/modern-swift

Modern Attributes (Swift 5.9-6.2)

New attributes introduced in recent Swift versions.

@preconcurrency

Suppresses strict concurrency warnings for legacy code during migration.

On Imports

// Suppress warnings from dependencies not yet updated for Swift 6
@preconcurrency import LegacyNetworking

// Use LegacyNetworking types without Sendable warnings

On Protocols

// Allow non-Sendable types to conform during migration
@preconcurrency
protocol DataSource {
    func fetchData() async -> Data
}

// Classes can conform without Sendable requirement
class LocalDataSource: DataSource {
    func fetchData() async -> Data { ... }
}

On Types

// Mark type as "will be Sendable eventually"
@preconcurrency
class LegacyManager {
    var state: String = ""
}

@backDeployed

Makes new API implementations available on older OS versions.

extension String {
    // Available on iOS 13+, but implemented on iOS 17+
    @backDeployed(before: iOS 17)
    @available(iOS 13, *)
    func trimmed() -> String {
        trimmingCharacters(in: .whitespaces)
    }
}

// iOS 13-16: Uses the provided implementation
// iOS 17+: Uses system implementation (if different)

When to Use

• Library evolution
• Backporting system APIs
• Gradual feature rollout

package Access Control (Swift 5.9)

New access level between internal and public.

// MyLibrary/Sources/Core/User.swift
package struct User {
    package let id: String
    package let name: String
}

// MyLibrary/Sources/Networking/API.swift
package func fetchUser() -> User {
    // Visible within MyLibrary package
}

// App/main.swift
import MyLibrary
// User and fetchUser are NOT visible here

Access Levels Summary

private < fileprivate < internal < package < public < open

@available(*, noasync)

Prevents async usage of specific APIs.

@available(*, noasync)
func dangerousBlockingOperation() {
    // Blocks thread - don't call from async context
    Thread.sleep(forTimeInterval: 5)
}

// ❌ Compile error in async context
async {
    dangerousBlockingOperation()
}

Use Cases

• Mark blocking I/O
• Prevent deadlocks
• Legacy synchronous APIs

@_exported (Underscore Attributes)

Re-exports a module's public API.

// In your module
@_exported import Foundation

// Users importing your module get Foundation too
import MyModule
// Can use Foundation types without separate import

Warning: @_exported is underscored = not stable API. Use sparingly.

Macro Attributes

See macros.md for:

• @attached(member)
• @attached(peer)
• @attached(accessor)
• @attached(memberAttribute)
• @attached(conformance)
• @freestanding(expression)

Migration Patterns

Swift 5.x to Swift 6

// 1. Add @preconcurrency to imports
@preconcurrency import LegacySDK

// 2. Use package for internal APIs
package func helperMethod() { }

// 3. Mark blocking code
@available(*, noasync)
func blockingWork() { }

Library Evolution

// Backport new features
@backDeployed(before: iOS 18)
@available(iOS 15, *)
func modernFeature() { }