running it with ./twice test_once_2 would run only the last.

To indicate that a test is supposed to fail instead of pass, you can give it a #[should_fail] attribute.

use std;

fn divide(a: float, b: float) -> float {
    if b == 0f { fail; }
    a / b
}

#[test]
#[should_fail]
fn divide_by_zero() { divide(1f, 0f); }

To disable a test completely, add an #[ignore] attribute. Running a test runner (the program compiled with --test) with an --ignored command-line flag will cause it to also run the tests labelled as ignored.

A program compiled as a test runner will have the configuration flag test defined, so that you can add code that won't be included in a normal compile with the #[cfg(test)] attribute (see conditional compilation).