rust-testing — Claude skill: Rust tests the TDD way

---
name: rust-testing
description: Rust testing patterns including unit tests, integration tests, async testing, property-based testing, mocking, and coverage. Follows TDD methodology. Use when writing Rust tests — unit, integration, async, property-based, or coverage.
metadata:
  origin: ECC
---

# Rust Testing Patterns

Comprehensive Rust testing patterns for writing reliable, maintainable tests following TDD methodology.

## When to Use

- Writing new Rust functions, methods, or traits
- Adding test coverage to existing code
- Creating benchmarks for performance-critical code
- Implementing property-based tests for input validation
- Following TDD workflow in Rust projects

## How It Works

1. **Identify target code** — Find the function, trait, or module to test
2. **Write a test** — Use `#[test]` in a `#[cfg(test)]` module, rstest for parameterized tests, or proptest for property-based tests
3. **Mock dependencies** — Use mockall to isolate the unit under test
4. **Run tests (RED)** — Verify the test fails with the expected error
5. **Implement (GREEN)** — Write minimal code to pass
6. **Refactor** — Improve while keeping tests green
7. **Check coverage** — Use cargo-llvm-cov, target 80%+

## TDD Workflow for Rust

### The RED-GREEN-REFACTOR Cycle

```
RED     → Write a failing test first
GREEN   → Write minimal code to pass the test
REFACTOR → Improve code while keeping tests green
REPEAT  → Continue with next requirement
```

### Step-by-Step TDD in Rust

```rust
// RED: Write test first, use todo!() as placeholder
pub fn add(a: i32, b: i32) -> i32 { todo!() }

#[cfg(test)]
mod tests {
    use super::*;
    #[test]
    fn test_add() { assert_eq!(add(2, 3), 5); }
}
// cargo test → panics at 'not yet implemented'
```

```rust
// GREEN: Replace todo!() with minimal implementation
pub fn add(a: i32, b: i32) -> i32 { a + b }
// cargo test → PASS, then REFACTOR while keeping tests green
```

## Unit Tests

### Module-Level Test Organization

```rust
// src/user.rs
pub struct User {
    pub name: String,
    pub email: String,
}

impl User {
    pub fn new(name: impl Into<String>, email: impl Into<String>) -> Result<Self, String> {
        let email = email.into();
        if !email.contains('@') {
            return Err(format!("invalid email: {email}"));
        }
        Ok(Self { name: name.into(), email })
    }

    pub fn display_name(&self) -> &str {
        &self.name
    }
}

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

    #[test]
    fn creates_user_with_valid_email() {
        let user = User::new("Alice", "[email protected]").unwrap();
        assert_eq!(user.display_name(), "Alice");
        assert_eq!(user.email, "[email protected]");
    }

    #[test]
    fn rejects_invalid_email() {
        let result = User::new("Bob", "not-an-email");
        assert!(result.is_err());
        assert!(result.unwrap_err().contains("invalid email"));
    }
}
```

### Assertion Macros

```rust
assert_eq!(2 + 2, 4);                                    // Equality
assert_ne!(2 + 2, 5);                                    // Inequality
assert!(vec![1, 2, 3].contains(&2));                     // Boolean
assert_eq!(value, 42, "expected 42 but got {value}");    // Custom message
assert!((0.1_f64 + 0.2 - 0.3).abs() < f64::EPSILON);   // Float comparison
```

## Error and Panic Testing

### Testing `Result` Returns

```rust
#[test]
fn parse_returns_error_for_invalid_input() {
    let result = parse_config("}{invalid");
    assert!(result.is_err());

    // Assert specific error variant
    let err = result.unwrap_err();
    assert!(matches!(err, ConfigError::ParseError(_)));
}

#[test]
fn parse_succeeds_for_valid_input() -> Result<(), Box<dyn std::error::Error>> {
    let config = parse_config(r#"{"port": 8080}"#)?;
    assert_eq!(config.port, 8080);
    Ok(()) // Test fails if any ? returns Err
}
```

### Testing Panics

```rust
#[test]
#[should_panic]
fn panics_on_empty_input() {
    process(&[]);
}

#[test]
#[should_panic(expected = "index out of bounds")]
fn panics_with_specific_message() {
    let v: Vec<i32> = vec![];
    let _ = v[0];
}
```

