Add swift-testing skill with framework basics

swift-testing/SKILL.md

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+---
+name: swift-testing
+description: Swift Testing framework (@Test, @Suite, #expect, #require) for Swift 5.9+/Xcode 16+. Use when writing tests, migrating from XCTest, or fixing Swift Testing failures.
+---
+
+# Swift Testing
+
+## Overview
+
+Write unit tests using Apple's Swift Testing framework with `@Test`, `@Suite`, `#expect`, and `#require`. Replaces XCTest for non-UI tests.
+
+## Quick reference
+
+| XCTest | Swift Testing |
+|--------|---------------|
+| `import XCTest` | `import Testing` |
+| `class FooTests: XCTestCase` | `@Suite struct FooTests` |
+| `func testBar()` | `@Test func bar()` |
+| `XCTAssertEqual(a, b)` | `#expect(a == b)` |
+| `XCTAssertNil(x)` | `#expect(x == nil)` |
+| `XCTAssertThrowsError` | `#expect(throws:)` |
+| `XCTUnwrap(x)` | `try #require(x)` |
+| `setUpWithError()` | `init() throws` |
+| `tearDown()` | `deinit` |
+
+## Workflow
+
+### 1. Set up the test suite
+
+Create a struct with `@Suite` and use `init()` for setup:
+
+```swift
+import Testing
+
+@Suite struct UserServiceTests {
+    let sut: UserService
+    let mockRepo: MockRepository
+
+    init() {
+        mockRepo = MockRepository()
+        sut = UserService(repository: mockRepo)
+    }
+}
+```
+
+### 2. Write test functions
+
+Add `@Test` to test functions. Remove the `test` prefix:
+
+```swift
+@Test func fetchUser_returnsValidUser() async throws {
+    let user = try await sut.fetchUser(id: "123")
+    #expect(user.name == "John")
+}
+```
+
+### 3. Use assertions
+
+- `#expect(condition)` — soft assertion, test continues on failure
+- `try #require(optional)` — hard assertion, test stops if nil
+
+```swift
+#expect(result == expected)
+#expect(isValid)
+let value = try #require(optional)
+
+#expect(throws: ValidationError.self) {
+    try validate(badInput)
+}
+```
+
+### 4. Add parameterized tests
+
+Run the same test with multiple inputs:
+
+```swift
+@Test(arguments: ["", " ", "   "])
+func validate_rejectsBlankStrings(_ input: String) {
+    #expect(!isValid(input))
+}
+
+@Test(arguments: [
+    (input: "hello", expected: 5),
+    (input: "", expected: 0)
+])
+func count_returnsCorrectLength(input: String, expected: Int) {
+    #expect(input.count == expected)
+}
+```
+
+### 5. Apply traits for test control
+
+```swift
+@Test(.enabled(if: ProcessInfo.processInfo.environment["CI"] != nil))
+func onlyOnCI() { }
+
+@Test(.disabled("Waiting for backend fix"))
+func brokenEndpoint() { }
+
+@Test(.tags(.slow, .network))
+func networkHeavyTest() { }
+
+@Test(.timeLimit(.minutes(1)))
+func mustCompleteFast() { }
+
+@Suite(.serialized)
+struct DatabaseTests {
+    @Test func insert() { }
+    @Test func delete() { }
+}
+```
+
+## Common mistakes
+
+| Mistake | Fix |
+|---------|-----|
+| `#expect(x == nil)` on `T??` | Use `try #require(x)` first to unwrap outer optional |
+| Test not running | Ensure `@Test` attribute is present |
+| Shared state between tests | Use `init()` for setup, avoid `static var` |
+| Async test not awaited | Mark test `async`, use `await` |
+| XCTest assertions in Swift Testing | Replace with `#expect` / `#require` |
+
+## Reference material
+
+- See `references/swift-testing-basics.md` for core concepts and syntax.
+- See `references/migration-from-xctest.md` for migration strategies.
+- See `references/advanced-patterns.md` for parameterized tests, traits, and mocking.

swift-testing/references/swift-testing-basics.md

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+# Swift Testing Basics
+
+## Core Components
+
+### @Test
+
+```swift
+@Test func userCanLogin() { }
+@Test func emptyInput_returnsError() { }
+@Test("User can update their profile") func updateProfile() { }
+```
+
+**Requirements:**
+- Must be a function (not a property or subscript)
+- Can be `async` and/or `throws`
+- Can be instance method or free function
+- No parameters unless using parameterized testing
+
+### @Suite
+
+```swift
+@Suite struct AuthenticationTests {
+    @Test func login() { }
+    @Test func logout() { }
+}
+```
+
+**Suite features:**
+- Can be `struct`, `class`, `actor`, or `enum`
+- `struct` recommended (value semantics, fresh instance per test)
+- Can nest suites for hierarchy
+- Supports traits for suite-wide configuration
+
+### #expect
+
+```swift
+#expect(value == 42)
+#expect(array.isEmpty)
+#expect(string.contains("hello"))
+#expect(!isDisabled)
+```
+
+Soft assertion—test continues on failure.
+
+**Comparisons:**
+```swift
+#expect(a == b)
+#expect(a != b)
+#expect(a > b)
+#expect(a === b)
+```
+
+**Errors:**
+```swift
+#expect(throws: (any Error).self) {
+    try riskyOperation()
+}
+
+#expect(throws: NetworkError.self) {
+    try fetch()
+}
+
+#expect {
+    try validate("")
+} throws: { error in
+    error as? ValidationError == .empty
+}
+
+#expect(throws: Never.self) {
+    try safeOperation()
+}
+```
+
+### #require
+
+Hard assertion—test stops on failure. Use for unwrapping.
+
+```swift
+let user = try #require(await fetchUser())
+#expect(user.name == "John")
+
+try #require(array.count > 0)
+let first = array[0]
+
+let viewModel = try #require(controller.viewModel as? ProfileViewModel)
+```
+
+## Lifecycle
+
+```swift
+@Suite struct ServiceTests {
+    let service: MyService
+    let mockRepository: MockRepository
+
+    init() {
+        mockRepository = MockRepository()
+        service = MyService(repository: mockRepository)
+    }
+
+    @Test func fetchData() async { }
+}
+```
+
+`init()` runs before each test. Each test gets a fresh instance.
+
+### Teardown (class only)
+
+```swift
+@Suite class ResourceTests {
+    var tempFile: URL?
+
+    init() throws {
+        tempFile = try createTempFile()
+    }
+
+    deinit {
+        if let tempFile {
+            try? FileManager.default.removeItem(at: tempFile)
+        }
+    }
+}
+```
+
+## Async
+
+```swift
+@Test func asyncFetch() async throws {
+    let data = try await api.fetchData()
+    #expect(data.count > 0)
+}
+```
+
+```swift
+@Test(.timeLimit(.seconds(5)))
+func mustCompleteFast() async { }
+
+@Suite(.serialized)
+struct OrderDependentTests {
+    @Test func first() { }
+    @Test func second() { }
+}
+```
+
+## Running Tests
+
+**Xcode:** Cmd+U (all), click diamond (individual)
+
+**Command line:**
+```bash
+swift test
+swift test --filter UserTests
+swift test --filter .tags:slow
+swift test --skip .tags:slow
+```
+