axiom-swift-testing

# Swift Testing

## Overview

Swift Testing is Apple's modern testing framework introduced at WWDC 2024. It uses Swift macros (`@Test`, `#expect`) instead of naming conventions, runs tests in parallel by default, and integrates seamlessly with Swift concurrency.

**Core principle**: Tests should be fast, reliable, and expressive. The fastest tests run without launching your app or simulator.

## The Speed Hierarchy

Tests run at dramatically different speeds depending on how they're configured:

| Configuration | Typical Time | Use Case |
|---------------|--------------|----------|
| `swift test` (Package) | ~0.1s | Pure logic, models, algorithms |
| Host Application: None | ~3s | Framework code, no UI dependencies |
| Bypass app launch | ~6s | App target but skip initialization |
| Full app launch | 20-60s | UI tests, integration tests |

**Key insight**: Move testable logic into Swift Packages or frameworks, then test with `swift test` or "None" host application.

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## Building Blocks

### @Test Functions

```swift
import Testing

@Test func videoHasCorrectMetadata() {
    let video = Video(named: "example.mp4")
    #expect(video.duration == 120)
}
```

**Key differences from XCTest**:
- No `test` prefix required — `@Test` attribute is explicit
- Can be global functions, not just methods in a class
- Supports `async`, `throws`, and actor isolation
- Each test runs on a fresh instance of its containing suite

### #expect and #require

```swift
// Basic expectation — test continues on failure
#expect(result == expected)
#expect(array.isEmpty)
#expect(numbers.contains(42))

// Required expectation — test stops on failure
let user = try #require(await fetchUser(id: 123))
#expect(user.name == "Alice")

// Unwrap optionals safely
let first = try #require(items.first)
#expect(first.isValid)
```

**Why #expect is better than XCTAssert**:
- Captures source code and sub-values automatically
- Single macro handles all operators (==, >, contains, etc.)
- No need for specia