## Integration Tests

### File Structure

```text
my_crate/
├── src/
│   └── lib.rs
├── tests/              # Integration tests
│   ├── api_test.rs     # Each file is a separate test binary
│   ├── db_test.rs
│   └── common/         # Shared test utilities
│       └── mod.rs
```

### Writing Integration Tests

```rust
// tests/api_test.rs
use my_crate::{App, Config};

#[test]
fn full_request_lifecycle() {
    let config = Config::test_default();
    let app = App::new(config);

    let response = app.handle_request("/health");
    assert_eq!(response.status, 200);
    assert_eq!(response.body, "OK");
}
```

## Async Tests

### With Tokio

```rust
#[tokio::test]
async fn fetches_data_successfully() {
    let client = TestClient::new().await;
    let result = client.get("/data").await;
    assert!(result.is_ok());
    assert_eq!(result.unwrap().items.len(), 3);
}

#[tokio::test]
async fn handles_timeout() {
    use std::time::Duration;
    let result = tokio::time::timeout(
        Duration::from_millis(100),
        slow_operation(),
    ).await;

    assert!(result.is_err(), "should have timed out");
}
```

## Test Organization Patterns

### Parameterized Tests with `rstest`

```rust
use rstest::{rstest, fixture};

#[rstest]
#[case("hello", 5)]
#[case("", 0)]
#[case("rust", 4)]
fn test_string_length(#[case] input: &str, #[case] expected: usize) {
    assert_eq!(input.len(), expected);
}

// Fixtures
#[fixture]
fn test_db() -> TestDb {
    TestDb::new_in_memory()
}

#[rstest]
fn test_insert(test_db: TestDb) {
    test_db.insert("key", "value");
    assert_eq!(test_db.get("key"), Some("value".into()));
}
```

### Test Helpers

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

    /// Creates a test user with sensible defaults.
    fn make_user(name: &str) -> User {
        User::new(name, &format!("{name}@test.com")).unwrap()
    }

    #[test]
    fn user_display() {
        let user = make_user("alice");
        assert_eq!(user.display_name(), "alice");
    }
}
```

## Property-Based Testing with `proptest`

### Basic Property Tests

```rust
use proptest::prelude::*;

proptest! {
    #[test]
    fn encode_decode_roundtrip(input in ".*") {
        let encoded = encode(&input);
        let decoded = decode(&encoded).unwrap();
        assert_eq!(input, decoded);
    }

    #[test]
    fn sort_preserves_length(mut vec in prop::collection::vec(any::<i32>(), 0..100)) {
        let original_len = vec.len();
        vec.sort();
        assert_eq!(vec.len(), original_len);
    }

    #[test]
    fn sort_produces_ordered_output(mut vec in prop::collection::vec(any::<i32>(), 0..100)) {
        vec.sort();
        for window in vec.windows(2) {
            assert!(window[0] <= window[1]);
        }
    }
}
```

### Custom Strategies

```rust
use proptest::prelude::*;

fn valid_email() -> impl Strategy<Value = String> {
    ("[a-z]{1,10}", "[a-z]{1,5}")
        .prop_map(|(user, domain)| format!("{user}@{domain}.com"))
}

proptest! {
    #[test]
    fn accepts_valid_emails(email in valid_email()) {
        assert!(User::new("Test", &email).is_ok());
    }
}
```

## Mocking with `mockall`

### Trait-Based Mocking

```rust
use mockall::{automock, predicate::eq};

#[automock]
trait UserRepository {
    fn find_by_id(&self, id: u64) -> Option<User>;
    fn save(&self, user: &User) -> Result<(), StorageError>;
}

#[test]
fn service_returns_user_when_found() {
    let mut mock = MockUserRepository::new();
    mock.expect_find_by_id()
        .with(eq(42))
        .times(1)
        .returning(|_| Some(User { id: 42, name: "Alice".into() }));

    let service = UserService::new(Box::new(mock));
    let user = service.get_user(42).unwrap();
    assert_eq!(user.name, "Alice");
}

