📦 deps(thirdparty): update snapshots

2026-05-29 08:33:53 +00:00
parent fdb52f1e96
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+# Test Quality Metrics
+
+Comprehensive guide to measuring and improving test quality through coverage and mutation testing.
+
+## Coverage Metrics
+
+### Line Coverage
+
+Percentage of code lines executed during test runs.
+
+**Target**: ≥80% overall, ≥90% for critical paths
+
+**TypeScript/Bun**:
+
+```bash
+bun test --coverage
+
+# Output
+Coverage Summary:
+  Statements   : 85.2% ( 1420/1667 )
+  Branches     : 78.5% ( 314/400 )
+  Functions    : 82.1% ( 156/190 )
+  Lines        : 85.2% ( 1420/1667 )
+```
+
+**Rust**:
+
+```bash
+# Using cargo-tarpaulin
+cargo tarpaulin --out Html --output-dir coverage/
+
+# Using cargo-llvm-cov
+cargo llvm-cov --html
+```
+
+### Branch Coverage
+
+Percentage of decision branches (if/else, switch, ternary) executed.
+
+**Target**: ≥75%
+
+Example showing uncovered branch:
+
+```typescript
+function divide(a: number, b: number): number {
+  if (b === 0) {  // Branch covered
+    throw new Error('Division by zero')
+  }
+  return a / b    // Branch covered
+}
+
+// Test only covers success path
+test('divides numbers', () => {
+  expect(divide(10, 2)).toBe(5)
+})
+
+// Coverage: 50% branches (only success branch covered)
+```
+
+Fix with both branches:
+
+```typescript
+test('divides numbers', () => {
+  expect(divide(10, 2)).toBe(5)
+})
+
+test('throws on division by zero', () => {
+  expect(() => divide(10, 0)).toThrow('Division by zero')
+})
+
+// Coverage: 100% branches
+```
+
+### Function Coverage
+
+Percentage of functions called during tests.
+
+**Target**: ≥80%
+
+Uncovered functions often indicate:
+- Dead code that should be removed
+- Missing test cases
+- Helper functions only used in uncovered paths
+
+### Interpreting Coverage
+
+High coverage ≠ high quality. Coverage shows what's tested, not how well.
+
+**Example of misleading coverage**:
+
+```typescript
+function processPayment(amount: number): Result {
+  if (amount <= 0) {
+    return { type: 'error', code: 'INVALID_AMOUNT' }
+  }
+
+  const result = chargeCard(amount)
+  return { type: 'success', transactionId: result.id }
+}
+
+// Bad test with 100% coverage
+test('processes payment', () => {
+  processPayment(100)
+  processPayment(-10)
+})
+
+// No assertions! 100% coverage but 0% verification
+```
+
+Coverage shows code was executed, not that it was verified correct.
+
+## Mutation Testing
+
+Mutation testing verifies test quality by introducing small bugs and checking if tests catch them.
+
+### How It Works
+
+1. **Mutant Generation**: Tool mutates source code (e.g., `===` → `!==`, `+` → `-`)
+2. **Test Execution**: Run tests against each mutant
+3. **Classification**:
+   - **Killed**: Test fails (good — test caught the bug)
+   - **Survived**: Test passes (bad — test missed the bug)
+   - **Timeout**: Mutant caused infinite loop
+   - **No Coverage**: Line not executed by tests
+
+### Mutation Score
+
+```
+Mutation Score = (Killed Mutants / Total Mutants) × 100%
+```
+
+**Target**: ≥75%
+
+### TypeScript Mutation Testing
+
+Using Stryker:
+
+**Install**:
+
+```bash
+bun add -d @stryker-mutator/core @stryker-mutator/typescript-checker
+```
+
+**Configuration** (`stryker.conf.json`):
+
+```json
+{
+  "mutator": "typescript",
+  "packageManager": "bun",
+  "reporters": ["html", "clear-text", "progress"],
+  "testRunner": "bun",
+  "coverageAnalysis": "perTest",
+  "mutate": [
+    "src/**/*.ts",
+    "!src/**/*.test.ts",
+    "!src/**/*.spec.ts"
+  ],
