Swift 6 is still one of the most significant updates in the history of Apple’s programming language. It improved concurrency, safety, and performance, making Swift more reliable and scalable across modern hardware and complex apps.
The release undoubtedly brought a host of powerful new features, including enhanced support for concurrent execution, stricter safety rules, and some long-awaited enhancements that developers had been requesting.
Before we dive into our go-to Swift 6 improvements, let’s take a quick tour through how Swift evolved leading up to this release.
A Look Back: A Quick Recap of Swift Evolution
Swift has evolved steadily since its debut in 2014. The Swift 5 series laid the groundwork for Swift 6 with incremental improvements in concurrency and type safety. Here’s a quick recap of the journey from Swift 5.1 to 5.10:
- Swift 5.2 (Mar 2020) enhanced key path expressions, improved diagnostic messages, and boosted compiler performance.
- Swift 5.6 (Mar 2022) continued that journey with more robust support for type placeholders and existential Any, and added plugin support for Swift Package Manager.
- Swift 5.7 (Sep 2022) brought in optional shorthand syntax, built-in Swift Regex, and made Swift's standard library leaner and faster. C interoperability also saw meaningful improvements
- Swift 5.9 (Sep 2023) delivered type and value parameter packs, removing the infamous "10 views in a VStack" limitation in SwiftUI. It also introduced macros, new if/switch expressions, and allowed discarding task groups for cleaner concurrent code.
- Swift 5.10 (Mar 2024) focused heavily on safety. It introduced strict concurrency for global and static variables, preventing data races in multithreaded environments. The update also plugged a major hole in default value isolation using isolated default value expressions.
Our Go-To Features of Swift 6
count(where:): A More Efficient Way to Filter and Count
With the acceptance of SE-0220, Swift now includes the count(where:) method for sequences. This allows you to count elements matching a condition—without using filter, which creates a temporary array in memory.
This is a small but powerful change, especially when working with large collections. It improves performance and reduces memory overhead by doing a single-pass iteration.
Count even numbers in an array:
let numbers = [3, 6, 9, 12, 15]
let evenCount = numbers.count { $0.isMultiple(of: 2) }
// print: [6, 12]
Count lines containing the word “error”:
let logs = ["info", "warning", "error: not found", "error: timeout", "success"]
let errorCount = logs.count { $0.contains("error") }
// print: ["error: not found", "error: timeout"]
The method works with any type that conforms to Sequence, including Array, Set, and Dictionary.
Typed Throws: Safer and More Predictable Error Handling
With SE-0413, Swift 6 introduces typed throws – a long-awaited feature that lets you specify the exact error type a function can throw.
This makes your code more robust and helps the compiler catch unhandled errors at compile time. It also enables you to write cleaner catch blocks that only handle specific error types.
Example:
enum DownloadError: Error {
case timeout
}
func fetchData(from url: String) throws(DownloadError) {
let isTimeout = true
if isTimeout {
throw .timeout
}
}
Typed throws help especially in embedded systems or safety-critical environments where every possible error must be accounted for. However, for public APIs or libraries, typed throws should be used with care. If your error list grows later, changing the error type will be a breaking change.
Remember:
- Only one error type is allowed: throws(MyError).
- throws(any Error) behaves like traditional throws.
- throws(Never) means the function is guaranteed not to throw.
Parameter Pack Iteration: Powerful Tuple Metaprogramming
With SE-0408, Swift now supports iteration over parameter packs, building on the value and type parameter packs introduced in Swift 5.9.
Before Swift 6, comparing tuples of arbitrary length was impossible beyond six elements. This update changes that by letting you iterate over tuple elements generically.
Comparing tuples of any size:
func == <each T: Equatable>(lhs: (repeat each T), rhs: (repeat each T)) -> Bool {
for (l, r) in repeat (each lhs, each rhs) {
if l != r { return false }
}
return true
}
This eliminates boilerplate and makes tuple-based APIs far more scalable. It also opens the door for more advanced type-level metaprogramming in SwiftUI, such as iterating over children in a TupleView.
Future directions might include extending functions like zip() to work with any number of sequences, not just two, unlocking new flexibility for generic algorithms.
