Rust Tdd

Rust TDD Workflow

Process (Red-Green-Refactor)

1. Red Phase

Write a failing test first that describes the desired behavior:

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

    #[test]
    fn test_new_feature() {
        let result = new_function(input);
        assert_eq!(result, expected_output);
    }
}

Verify the test fails: cargo test -p <crate> <test_name>

2. Green Phase

Implement the minimum code to make the test pass:

• Write only enough code to pass the test
• Don't over-engineer or add extra features
• Focus on correctness, not elegance

Verify: cargo test -p <crate>

3. Refactor Phase

Improve the code while keeping tests green:

• Remove duplication
• Improve naming
• Simplify logic
• Extract functions if needed

After each change: cargo test -p <crate>

Test Commands

# Run specific test
cargo test -p <crate> <test_name> -- --nocapture

# Run all tests in crate
cargo test -p <crate> --all-features

# Run with output
cargo test -p <crate> -- --nocapture

# Run doc tests
cargo test -p <crate> --doc

# Continuous testing
cargo watch -x "test -p <crate>"

Test Patterns

Basic Test

#[test]
fn test_success_case() {
    let result = function_under_test(42);
    assert_eq!(result, 84);
}

Result-returning Test

#[test]
fn test_with_result() -> Result<(), Box<dyn std::error::Error>> {
    let result = fallible_function()?;
    assert_eq!(result, expected);
    Ok(())
}

Panic Test

#[test]
#[should_panic(expected = "index out of bounds")]
fn test_panic_case() {
    function_that_panics();
}

Async Test

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

Property-based Test (with proptest)

use proptest::prelude::*;

proptest! {
    #[test]
    fn test_property(x in 0..100i32) {
        let result = function(x);
        prop_assert!(result >= 0);
    }
}

Final Verification

After completing TDD cycle:

cargo clippy -p <crate> -- -D warnings
cargo fmt -p <crate> -- --check
cargo test -p <crate> --all-features

Nebula-specific Patterns

• Each crate has its own error type via thiserror
• Use #[tokio::test] for async tests
• Place unit tests in same file under #[cfg(test)] module
• Place integration tests in tests/ directory of the crate