+  "thresholds": {
+    "high": 80,
+    "low": 60,
+    "break": 50
+  }
+}
+```
+
+**Run**:
+
+```bash
+bun x stryker run
+
+# Output
+Mutation testing complete:
+  Killed: 78
+  Survived: 12
+  Timeout: 2
+  No Coverage: 8
+  Mutation Score: 78.0%
+```
+
+**Common Mutations**:
+
+| Original | Mutant | Catches |
+|----------|--------|---------|
+| `===` | `!==` | Equality assertions |
+| `>` | `>=` | Boundary tests |
+| `+` | `-` | Arithmetic verification |
+| `&&` | `||` | Logic tests |
+| `true` | `false` | Boolean verification |
+| `0` | `1` | Zero handling |
+| `return x` | `return undefined` | Return value tests |
+
+**Example Analysis**:
+
+```typescript
+function calculateDiscount(price: number, isPremium: boolean): number {
+  if (isPremium) {
+    return price * 0.8  // 20% discount
+  }
+  return price
+}
+
+// Weak test
+test('calculates discount', () => {
+  calculateDiscount(100, true)
+  calculateDiscount(100, false)
+})
+
+// Mutation: 0.8 → 0.9
+// Status: Survived (no assertion)
+```
+
+Fix with assertions:
+
+```typescript
+test('applies 20% discount for premium users', () => {
+  expect(calculateDiscount(100, true)).toBe(80)
+})
+
+test('no discount for regular users', () => {
+  expect(calculateDiscount(100, false)).toBe(100)
+})
+
+// Mutation: 0.8 → 0.9
+// Status: Killed (test fails with 90 !== 80)
+```
+
+### Rust Mutation Testing
+
+Using `cargo-mutants`:
+
+**Install**:
+
+```bash
+cargo install cargo-mutants
+```
+
+**Run**:
+
+```bash
+cargo mutants
+
+# Output
+Mutation testing results:
+  caught: 45
+  missed: 5
+  timeout: 1
+  unviable: 2
+  score: 90.0%
+```
+
+**Common Mutations**:
+
+| Original | Mutant | Catches |
+|----------|--------|---------|
+| `==` | `!=` | Equality tests |
+| `>` | `>=` | Boundary tests |
+| `&&` | `||` | Logic tests |
+| `Some(x)` | `None` | Option handling |
+| `Ok(x)` | `Err(...)` | Result handling |
+| `+` | `-` | Arithmetic verification |
+
+**Example**:
+
+```rust
+fn calculate_discount(price: i32, is_premium: bool) -> i32 {
+    if is_premium {
+        price * 80 / 100  // 20% discount
+    } else {
+        price
+    }
+}
+
+// Weak test
+#[test]
+fn test_discount() {
+    calculate_discount(100, true);
+    calculate_discount(100, false);
+}
+
+// Mutation: 80 → 90
+// Status: missed (no assertion)
+```
+
+Fix:
+
+```rust
+#[test]
+fn applies_discount_for_premium() {
+    assert_eq!(calculate_discount(100, true), 80);
+}
+
+#[test]
+fn no_discount_for_regular() {
+    assert_eq!(calculate_discount(100, false), 100);
+}
+
+// Mutation: 80 → 90
+// Status: caught (assertion fails)
+```
+
+## Quality Standards Matrix
+
+| Metric | Minimum | Good | Excellent |
+|--------|---------|------|-----------|
+| Line Coverage | 70% | 80% | 90% |
+| Branch Coverage | 65% | 75% | 85% |
+| Function Coverage | 75% | 85% | 95% |
+| Mutation Score | 60% | 75% | 85% |
+| Test Execution Time | <10s | <5s | <2s |
+
+## Improving Test Quality
+
+### Weak Assertion Detection
+
+**Problem**: Tests execute code but don't verify results
+
+```typescript
+// ❌ Weak - no verification
+test('processes order', () => {
+  processOrder({ items: [item1, item2] })
+})
+```
+
+**Solution**:
+
+```typescript
+// ✓ Strong - verifies result
+test('processes order', () => {
+  const result = processOrder({ items: [item1, item2] })
+  expect(result.type).toBe('success')
+  expect(result.total).toBe(150)
+})
+```
+
+### Missing Edge Cases
+
+Use mutation testing to find gaps:
+
+```typescript
+function validateAge(age: number): boolean {
+  return age >= 18  // Mutant: >= → >