128-Bit Integers: High-Precision Number Crunching
Swift 6 adds support for 128-bit integers via SE-0425. The new types are:
- Int128: signed 128-bit integer
- UInt128: unsigned 128-bit integer
These types are fully integrated into Swift’s numeric ecosystem. They support all standard arithmetic, comparison, conversion, and bitwise operations.
Example:
let maxValue: Int128 = 170_141_183_460_469_231_731_687_303_715_884_105_727
This is a big win for developers working on cryptography, hash algorithms, or low-level data structures where 64-bit integers just aren’t enough. It also aligns Swift with other modern languages like Rust and C++ that already support 128-bit arithmetic.
Fine-Grained Access Control for import: Cleaner APIs and Better Encapsulation
Swift 6 also introduces a major quality-of-life improvement with SE-0409: the ability to specify access levels directly in import statements. This makes it easier to manage module visibility and avoid leaking internal dependencies through public APIs.
For example:
private import ImageProcessingKit
This means ImageFilters is available within the current file or module but cannot be re-exposed through a public API – ideal for layered architectures.
Let’s walk through a scenario:
Imagine you have a public struct in the ImageFilters module:
public struct EditedImage {
// ...
}
And you're using that in the PhotoEditor module:
// В PhotoEditor
public func applyFilter(to imageID: Int) -> EditedImage {
return EditedImage()
}
Previously, any module importing PhotoEditor would also gain access to EditedImage, even if you never intended to expose it. With Swift 6, you can fix this by limiting the import:
internal import ImageFilters
Now, if applyFilter() attempts to return EditedImage, the compiler will flag it, because you can’t expose types from a non-public import in a public API.
Other supported modifiers include: private, fileprivate, internal, package, and public.
Swift 6 sets internal as the new default for imports, promoting better encapsulation. In Swift 5, the default remains public for compatibility.
Work with Disjoint Ranges Using RangeSet: More Powerful Collection Slicing
Swift 6 adds a new tool to your collection toolbox: RangeSet, introduced via SE-0270. This type lets you operate on multiple non-contiguous ranges in a single pass – perfect for selectively updating or accessing disjoint elements in an array.
If you’ve used IndexSet in Foundation, this will feel familiar. But unlike IndexSet, RangeSet works with any Comparable type, not just Int.
Here’s an example. Suppose you have a list of books and want to find those with a rating above 85:
struct Book {
let title: String
let rating: Int
}
let library = [
Book(title: "The Silent Forest", rating: 88),
Book(title: "Deep Waters", rating: 76),
Book(title: "Stars Beyond", rating: 92),
Book(title: "Hidden Truths", rating: 81)
]
let highlyRatedIndices = library.indices { $0.rating > 85 }
for book in library[highlyRatedIndices] {
print("\"\(book.title)\" received a rating of \(book.rating)")
}
This works because Swift 6 now supports subscript access using RangeSet. The result is a DiscontiguousSlice, which, unlike regular Slice, doesn’t require contiguous elements.
Under the hood, RangeSet conforms to SetAlgebra, allowing powerful set operations like:
- union(_:) – merge overlapping or adjacent ranges
- intersection(_:) – find common ranges
- isSuperset(of:) – test if one RangeSet contains another
This enables more expressive and safer manipulation of collections, especially when filtering or modifying multiple elements conditionally.
Major Advancements for noncopyable Types
Noncopyable types were introduced in Swift 5.9, but Swift 6 takes them to a new level with SE-0427, SE-0429, and SE-0432.
These types represent values that are consumed once used, similar to Rust’s ownership model, and are critical for resource-safe programming.
Example:
struct Notification: ~Copyable {
var sender: String
private var content: String
consuming func display() {
print("\(sender): \(content)")
}
}
Swift 6 Improvements for noncopyable Types:
SE-0427: Noncopyable Generics & Protocols
Types are Copyable by default. To declare one as noncopyable, explicitly mark it with ~Copyable.
You can now write:
struct SecureBox<T: ~Copyable> { var content: T }
This allows noncopyable generics and protocols, enabling more flexible design while preserving uniqueness constraints.
