Building reliable systems in Rust demands thorough testing. I’ve found that combining multiple strategies creates a robust safety net. Here’s how I approach testing across different system layers:

Mocking Dependencies with Trait Objects
Testing components in isolation requires replacing real dependencies. I define traits for external services and create test implementations. This approach maintains type safety while allowing controlled test scenarios.

trait PaymentProcessor {
    fn charge(&self, amount: f64) -> Result<(), String>;
}

struct MockProcessor;
impl PaymentProcessor for MockProcessor {
    fn charge(&self, _: f64) -> Result<(), String> {
        Ok(()) // Always succeeds in tests
    }
}

struct ProductionProcessor;
impl PaymentProcessor for ProductionProcessor {
    fn charge(&self, amount: f64) -> Result<(), String> {
        // Actual payment gateway integration
    }
}

#[test]
fn test_order_processing() {
    let processor = MockProcessor;
    let order = Order::new(processor);
    assert!(order.process(100.0).is_ok());
}

Property-Based Testing
For critical algorithms, I validate mathematical properties with generated inputs. The proptest crate helps me test edge cases I might overlook.

proptest! {
    #[test]
    fn string_reversal_identity(s in ".*") {
        let reversed = s.chars().rev().collect::<String>();
        let double_reversed = reversed.chars().rev().collect::<String>();
        prop_assert_eq!(s, double_reversed);
    }
}

Fuzz Testing with Arbitrary Data
Security-sensitive parsers benefit from random input testing. I use libFuzzer through cargo-fuzz to expose panic scenarios.

#[fuzz]
fn test_image_parser(data: &[u8]) {
    if let Ok(img) = Image::parse(data) {
        assert!(!img.dimensions().is_empty());
    }
}

Concurrency Testing with Loom
Testing thread interactions requires exploring execution permutations. Loom’s model checker helps verify synchronization primitives.

loom::model(|| {
    let lock = Arc::new(Mutex::new(0));
    let lock_clone = Arc::clone(&lock);

    let t1 = loom::thread::spawn(move || {
        let mut guard = lock_clone.lock().unwrap();
        *guard += 1;
    });

    let t2 = loom::thread::spawn(move || {
        let mut guard = lock.lock().unwrap();
        *guard += 1;
    });

    t1.join().unwrap();
    t2.join().unwrap();
    assert_eq!(*lock.lock().unwrap(), 2);
});

Golden File Testing
When maintaining output formats, I compare against known-good examples. This catches unintended formatting changes.

#[test]
fn generate_config_template() {
    let config = Config::default();
    let output = config.generate();
    let expected = fs::read_to_string("tests/golden/config.toml").unwrap();
    assert_eq!(output, expected);
}

Benchmarking Critical Paths
Performance validation requires precise measurement. Criterion.rs provides statistical rigor for optimization work.

fn bench_compression(c: &mut Criterion) {
    let data = vec![0u8; 10_000];
    c.bench_function("zstd_compress", |b| {
        b.iter(|| zstd::encode(&data, 3).unwrap())
    });
}

Error Injection Testing
Resilient systems handle failures gracefully. I implement fault-injecting versions of traits to test recovery paths.

struct FlakyNetwork {
    failure_rate: f32,
}

impl NetworkService for FlakyNetwork {
    fn send(&self, _: &[u8]) -> Result<(), IoError> {
        if rand::random::<f32>() < self.failure_rate {
            Err(IoError::new(ErrorKind::ConnectionAborted, "simulated"))
        } else {
            Ok(())
        }
    }
}

#[test]
fn test_retry_mechanism() {
    let net = FlakyNetwork { failure_rate: 0.7 };
    let client = Client::new(net);
    assert!(client.send_with_retries(b"data", 5).is_ok());
}

Contract Testing with Consumer-Driven Pacts
For microservices, I verify API agreements using Pact. This prevents integration breakages.

#[tokio::test]
async fn test_auth_service_contract() {
    let pact = PactBuilder::new("web_frontend", "auth_service")
        .interaction("valid login", |mut i| async {
            i.request.post().path("/login")
                .json_body(json!({"user": "admin", "pass": "secret"}));
            i.response.ok().json_body(json!({"token": "abc123"}));
            i
        })
        .build();
    pact.verify().await;
}

Each strategy targets specific failure modes. Mocking isolates components, property tests verify invariants, fuzzing exposes input handling flaws, and Loom checks concurrency logic. Golden files preserve output stability, benchmarks maintain performance, error injection validates resilience, and contract tests ensure API compatibility. Combining these approaches provides comprehensive coverage across the testing pyramid. I start with unit tests using mocks, add property-based validation for core logic, include fuzzing for parsers, and use Loom for concurrent code. Integration tests employ golden files and contract verification, while benchmarks and fault injection cover operational aspects. This layered approach catches issues early while maintaining system reliability through changes. Remember to run tests frequently - I integrate them in CI pipelines with cargo test and specialized tool executions.