+}
+
+// Current test
+test('validates age', () => {
+  expect(validateAge(20)).toBe(true)
+  expect(validateAge(16)).toBe(false)
+})
+
+// Mutation survived: >= → >
+// Missing: boundary test for exactly 18
+```
+
+Add boundary test:
+
+```typescript
+test('accepts exactly 18', () => {
+  expect(validateAge(18)).toBe(true)
+})
+
+// Now mutation is caught
+```
+
+### Test Redundancy
+
+Multiple tests verifying same thing:
+
+```typescript
+// Redundant tests
+test('validates positive number', () => {
+  expect(isPositive(5)).toBe(true)
+})
+
+test('validates another positive number', () => {
+  expect(isPositive(10)).toBe(true)
+})
+
+test('validates yet another positive number', () => {
+  expect(isPositive(100)).toBe(true)
+})
+```
+
+Consolidate:
+
+```typescript
+test.each([5, 10, 100])('validates positive number %i', (num) => {
+  expect(isPositive(num)).toBe(true)
+})
+```
+
+## Continuous Quality Monitoring
+
+### CI/CD Integration
+
+**TypeScript**:
+
+```yaml
+# .github/workflows/test.yml
+- name: Run tests with coverage
+  run: bun test --coverage
+
+- name: Check coverage thresholds
+  run: |
+    coverage=$(bun test --coverage --json | jq '.coverage.total.statements.pct')
+    if (( $(echo "$coverage < 80" | bc -l) )); then
+      echo "Coverage $coverage% below 80% threshold"
+      exit 1
+    fi
+
+- name: Run mutation testing
+  run: bun x stryker run
+  # Fail if mutation score < 75%
+```
+
+**Rust**:
+
+```yaml
+# .github/workflows/test.yml
+- name: Run tests with coverage
+  run: cargo tarpaulin --fail-under 80
+
+- name: Run mutation testing
+  run: cargo mutants
+  continue-on-error: true  # Warning only initially
+```
+
+### Tracking Over Time
+
+Monitor trends:
+
+```bash
+# Generate coverage badge
+coverage=$(bun test --coverage --json | jq '.coverage.total.statements.pct')
+echo "Coverage: $coverage%" > coverage.txt
+
+# Track mutation score
+mutation=$(bun x stryker run --json | jq '.mutationScore')
+echo "Mutation Score: $mutation%" > mutation.txt
+```
+
+## Advanced Techniques
+
+### Differential Coverage
+
+Only measure coverage on changed code:
+
+```bash
+# Get changed files
+git diff --name-only main... > changed.txt
+
+# Run coverage on changed files
+bun test --coverage --changed-files changed.txt
+```
+
+### Coverage Ratcheting
+
+Prevent coverage from decreasing:
+
+```bash
+# Save current coverage
+current=$(bun test --coverage --json | jq '.coverage.total.statements.pct')
+echo "$current" > .baseline-coverage
+
+# On future runs, compare
+baseline=$(cat .baseline-coverage)
+if (( $(echo "$current < $baseline" | bc -l) )); then
+  echo "Coverage decreased from $baseline% to $current%"
+  exit 1
+fi
+```
+
+### Mutation Testing Optimization
+
+Run only on changed code:
+
+```bash
+# Stryker incremental mode
+bun x stryker run --incremental
+
+# cargo-mutants on specific files
+cargo mutants --file src/auth/mod.rs
+```
+
+## Common Pitfalls
+
+### Pitfall 1: Chasing 100% Coverage
+
+**Problem**: Diminishing returns past 90%, testing trivial code
+
+```typescript
+// Trivial getter - not worth testing
+class User {
+  get email(): string {
+    return this._email
+  }
+}
+```
+
+**Solution**: Focus on behavior, not line count. Exclude trivial code from coverage requirements.
+
+### Pitfall 2: Gaming Metrics
+
+**Problem**: Tests that execute code without verification
+
+```typescript
+// ❌ High coverage, zero value
+test('calls all functions', () => {
+  func1()
+  func2()
+  func3()
+})
+```
+
+**Solution**: Use mutation testing to catch weak assertions.
