Unit Tests

Rust and Cargo come with a simple unit test framework. Tests are marked with #[test]. Unit tests are often put in a nested tests module, using #[cfg(test)] to conditionally compile them only when building tests.

rust,editable

# // Copyright 2022 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
fn first_word(text: &str) -> &str {
    match text.find(' ') {
        Some(idx) => &text[..idx],
        None => &text,
    }
}

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

    #[test]
    fn test_empty() {
        assert_eq!(first_word(""), "");
    }

    #[test]
    fn test_single_word() {
        assert_eq!(first_word("Hello"), "Hello");
    }

    #[test]
    fn test_multiple_words() {
        assert_eq!(first_word("Hello World"), "Hello");
    }
}

This lets you unit test private helpers.
The #[cfg(test)] attribute is only active when you run cargo test.