SE-0429: Partial Consumption Support
You can now access parts of noncopyable structs without consuming the whole value:
struct Envelope: ~Copyable {
var notification: Notification
consuming func open() {
notification.display() // previously an error, now okay
}
}
SE-0432: switch Now Supports Noncopyables
You can also pattern-match noncopyable types in switch statements, with 'where' clauses:
enum SecureItem: ~Copyable {
case labeled(Envelope)
case unlabeled(Notification)
}
switch consume item {
case .unlabeled(let note) where note.sender == "System Admin":
print("Alert: check system logs")
note.display()
default:
break
}
These features bring noncopyable types much closer to being as ergonomic as regular ones, while still preventing unintended copies of unique or sensitive data – ideal for secure contexts like key handling, file descriptors, or embedded systems.
Introducing BitwiseCopyable: Faster, Safer Low-Level Optimizations
Swift 6 also adds a new protocol, BitwiseCopyable, with SE-0426. This marker tells the compiler that a type can safely be copied bit-for-bit in memory, no need to run a copy initializer.
The result is faster generated code, especially for simple value types.
Swift automatically marks many built-in types as BitwiseCopyable, including:
- Int, Double, Bool
- StaticString, Duration
- RawOptionSet enums
- Most structs without reference types or non-trivial copy logic
You usually don’t have to declare it manually, but there are important caveats:
- If your public or package type is marked @frozen, Swift may infer BitwiseCopyable automatically.
- If not, the compiler will not infer it – you must explicitly conform or suppress it.
To prevent automatic inference:
@frozen
public enum LogLevel: ~BitwiseCopyable {
case debug, info, warning, error
}
Note: this override must be declared at the type’s definition, not in an extension.
This lets Swift generate more optimized machine code without sacrificing safety or breaking ABI compatibility. For library authors dealing with frozen public types, this is a critical new tool for managing performance and memory layout.
Seamless C++ Interoperability: Swift 6 Bridges the Gap
Swift 6 takes a big step forward in C++ interoperability. While Swift 5.9 laid the groundwork with initial two-way interop, Swift 6 significantly extends these capabilities, making it easier to work across language boundaries in real-world, multi-platform codebases.
Here’s what’s new:
Support for C++ Move-Only Types
Swift 6 can now accurately represent C++ types that are non-copyable – those lacking a copy constructor. These are imported into Swift as ~Copyable, preserving their move-only semantics and preventing misuse.
You can even force Swift to treat a type as non-copyable using a SWIFT_NONCOPYABLE annotation in your C++ code, even if a copy constructor exists:
struct SWIFT_NONCOPYABLE MyBuffer {
MyBuffer(const MyBuffer&) = default;
};
This allows Swift to better reflect your C++ intent, especially for types managing unique system resources (like file descriptors or GPU buffers).
Calling Virtual Methods from Swift
Swift can now call virtual methods on C++ types, so long as the C++ class is marked with SWIFT_SHARED_REFERENCE or SWIFT_IMMORTAL_REFERENCE. This unlocks full access to C++ polymorphism from Swift, including method overriding and dynamic dispatch.
You get the power of virtual function tables in Swift code, no extra glue needed.
Default Parameters Now Work
Functions or methods in C++ with default arguments now correctly import into Swift. That means you can call them with fewer parameters, and Swift will automatically apply the defaults just like C++ would.
C++ function with defaults:
void configure(int level = 3);
Swift usage:
configure() // level will be equal to 3
STL Support: std::optional, std::map, and More
Swift 6 expands its support for standard C++ library types, including containers like std::optional, std::vector, and std::map. This means you can use popular data structures from C++ directly in Swift – perfect for hybrid codebases or transitioning legacy logic incrementally.
Embedded Swift: Compact Code for Microcontrollers
With Swift 6, Apple introduces a preview of Embedded Swift – a specialized subset of the language tailored for constrained environments like microcontrollers and bare-metal systems.
Designed for zero-runtime, low-footprint applications, Embedded Swift allows developers to write Swift for platforms that traditionally rely on C or assembly. It’s aimed at ARM Cortex-M, RISC-V, and similar architectures.
Key features:
- No runtime overhead: no metadata, dynamic type system, or heap allocation required.
- Static memory layout: perfect for environments where memory must be fully known at compile time.
- Lean binaries: only the necessary code is emitted – ideal for firmware or bootloaders.