+
+### Pitfall 3: Slow Mutation Testing
+
+**Problem**: Full mutation testing takes hours
+
+**Solution**: Run incrementally or in CI only:
+
+```bash
+# Local: Quick feedback on changed files
+bun x stryker run --mutate "src/auth/**/*.ts"
+
+# CI: Full suite
+bun x stryker run
+```
+
+## Quality Metrics Dashboard
+
+Example report format:
+
+```
+Test Quality Report
+===================
+
+Coverage:
+  Statements: 85.2% ░░░░░░░░▓▓
+  Branches:   78.5% ░░░░░░░▓▓▓
+  Functions:  82.1% ░░░░░░░░▓▓
+
+Mutation Testing:
+  Score:      78.0% ░░░░░░░▓▓▓
+  Killed:     78
+  Survived:   12
+  No Cov:     8
+
+Performance:
+  Unit Tests: 2.3s  ✓
+  Total:      8.7s  ✓
+
+Status: ✓ All thresholds met
+```
+
+## Actionable Improvement Plan
+
+1. **Week 1**: Establish baseline
+   - Run coverage analysis
+   - Run mutation testing
+   - Document current state
+
+2. **Week 2-3**: Fix critical gaps
+   - Add tests for uncovered critical paths
+   - Fix survived mutants in high-risk code
+   - Target 80% coverage, 75% mutation score
+
+3. **Week 4**: Automate
+   - Add CI coverage checks
+   - Set up coverage ratcheting
+   - Schedule weekly mutation testing
+
+4. **Ongoing**: Maintain
+   - Review coverage on each PR
+   - Run mutation testing monthly
+   - Gradually raise thresholds
+
+## Resources
+
+TypeScript:
+- [Stryker Documentation](https://stryker-mutator.io)
+- [Bun Test Coverage](https://bun.sh/docs/cli/test#coverage)
+
+Rust:
+- [cargo-tarpaulin](https://github.com/xd009642/tarpaulin)
+- [cargo-llvm-cov](https://github.com/taiki-e/cargo-llvm-cov)
+- [cargo-mutants](https://github.com/sourcefrog/cargo-mutants)
@@ -0,0 +1,804 @@
+# Test Patterns Reference
+
+Comprehensive test patterns for TypeScript/Bun and Rust.
+
+## TypeScript/Bun Patterns
+
+### Basic Test Structure
+
+```typescript
+import { describe, test, expect } from 'bun:test'
+
+describe('Module or Feature Name', () => {
+  test('describes specific behavior', () => {
+    // Arrange
+    const input = createTestInput()
+
+    // Act
+    const result = functionUnderTest(input)
+
+    // Assert
+    expect(result).toBe(expected)
+  })
+})
+```
+
+### Discriminated Unions for Test Scenarios
+
+Use discriminated unions to make test scenarios type-safe:
+
+```typescript
+type TestScenario =
+  | { type: 'success'; input: ValidInput; expected: Output }
+  | { type: 'error'; input: InvalidInput; expectedError: ErrorCode }
+  | { type: 'edge-case'; input: EdgeInput; expected: Output }
+
+test.each<TestScenario>([
+  {
+    type: 'success',
+    input: { value: 100 },
+    expected: { result: 100 },
+  },
+  {
+    type: 'error',
+    input: { value: -1 },
+    expectedError: 'NEGATIVE_VALUE',
+  },
+  {
+    type: 'edge-case',
+    input: { value: 0 },
+    expected: { result: 0 },
+  },
+])('handles $type scenario', async (scenario) => {
+  const result = await processValue(scenario.input)
+
+  if (scenario.type === 'success' || scenario.type === 'edge-case') {
+    expect(result).toEqual(scenario.expected)
+  } else {
+    expect(result.error).toBe(scenario.expectedError)
+  }
+})
+```
+
+### Type-Safe Test Builders
+
+Create fluent builders for complex test data:
+
+```typescript
+class UserBuilder {
+  private data: Partial<User> = {
+    id: 'test-id',
+    email: 'test@example.com',
+    role: 'user',
+    createdAt: new Date('2024-01-01'),
+  }
+
+  withId(id: string): this {
+    this.data.id = id
+    return this
+  }
+
+  withEmail(email: string): this {
+    this.data.email = email
+    return this
+  }
+
+  withRole(role: UserRole): this {
+    this.data.role = role
+    return this
+  }
+
+  asAdmin(): this {
+    return this.withRole('admin')
+  }
+
+  build(): User {
+    return this.data as User
+  }
+}
+
+// Usage
