What's New in Swift 6.3 & 6.4

import Foundation
import Swift

// MARK: - 1. anyAppleOS availability
// Before Swift 6.4 you needed:
// @available(iOS 18, macOS 15, watchOS 11, tvOS 18, visionOS 2, *)
// Now:
@available(anyAppleOS 26, *)
func fetchLatestManifest() async throws -> Data {
    let url = URL(string: "https://example.com/manifest.json")!
    let (data, _) = try await URLSession.shared.data(from: url)
    return data
}

// MARK: - 2. Task Cancellation Shield
// Guarantees a critical write completes even if the parent task is cancelled.
func persistPendingChanges(records: [String]) async throws {
    // Expensive network work here (respects cancellation) ...
    try Task.checkCancellation()

    // Shield ensures the flush to disk always finishes.
    await Task.withCancellationShield {
        for record in records {
            // Simulate writing each record to disk
            print("Writing record: \(record)")
        }
        print("All records persisted safely.")
    }
}

// MARK: - 3. Dictionary.mapKeyedValues
// Old approach required building a new dictionary manually when the key was needed.
let inventory: [String: Int] = ["apple": 3, "banana": 7, "cherry": 1]

// New in Swift 6.4: both key and value are passed to the closure.
let discountedLabels: [String: String] = inventory.mapKeyedValues { key, value in
    value > 5 ? "\(key.capitalized): SALE" : "\(key.capitalized): \(value) left"
}

// Expected output:
// ["Apple": "Apple: 3 left", "Banana": "Banana: SALE", "Cherry": "Cherry: 1 left"]
print(discountedLabels)

// MARK: - 4. @diagnose — suppress a deprecated-declaration warning locally
@diagnose(ignore: "deprecated_declaration")
func legacyEntryPoint() {
    // Calls a deprecated API while migration is in progress.
    // The warning is silenced only inside this function.
    print("Using legacy path temporarily.")
}