Rust Testing

Comprehensive guide for testing Rust code effectively.

Quick Start

Write unit tests inline with your code:

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

    #[test]
    fn it_works() {
        let result = add(2, 2);
        assert_eq!(result, 4);
    }

    #[test]
    #[should_panic(expected = "divide by zero")]
    fn test_panic() {
        divide(10, 0);
    }
}

Run tests with:

cargo test              # Run all tests
cargo test test_name    # Run specific test
cargo test -- --nocapture  # Show println output

Test Organization

Unit tests - In the same file as the code:

• •Use #[cfg(test)] module
• •Test private functions
• •Fast, isolated

Integration tests - In tests/ directory:

• •Test public API only
• •Each file is a separate crate
• •Example: tests/integration_test.rs

Doc tests - In documentation comments:

/// Adds two numbers
///
/// ```
/// assert_eq!(add(2, 2), 4);
/// ```
pub fn add(a: i32, b: i32) -> i32 {
    a + b
}

Common Patterns

Testing Results:

#[test]
fn test_result() -> Result<(), String> {
    let value = parse_config("config.toml")?;
    assert_eq!(value.port, 8080);
    Ok(())
}

Testing with setup/teardown:

use tempfile::TempDir;

#[test]
fn test_with_cleanup() {
    let dir = TempDir::new().unwrap();
    // Test with temp directory
    // Automatically cleaned up on drop
}

Async tests (with tokio):

#[tokio::test]
async fn test_async() {
    let result = fetch_data().await;
    assert!(result.is_ok());
}

Best Practices

1. •One assertion per test - Makes failures clear
2. •Test edge cases - Empty inputs, boundaries, errors
3. •Use descriptive names - test_parse_empty_string_returns_error
4. •Avoid test interdependence - Each test should be isolated
5. •Mock external dependencies - Use traits for dependency injection

Useful Attributes

• •#[test] - Mark as test function
• •#[should_panic] - Expect panic
• •#[ignore] - Skip by default (run with cargo test -- --ignored)
• •#[cfg(test)] - Compile only in test mode

Coverage

Generate coverage with cargo-tarpaulin:

cargo install cargo-tarpaulin
cargo tarpaulin --out Html

Aim for >80% coverage on critical paths.