+const adminUser = new UserBuilder()
+  .withEmail('admin@example.com')
+  .asAdmin()
+  .build()
+```
+
+Generic builder for flexibility:
+
+```typescript
+class Builder<T> {
+  constructor(private defaults: T) {}
+
+  with<K extends keyof T>(key: K, value: T[K]): this {
+    this.defaults = { ...this.defaults, [key]: value }
+    return this
+  }
+
+  build(): T {
+    return { ...this.defaults }
+  }
+}
+
+// Usage
+const userBuilder = new Builder<User>({
+  id: 'test-id',
+  email: 'test@example.com',
+  role: 'user',
+})
+
+const admin = userBuilder.with('role', 'admin').build()
+```
+
+### Const Assertions for Test Data
+
+Type-safe test data with const assertions:
+
+```typescript
+const validInputs = [
+  { input: 'hello', expected: 'HELLO' },
+  { input: 'world', expected: 'WORLD' },
+  { input: '', expected: '' },
+] as const
+
+test.each(validInputs)(
+  'transforms $input to $expected',
+  ({ input, expected }) => {
+    expect(transform(input)).toBe(expected)
+  }
+)
+```
+
+### Async Testing Patterns
+
+Promise rejection:
+
+```typescript
+test('rejects with error for invalid input', async () => {
+  const promise = fetchUser('invalid-id')
+
+  await expect(promise).rejects.toThrow(UserNotFoundError)
+  await expect(promise).rejects.toThrow('User not found')
+})
+```
+
+Async/await with error handling:
+
+```typescript
+test('handles async errors gracefully', async () => {
+  const result = await processData('invalid').catch(err => ({
+    error: err.message,
+  }))
+
+  expect(result.error).toBe('Invalid data')
+})
+```
+
+Timeout handling:
+
+```typescript
+test('times out slow operations', async () => {
+  const promise = slowOperation()
+
+  await expect(
+    Promise.race([
+      promise,
+      new Promise((_, reject) =>
+        setTimeout(() => reject(new Error('Timeout')), 100)
+      ),
+    ])
+  ).rejects.toThrow('Timeout')
+})
+```
+
+### Mocking with Bun
+
+Module mocking:
+
+```typescript
+import { mock } from 'bun:test'
+
+// Mock entire module
+mock.module('./database', () => ({
+  query: mock(() => Promise.resolve({ rows: [] })),
+  connect: mock(() => Promise.resolve()),
+}))
+
+// Use in test
+test('handles database errors', async () => {
+  const { query } = await import('./database')
+
+  query.mockImplementationOnce(() => Promise.reject(new Error('DB Error')))
+
+  const result = await fetchUsers()
+  expect(result.error).toBe('DB Error')
+})
+```
+
+Function mocking:
+
+```typescript
+const mockFetch = mock(async (url: string) => ({
+  ok: true,
+  json: async () => ({ data: 'test' }),
+}))
+
+test('fetches data successfully', async () => {
+  const result = await fetchData('https://api.example.com', mockFetch)
+
+  expect(mockFetch).toHaveBeenCalledWith('https://api.example.com')
+  expect(result.data).toBe('test')
+})
+```
+
+### Snapshot Testing
+
+Simple snapshots:
+
+```typescript
+test('serializes user correctly', () => {
+  const user = new UserBuilder().build()
+
+  expect(JSON.stringify(user, null, 2)).toMatchSnapshot()
+})
+```
+
+Inline snapshots:
+
+```typescript
+test('formats error message', () => {
+  const error = new ValidationError('Invalid email')
+
+  expect(error.message).toMatchInlineSnapshot(`"Invalid email"`)
+})
+```
+
+### Parameterized Tests
+
+Basic parameterization:
+
+```typescript
+test.each([
+  [1, 1],
+  [2, 4],
+  [3, 9],
+  [4, 16],
+])('square(%i) returns %i', (input, expected) => {
+  expect(square(input)).toBe(expected)
+})
+```
+
+Object-based parameterization:
+
+```typescript
+test.each([
+  { input: 5, expected: 25, description: 'positive number' },
+  { input: -3, expected: 9, description: 'negative number' },
+  { input: 0, expected: 0, description: 'zero' },
+])('square($input) for $description', ({ input, expected }) => {