Building for a microcontroller:
swiftc -target riscv32 -O -embedded -o firmware.elf main.swift
While still experimental and not yet production-ready, Embedded Swift opens the door to writing expressive, memory-safe firmware in a high-level language. It’s perfect for hobbyists, embedded developers, or anyone tired of fighting undefined behavior in C.
Debug Smarter with @DebugDescription
In Swift 6, debugging becomes more customizable thanks to the new @DebugDescription macro introduced in SE-0431. This macro lets you fine-tune how your custom types appear in LLDB or the Xcode/VSCode debugger UI – without requiring custom summary scripts or runtime code.
It enhances types conforming to CustomDebugStringConvertible, which already define a debugDescription string.
Example usage:
@DebugDescription
struct Organization: CustomDebugStringConvertible {
var id: String
var name: String
var manager: Person
var debugDescription: String {
"#\(id) \(name) [\(manager.name)]"
}
}
Now, when using the p command in LLDB, the object will be displayed in the following format:
(lldb) p myOrg
(Organization) myOrg = "`#100 Worldwide Travel [Jonathan Swift]`"
You can still override debugDescription if needed, but @DebugDescription provides a clean, declarative way to define summaries visible in debugger views, making debugging faster and more intuitive.
Faster LLDB Debugging with Explicit Module Support
Swift 6 also brings dramatic performance improvements to LLDB, especially for builds using explicit modules. Previously, LLDB would attempt to recompile missing Clang modules on the fly, causing slowdowns when evaluating expressions like p or po.
Now, LLDB can load modules directly from the build’s artifacts – no recompilation needed. This improves:
- Startup time of debugging sessions
- Speed of first expression evaluation
- Reliability of symbol resolution in complex builds
To take advantage of this, ensure your project uses explicit module builds, especially if you rely on heavy Clang interop (e.g., importing system libraries or bridging headers).
This change is a huge win for Swift+iOS teams who’ve experienced sluggish debugger behavior after a clean build or when switching Xcode versions.
Unified Foundation Across Platforms
In Swift 6, the Foundation framework has been fully unified across all supported platforms. The modern, portable implementation, now written entirely in Swift, delivers greater consistency, enhanced reliability, and is open source.
Since Swift 5.9, Apple platforms like macOS and iOS already adopted the Swift-based Foundation. With Swift 6, Linux and Windows now benefit from this same foundation layer, enabling true cross-platform consistency.
Core types like JSONDecoder, URL, Calendar, FileManager, and ProcessInfo have been completely rewritten in Swift. These now share a unified implementation across macOS 15 and iOS 18, boosting performance and reliability across the board.
Swift 6 also brings many modern APIs – such as FormatStyle, ParseStrategy, Predicate, and JSON5 – to all supported platforms. In addition, new APIs introduced in Swift 6 include:
- Expression
- Extended calendar enumerations
- Recurrence rules for calendars
- Improved formatting styles
These APIs are available on macOS, iOS, Linux, and Windows, with contributions from the Swift open-source community.
For apps using Foundation on Linux or Windows, these enhancements are available automatically. And for developers concerned about binary size, the new FoundationEssentials library offers a lightweight subset of Foundation that excludes localization and internationalization features, reducing file size while keeping core functionality intact.
Introducing Swift Testing
Swift 6 introduces a brand-new built-in testing framework: Swift Testing. Designed from the ground up specifically for Swift, it offers expressive and intuitive APIs that make writing, organizing, and scaling tests easier than ever.
Key features:
- Macros-first design: Swift Testing heavily uses macros for defining tests. Use @Test for individual tests and @Suite for test groups. Both accept configuration arguments to fine-tune behavior.
- Rich failure output: Macros like #expect and #require capture detailed expression data and sub-values to provide clear, readable failure messages.
- Parameterization support: Easily run tests with multiple data sets—ideal for larger projects.
Example:
@Test("Continents mentioned in the video", arguments: [
"Beach",
"By the lake",
"Camping in the forest"
])
func mentionedContinents(videoName: String) async throws {
let videoLibrary = try await VideoLibrary()
let video = try #require(await videoLibrary.video(named: videoName))
#expect(video.mentionedContinents.count <= 3)
}
Swift Testing is fully integrated into the Swift 6 toolchain. Just import Testing – no need to declare an external dependency. The macro implementations are built-in, so there's no need to compile swift-syntax separately.
