name: quality-unit-testing
description: Write high-quality Rust unit tests following best practices. Use when writing new tests, reviewing test code, or improving test quality. Emphasizes clear naming, AAA pattern, isolation, and deployment confidence.

Quality Unit Testing for Rust

Write tests that catch real bugs and provide deployment confidence.

Core Principles

Quality over coverage: Tests should catch real bugs, not just boost percentages.

Quick Reference

Naming: test_<function>_<scenario>_<expected>

#[test]
fn test_process_payment_insufficient_funds_returns_error()

#[tokio::test]
async fn test_withdraw_valid_amount_decreases_balance()

AAA Pattern (Arrange-Act-Assert)

#[test]
fn test_account_withdraw_decreases_balance() {
    // Arrange
    let mut account = Account::new(100);

    // Act
    let result = account.withdraw(30);

    // Assert
    assert!(result.is_ok());
    assert_eq!(account.balance(), 70);
}

Isolation

✓ Mock: APIs, databases, file systems, external services
✗ Don't mock: Value types, pure functions, code under test

Single Responsibility

Each test verifies ONE behavior with ONE reason to fail.

Rust-Specific Patterns

// Async tests
#[tokio::test]
async fn test_async_operation() { /* ... */ }

// Result-based tests
#[test]
fn test_operation() -> anyhow::Result<()> { /* ... */ }

// Test builders
let episode = TestEpisodeBuilder::new()
    .with_task("Test task")
    .completed(true)
    .build();

// RAII cleanup
struct TestDb(TempDir);
impl Drop for TestDb { fn drop(&mut self) { /* auto cleanup */ } }

Success Metrics

✓ Deploy without manual testing
✓ Test failures pinpoint exact problems
✓ Refactoring doesn't break unrelated tests
✓ Tests run in milliseconds

Workflow

Creating Tests:
1. Understand the code behavior
2. Identify risks
3. Write failing test first (red-green-refactor)
4. Apply AAA pattern
5. Isolate dependencies
6. Verify speed (milliseconds)

Reviewing Tests:
1. Run analysis script
2. Check naming conventions
3. Ensure isolation
4. Confirm single responsibility