+  expect(square(input)).toBe(expected)
+})
+```
+
+### Focused Testing
+
+Run specific tests:
+
+```typescript
+// Only run this test
+test.only('current feature under development', () => {
+  // Fast feedback during active development
+})
+
+// Skip slow tests during TDD
+test.skip('slow integration test', () => {
+  // Run in CI but not during rapid TDD cycles
+})
+
+// Mark test as work in progress
+test.todo('implement rate limiting')
+```
+
+### Parallel Test Execution
+
+Run independent tests in parallel:
+
+```typescript
+describe.concurrent('Independent Operations', () => {
+  test('operation 1', async () => {
+    const result = await independentOp1()
+    expect(result).toBeDefined()
+  })
+
+  test('operation 2', async () => {
+    const result = await independentOp2()
+    expect(result).toBeDefined()
+  })
+
+  test('operation 3', async () => {
+    const result = await independentOp3()
+    expect(result).toBeDefined()
+  })
+})
+```
+
+### Error Testing Patterns
+
+Exception testing:
+
+```typescript
+test('throws error for invalid input', () => {
+  expect(() => processData(null)).toThrow(ValidationError)
+  expect(() => processData(null)).toThrow('Input cannot be null')
+})
+```
+
+Error result testing:
+
+```typescript
+test('returns error result for invalid input', () => {
+  const result = processData(null)
+
+  expect(result.type).toBe('error')
+  if (result.type === 'error') {
+    expect(result.code).toBe('INVALID_INPUT')
+    expect(result.message).toContain('null')
+  }
+})
+```
+
+## Rust Patterns
+
+### Basic Test Structure
+
+```rust
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_basic_functionality() {
+        // Arrange
+        let input = setup_test_input();
+
+        // Act
+        let result = function_under_test(input);
+
+        // Assert
+        assert_eq!(result, expected);
+    }
+}
+```
+
+### Property-Based Testing
+
+Using `proptest`:
+
+```rust
+use proptest::prelude::*;
+
+proptest! {
+    #[test]
+    fn password_hash_is_deterministic(password in "[a-zA-Z0-9]{8,32}") {
+        let hash1 = hash_password(&password);
+        let hash2 = hash_password(&password);
+        prop_assert_eq!(hash1, hash2);
+    }
+
+    #[test]
+    fn email_validation_never_panics(email in ".*") {
+        let result = validate_email(&email);
+        // Should always return Ok or Err, never panic
+        prop_assert!(result.is_ok() || result.is_err());
+    }
+
+    #[test]
+    fn parse_then_serialize_is_identity(value in 0..1000) {
+        let serialized = serialize_value(value);
+        let parsed = parse_value(&serialized).unwrap();
+        prop_assert_eq!(value, parsed);
+    }
+}
+```
+
+### Test Builders
+
+Rust builder pattern:
+
+```rust
+#[derive(Default)]
+struct UserBuilder {
+    id: Option<String>,
+    email: Option<String>,
+    role: Option<Role>,
+}
+
+impl UserBuilder {
+    fn new() -> Self {
+        Self::default()
+    }
+
+    fn with_id(mut self, id: impl Into<String>) -> Self {
+        self.id = Some(id.into());
+        self
+    }
+
+    fn with_email(mut self, email: impl Into<String>) -> Self {
+        self.email = Some(email.into());
+        self
+    }
+
+    fn with_role(mut self, role: Role) -> Self {
+        self.role = Some(role);
+        self
+    }
+
+    fn as_admin(self) -> Self {
+        self.with_role(Role::Admin)
+    }
+
+    fn build(self) -> User {
+        User {
+            id: self.id.unwrap_or_else(|| "test-id".to_string()),
+            email: self.email.unwrap_or_else(|| "test@example.com".to_string()),
+            role: self.role.unwrap_or(Role::User),
+        }
+    }
+}
+
+// Usage
+#[test]
+fn test_admin_permissions() {
+    let admin = UserBuilder::new()
+        .with_email("admin@example.com")
