Swift 6 Release - Achieving Complete Concurrency Safety

Swift 6 Overview

In September 2024, Swift 6 was released alongside Xcode 16. The biggest feature of this version is the ability to completely prevent data races at compile time.

Reference: Swift.org - Swift 6

Complete Data Race Prevention

Swift 6 Language Mode

In Swift 6, data races become compile errors.

// Enable Swift 6 mode in Package.swift
// swift-tools-version: 6.0

var counter = 0

func increment() async {
    counter += 1  // Error: Potential data race
}

Stricter Sendable

// Cannot pass non-Sendable objects to concurrent context
class UnsafeClass {
    var value = 0
}

func example() async {
    let obj = UnsafeClass()

    Task {
        obj.value += 1  // Error: UnsafeClass is not Sendable
    }
}

// Solution 1: Conform to Sendable
final class SafeClass: Sendable {
    let value: Int  // Make immutable
    init(value: Int) { self.value = value }
}

// Solution 2: Use an actor
actor Counter {
    var value = 0
    func increment() { value += 1 }
}

Reference: Swift Concurrency

Typed throws

You can now explicitly specify error types.

// Traditional: any error
func legacyParse() throws -> Data {
    throw ParseError.invalidFormat
}

// Swift 6: typed throws
enum ParseError: Error {
    case invalidFormat
    case emptyInput
}

func parse() throws(ParseError) -> Data {
    throw .invalidFormat
}

// Caller knows the type
do {
    let data = try parse()
} catch {
    // error type is ParseError
    switch error {
    case .invalidFormat:
        print("Format error")
    case .emptyInput:
        print("Empty input")
    }
}

never for Functions That Never Fail

// Never throws an error
func safeParse() throws(Never) -> Data {
    return Data()
}

// Can be called without try
let data = safeParse()

128-bit Integers

let big: Int128 = 170_141_183_460_469_231_731_687_303_715_884_105_727
let ubig: UInt128 = 340_282_366_920_938_463_463_374_607_431_768_211_455

print(big.bitWidth)  // 128

Embedded Swift

A Swift subset for resource-constrained environments (such as microcontrollers).

// Embedded Swift code
@main
struct LEDBlink {
    static func main() {
        let led = DigitalOutput(pin: 13)

        while true {
            led.toggle()
            sleep(milliseconds: 500)
        }
    }
}

# Build with Embedded Swift
swiftc -enable-experimental-feature Embedded -target armv7em-none-eabi main.swift

Reference: Embedded Swift Vision

C++ Interoperability Improvements

C++ to Swift

// C++ header
class CppCounter {
public:
    CppCounter(int initial);
    void increment();
    int getValue() const;
private:
    int value;
};

// Use from Swift
import CppCounter

let counter = CppCounter(42)
counter.increment()
print(counter.getValue())  // 43

Swift to C++

// Swift
@_expose(Cxx)
public func swiftFunction() -> Int {
    return 42
}

// Call from C++
#include "MyModule-Swift.h"

int result = swiftFunction();

Reference: Swift C++ Interoperability

Mutex and Atomic for Synchronization

import Synchronization

// Mutex
let mutex = Mutex(initialValue: 0)

mutex.withLock { value in
    value += 1
}

// Atomic
let atomic = Atomic<Int>(0)

atomic.wrappingAdd(1, ordering: .relaxed)
let current = atomic.load(ordering: .acquiring)

Gradual Migration

Warnings in Swift 5 Mode

// Package.swift
let package = Package(
    swiftLanguageVersions: [.v5],  // Still Swift 5
    // ...
)

// Enable concurrency warnings
swiftSettings: [
    .enableUpcomingFeature("StrictConcurrency")
]

Per-File Migration

// Migrate only specific files to Swift 6 mode
// Add at the beginning of the file
// swift-tools-version: 6.0

Integration with Xcode 16

Build Settings

SWIFT_VERSION = 6.0
SWIFT_STRICT_CONCURRENCY = complete

New Warnings and Fix Suggestions

// Xcode suggests automatic fixes
class MyClass {
    var data: [Int] = []

    func process() async {
        // Warning + Fix-it: Add @MainActor
        data.append(1)
    }
}

Performance Improvements

Summary

Swift 6 raises the language’s safety to a new level.

• Data Race Prevention: Completely detected at compile time
• Typed throws: Improved error handling
• 128-bit Integers: Support for large numbers
• Embedded Swift: Embedded systems support
• C++ Interoperability: Deeper integration

Existing projects can migrate gradually, and compatibility with Swift 5 is maintained.