swift-testing

# Swift Testing

Swift Testing is the modern testing framework for Swift (Xcode 16+, Swift 6+). Prefer it over XCTest for all new unit tests. Use XCTest only for UI tests, performance benchmarks, and snapshot tests.

## Contents

- [Basic Tests](#basic-tests)
- [`@Test Traits`](#test-traits)
- [#expect and #require](#expect-and-require)
- [`@Suite and Test Organization`](#suite-and-test-organization)
- [Execution Model](#execution-model)
- [XCTest Migration Boundaries](#xctest-migration-boundaries)
- [Known Issues](#known-issues)
- [Additional Patterns](#additional-patterns)
- [Common Mistakes](#common-mistakes)
- [Test Attachments](#test-attachments)
- [Exit Testing](#exit-testing)
- [Version-Gated APIs](#version-gated-apis)
- [Advanced API Review Checklist](#advanced-api-review-checklist)
- [Review Checklist](#review-checklist)
- [References](#references)

---

## Basic Tests

```swift
import Testing

@Test("User can update their display name")
func updateDisplayName() {
    var user = User(name: "Alice")
    user.name = "Bob"
    #expect(user.name == "Bob")
}
```

## `@Test` Traits

```swift
@Test("Validates email format")                                    // display name
@Test(.tags(.validation, .email))                                  // tags
@Test(.disabled("Server migration in progress"))                   // disabled
@Test(.enabled(if: ProcessInfo.processInfo.environment["CI"] != nil)) // conditional
@Test(.bug("https://github.com/org/repo/issues/42"))               // bug reference
@Test(.timeLimit(.minutes(1)))                                     // time limit
@Test("Timeout handling", .tags(.networking), .timeLimit(.seconds(30))) // combined
```

## #expect and #require

```swift
// #expect records failure but continues execution
#expect(result == 42)
#expect(name.isEmpty == false)
#expect(items.count > 0, "Items should not be empty")

// #expect with error type checking
#expect(throws: ValidationError.self) {
    try validate(email: "not-an-email")
}

// #expect with specific error value
#expect {
    try validate(email: "")
} throws: { error in
    guard let err = error as? ValidationError else { return false }
    return err == .empty
}

// #require records failure AND stops test (like XCTUnwrap)
let user = try #require(await fetchUser(id: 1))
#expect(user.name == "Alice")

// #require for optionals -- unwraps or fails
let first = try #require(items.first)
#expect(first.isValid)
```

**Rule: Use `#require` when subsequent assertions depend on the value. Use `#expect` for independent checks.**

## `@Suite` and Test Organization

See [references/testing-patterns.md](references/testing-patterns.md) for suite organization, confirmation patterns, known-issue handling, and execution-model details.

## Execution Model

Swift Testing runs tests in parallel by default. Do not assume test order, shared suite instances, or exclusive access to mutable state unless you explicitly design for it.

```swift
@Suite(.serialized)
struct KeychainTests {
    @Test func storesToken() throws { /* ... */ }
    @Test func deletesToken() throws { /* ... */ }
}
```

Use `.serialized` when a test or suite must run one-at-a-time because it touches shared external state. It does not make unrelated tests outside that scope run serially.

**Rules:**
- Each test must set up its own state.
- Shared mutable globals are a bug unless protected or intentionally serialized.
- `@Suite(.serialized)` is for exclusive execution, not for expressing logical ordering between tests.
- If tests depend on sequence, combine them into one test or move the sequence into shared helper code.

## XCTest Migration Boundaries

Swift Testing unit tests do not inherit from `XCTestCase`. Declare `@Test` on free functions, global functions, or methods on suite types such as `struct`, `class`, or `actor`; use `static` or `class` methods when instance fixtures are not needed.

When reviewing migration code or plans, do not collapse every XCTest construct into `#expect`. Include a compact assertion-mapping note or table in the answer so required unwraps and unconditional manual failures are not lost, even when the user only says "replace every XCTAssert with #expect."

State coexistence explicitly: XCTest and Swift Testing can coexist during migration. Keep UI automation, performance benchmarks, and common snapshot-test flows on XCTest/XCUITest or snapshot tooling, and separate files or targets when that makes runner expectations clearer.

Migration defaults:
- `XCTAssert*` -> `#expect(...)`
- `XCTUnwrap` or any value required by later checks -> `try #require(...)`
- `XCTFail("...")` or manual unconditional issues -> `Issue.record("...")`
- UI tests, performance benchmarks, and common snapshot-test flows stay on XCTest/XCUITest or snapshot tooling.
- Put `@available` on individual `@Test` functions, not on suite types or their containing types.

```swift
let user = try #require(optionalUser)
#expect(user.isActive)

guard featureFlag.isEnabled else {
    Issue.record("Expected feature flag to be enabled")
    return
}
```

See [references/testing-patterns.md](references/testing-patterns.md) for migration examples and [references/testing-advanced.md](references/testing-advanced.md) for Swift/Xcode version gates.

## Known Issues

Mark expected failures so they do not cause test failure:

```swift
withKnownIssue("Propane tank is empty") {
    #expect(truck.grill.isHeating)
}

// Intermittent / flaky failures
withKnownIssue(isIntermittent: true) {
    #expect(service.isReachable)
}

// Conditional known issue
withKnownIssue {
    #expect(foodTruck.grill.isHeating)
} when: {
    !hasPropane
}
```

If no known issues are recorded, Swift Testing records a distinct issue notifying you the problem may be resolved.

## Additional Patterns

See [references/testing-patterns.md](references/testing-patterns.md) for parameterized tests, tags and suites, async testing, traits, and execution-model details.

## Test Attachments

Attach diagnostic data