+        .as_admin()
+        .build();
+
+    assert!(has_admin_access(&admin));
+}
+```
+
+### Async Testing
+
+Using `tokio::test`:
+
+```rust
+#[tokio::test]
+async fn authenticates_user_async() {
+    let credentials = Credentials {
+        email: "user@example.com".to_string(),
+        password: "password".to_string(),
+    };
+
+    let result = authenticate_async(&credentials).await;
+    assert!(result.is_ok());
+}
+
+#[tokio::test]
+#[should_panic(expected = "timeout")]
+async fn times_out_slow_operations() {
+    tokio::time::timeout(
+        Duration::from_millis(100),
+        very_slow_operation()
+    ).await.expect("timeout");
+}
+```
+
+### Result Testing
+
+Testing `Result` types:
+
+```rust
+#[test]
+fn returns_error_for_invalid_input() {
+    let result = process_data(None);
+
+    assert!(result.is_err());
+    assert!(matches!(result, Err(ProcessError::InvalidInput)));
+}
+
+#[test]
+fn returns_success_for_valid_input() {
+    let result = process_data(Some("valid"));
+
+    assert!(result.is_ok());
+    let value = result.unwrap();
+    assert_eq!(value, "processed");
+}
+```
+
+Using `assert_matches!` macro:
+
+```rust
+#[test]
+fn authenticates_with_valid_credentials() {
+    let result = authenticate(&valid_creds);
+
+    assert!(matches!(result, Ok(AuthResult::Success { .. })));
+}
+
+#[test]
+fn rejects_invalid_credentials() {
+    let result = authenticate(&invalid_creds);
+
+    assert!(matches!(result, Err(AuthError::InvalidCredentials)));
+}
+```
+
+### Documentation Tests
+
+Executable documentation:
+
+```rust
+/// Authenticates a user with credentials.
+///
+/// # Examples
+///
+/// ```
+/// use auth::{authenticate, Credentials};
+///
+/// let creds = Credentials {
+///     email: "user@example.com".to_string(),
+///     password: "password".to_string(),
+/// };
+///
+/// let result = authenticate(&creds);
+/// assert!(result.is_ok());
+/// ```
+///
+/// # Errors
+///
+/// Returns `AuthError::InvalidCredentials` if credentials are invalid:
+///
+/// ```
+/// use auth::{authenticate, Credentials, AuthError};
+///
+/// let bad_creds = Credentials {
+///     email: "wrong@example.com".to_string(),
+///     password: "wrong".to_string(),
+/// };
+///
+/// let result = authenticate(&bad_creds);
+/// assert!(matches!(result, Err(AuthError::InvalidCredentials)));
+/// ```
+pub fn authenticate(credentials: &Credentials) -> Result<AuthResult, AuthError> {
+    // Implementation
+}
+```
+
+### Snapshot Testing
+
+Using `insta`:
+
+```rust
+use insta::assert_snapshot;
+
+#[test]
+fn serializes_user_correctly() {
+    let user = User {
+        id: "test-id".to_string(),
+        email: "test@example.com".to_string(),
+        role: Role::Admin,
+    };
+
+    let serialized = serde_json::to_string_pretty(&user).unwrap();
+    assert_snapshot!(serialized);
+}
+```
+
+### Parameterized Tests
+
+Manual parameterization:
+
+```rust
+#[test]
+fn test_square() {
+    let test_cases = vec![
+        (5, 25),
+        (-3, 9),
+        (0, 0),
+        (10, 100),
+    ];
+
+    for (input, expected) in test_cases {
+        assert_eq!(square(input), expected, "Failed for input {}", input);
+    }
+}
+```
+
+Using `rstest`:
+
+```rust
+use rstest::rstest;
+
+#[rstest]
+#[case(5, 25)]
+#[case(-3, 9)]
+#[case(0, 0)]
+#[case(10, 100)]
+fn test_square(#[case] input: i32, #[case] expected: i32) {
+    assert_eq!(square(input), expected);
+}
+```
+
+### Mock Objects
+
+Using `mockall`:
+
+```rust
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use mockall::predicate::*;
+    use mockall::mock;
+
+    mock! {
+        Database {}
+
+        impl Database {
+            fn query(&self, sql: &str) -> Result<Vec<Row>, DbError>;