Swift Package Manager runs Swift Testing alongside existing XCTest tests (if present), and logs output from both. The library supports all officially supported Swift platforms: Apple platforms, Linux, and Windows.
Full Concurrency Checking by Default
Swift 6 now enables strict concurrency checking by default. The compiler automatically analyzes data access across async and multithreaded code, helping catch potential race conditions and thread-safety issues.
The core innovation here is SE-0414: Region-Based Isolation, which allows the compiler to reason about concurrency correctness even without explicit Sendable conformance. This dramatically reduces false positives and improves developer experience when working with asynchronous code.
Example:
class User {
var name = "Anonymous"
}
struct ContentView: View {
var body: some View {
Text("Hello, world!")
.task {
let user = User()
await loadData(for: user)
}
}
func loadData(for user: User) async {
print("Loading data for \(user.name)…")
}
}
Before Swift 6, this code might trigger a warning because the User type isn't Sendable. But with region-based isolation, the compiler understands that the object stays within a single task, no more warning.
Additional Concurrency Improvements
sending Keyword – SE-0430
Introduces the sending keyword to mark values transferred between isolated regions, giving developers fine-grained control over data movement and safety.
Dynamic Actor Isolation – SE-0423
Adds support for dynamic actor isolation, improving compatibility with Objective-C frameworks and legacy codebases.
Isolation Inheritance – SE-0420
Async functions now inherit isolation from their caller’s context unless explicitly stated. This reduces boilerplate like repeated @MainActor or @globalActor annotations.
Property Wrappers and @MainActor – SE-0401
Previously, marking a property wrapper with @MainActor implicitly made the entire type @MainActor. In Swift 6, this is no longer automatic – you must annotate explicitly.
Example:
@MainActor
class ViewModel: ObservableObject {
func logIn() { print("Logging in…") }
}
@MainActor
struct LoginView: View {
@StateObject private var model = ViewModel()
var body: some View {
Button("Log In", action: model.logIn)
}
}
Global Variables and Isolation – SE-0412
Global/static variables must now be:
- Immutable (let), or
- Actor-isolated (@MainActor, @globalActor), or
- Explicitly marked nonisolated.
Example:
struct Config {
static let maxRetries = 3
}
@MainActor
var sessionID = "ABC123"
nonisolated(unsafe) var cache = ["key": "value"]
Note: Use nonisolated(unsafe) only when absolutely certain the access is thread-safe.
Default Parameter Isolation – SE-0411
Default parameter values in initializers and functions now inherit the isolation of the surrounding function.
Example:
@MainActor
class Logger {}
@MainActor
class Manager {
init(logger: Logger = Logger()) {}
}
Creating Logger() here becomes part of the @MainActor context, since the initializer is marked as such.
Wrapping Up: Swift 6 Sets a New Standard
Swift 6 marks a pivotal moment in the evolution of Apple’s programming language, bringing long-anticipated features that elevate both developer experience and runtime safety across all platforms.
With its default strict concurrency checking, Swift now enforces safe multithreading practices while reducing false positives and boilerplate through innovations like region-based isolation and inheritance of actor contexts. This makes writing asynchronous code more intuitive, reliable, and future-proof.
The language’s new opt-in feature flags help eliminate legacy inconsistencies and boost cross-platform predictability. Improvements in type inference and macro capabilities further streamline Swift’s syntax and expressiveness without sacrificing control.
The fully unified Swift-based Foundation layer brings macOS, iOS, Linux, and Windows into a consistent ecosystem, with high-performance APIs available across the board. Developers now benefit from cross-platform parity without compromising speed or capability.
Finally, the introduction of Swift Testing makes test-driven development more accessible, scalable, and expressive with macro-powered syntax, parameterization, and full integration into the Swift toolchain.
Together, these advancements make Swift 6 not just a language update, but a forward-looking foundation for building safe, maintainable, and modern software on Apple platforms and far beyond.
Ready to build your custom iOS app with cutting-edge Swift 6 tech? Reach out or book a consultation right away to get started!
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