#[test]
fn service_returns_none_when_not_found() {
    let mut mock = MockUserRepository::new();
    mock.expect_find_by_id()
        .returning(|_| None);

    let service = UserService::new(Box::new(mock));
    assert!(service.get_user(99).is_none());
}
```

## Doc Tests

### Executable Documentation

```rust
/// Adds two numbers together.
///
/// # Examples
///
/// ```
/// use my_crate::add;
///
/// assert_eq!(add(2, 3), 5);
/// assert_eq!(add(-1, 1), 0);
/// ```
pub fn add(a: i32, b: i32) -> i32 {
    a + b
}

/// Parses a config string.
///
/// # Errors
///
/// Returns `Err` if the input is not valid TOML.
///
/// ```no_run
/// use my_crate::parse_config;
///
/// let config = parse_config(r#"port = 8080"#).unwrap();
/// assert_eq!(config.port, 8080);
/// ```
///
/// ```no_run
/// use my_crate::parse_config;
///
/// assert!(parse_config("}{invalid").is_err());
/// ```
pub fn parse_config(input: &str) -> Result<Config, ParseError> {
    todo!()
}
```

## Benchmarking with Criterion

```toml
# Cargo.toml
[dev-dependencies]
criterion = { version = "0.5", features = ["html_reports"] }

[[bench]]
name = "benchmark"
harness = false
```

```rust
// benches/benchmark.rs
use criterion::{black_box, criterion_group, criterion_main, Criterion};

fn fibonacci(n: u64) -> u64 {
    match n {
        0 | 1 => n,
        _ => fibonacci(n - 1) + fibonacci(n - 2),
    }
}

fn bench_fibonacci(c: &mut Criterion) {
    c.bench_function("fib 20", |b| b.iter(|| fibonacci(black_box(20))));
}

criterion_group!(benches, bench_fibonacci);
criterion_main!(benches);
```

## Test Coverage

### Running Coverage

```bash
# Install: cargo install cargo-llvm-cov (or use taiki-e/install-action in CI)
cargo llvm-cov                    # Summary
cargo llvm-cov --html             # HTML report
cargo llvm-cov --lcov > lcov.info # LCOV format for CI
cargo llvm-cov --fail-under-lines 80  # Fail if below threshold
```

### Coverage Targets

| Code Type | Target |
|-----------|--------|
| Critical business logic | 100% |
| Public API | 90%+ |
| General code | 80%+ |
| Generated / FFI bindings | Exclude |

## Testing Commands

```bash
cargo test                        # Run all tests
cargo test -- --nocapture         # Show println output
cargo test test_name              # Run tests matching pattern
cargo test --lib                  # Unit tests only
cargo test --test api_test        # Integration tests only
cargo test --doc                  # Doc tests only
cargo test --no-fail-fast         # Don't stop on first failure
cargo test -- --ignored           # Run ignored tests
```

## Best Practices

**DO:**
- Write tests FIRST (TDD)
- Use `#[cfg(test)]` modules for unit tests
- Test behavior, not implementation
- Use descriptive test names that explain the scenario
- Prefer `assert_eq!` over `assert!` for better error messages
- Use `?` in tests that return `Result` for cleaner error output
- Keep tests independent — no shared mutable state

**DON'T:**
- Use `#[should_panic]` when you can test `Result::is_err()` instead
- Mock everything — prefer integration tests when feasible
- Ignore flaky tests — fix or quarantine them
- Use `sleep()` in tests — use channels, barriers, or `tokio::time::pause()`
- Skip error path testing

## CI Integration

```yaml
# GitHub Actions
test:
  runs-on: ubuntu-latest
  steps:
    - uses: actions/checkout@v4
    - uses: dtolnay/rust-toolchain@stable
      with:
        components: clippy, rustfmt

    - name: Check formatting
      run: cargo fmt --check

    - name: Clippy
      run: cargo clippy -- -D warnings

    - name: Run tests
      run: cargo test

    - uses: taiki-e/install-action@cargo-llvm-cov

    - name: Coverage
      run: cargo llvm-cov --fail-under-lines 80
```

**Remember**: Tests are documentation. They show how your code is meant to be used. Write them clearly and keep them up to date.