+            fn execute(&self, sql: &str) -> Result<u64, DbError>;
+        }
+    }
+
+    #[test]
+    fn handles_empty_database() {
+        let mut mock_db = MockDatabase::new();
+        mock_db
+            .expect_query()
+            .with(eq("SELECT * FROM users"))
+            .returning(|_| Ok(vec![]));
+
+        let users = find_all_users(&mock_db);
+        assert_eq!(users.len(), 0);
+    }
+
+    #[test]
+    fn handles_database_error() {
+        let mut mock_db = MockDatabase::new();
+        mock_db
+            .expect_query()
+            .returning(|_| Err(DbError::ConnectionLost));
+
+        let result = find_all_users(&mock_db);
+        assert!(result.is_err());
+    }
+}
+```
+
+### Error Testing
+
+Custom error types:
+
+```rust
+#[test]
+fn returns_custom_error() {
+    let result = process_value(-1);
+
+    assert!(result.is_err());
+    let err = result.unwrap_err();
+    assert_eq!(err.to_string(), "Value cannot be negative");
+}
+
+#[test]
+fn error_contains_context() {
+    let result = parse_config("invalid");
+
+    match result {
+        Err(ConfigError::ParseError { line, message }) => {
+            assert_eq!(line, 1);
+            assert!(message.contains("invalid"));
+        }
+        _ => panic!("Expected ParseError"),
+    }
+}
+```
+
+### Integration Test Structure
+
+Separate integration tests in `tests/` directory:
+
+```rust
+// tests/integration/user_api.rs
+use my_crate::*;
+
+#[tokio::test]
+async fn test_user_registration_flow() {
+    // Setup test database
+    let db = setup_test_db().await;
+
+    // Create user
+    let user = register_user(&db, "test@example.com", "password").await.unwrap();
+
+    // Verify user created
+    let found = find_user(&db, user.id).await.unwrap();
+    assert_eq!(found.email, "test@example.com");
+
+    // Cleanup
+    cleanup_test_db(db).await;
+}
+```
+
+## Common Test Smells and Solutions
+
+### Test Smell: Setup Longer Than Test
+
+❌ Bad:
+
+```typescript
+test('processes order', () => {
+  const user = { id: '1', email: 'test@example.com', role: 'user', /* 10 more fields */ }
+  const product = { id: 'p1', name: 'Widget', price: 100, /* 8 more fields */ }
+  const cart = { items: [{ product, quantity: 2 }], /* 5 more fields */ }
+  const payment = { method: 'card', /* 6 more fields */ }
+
+  const result = processOrder(user, cart, payment)
+  expect(result.total).toBe(200)
+})
+```
+
+✓ Good:
+
+```typescript
+test('processes order', () => {
+  const order = new OrderBuilder().withQuantity(2).withPrice(100).build()
+
+  const result = processOrder(order)
+  expect(result.total).toBe(200)
+})
+```
+
+### Test Smell: Multiple Unrelated Assertions
+
+❌ Bad:
+
+```typescript
+test('user management', () => {
+  expect(createUser('test@example.com')).toBeDefined()
+  expect(findUser('1')).toEqual({ id: '1' })
+  expect(deleteUser('1')).toBe(true)
+})
+```
+
+✓ Good:
+
+```typescript
+test('creates user with valid email', () => {
+  expect(createUser('test@example.com')).toBeDefined()
+})
+
+test('finds user by id', () => {
+  expect(findUser('1')).toEqual({ id: '1' })
+})
+
+test('deletes user successfully', () => {
+  expect(deleteUser('1')).toBe(true)
+})
+```
+
+### Test Smell: Testing Implementation Details
+
+❌ Bad:
+
+```typescript
+test('caches results internally', () => {
+  const service = new UserService()
+  service.fetchUser('1')
+
+  expect(service._cache.has('1')).toBe(true) // Testing private implementation
+})
+```
+
+✓ Good:
+
+```typescript
+test('returns cached user on second fetch', async () => {
+  const service = new UserService()
+  const spy = mock.fn()
+
+  await service.fetchUser('1', spy)
+  await service.fetchUser('1', spy)
+
+  expect(spy).toHaveBeenCalledTimes(1) // Testing observable behavior
+})
+```