Swift 6.4 (Xcode 27)

Swift 6.4 ships with Xcode 27 (announced September 15, 2026). Use it only when the project's toolchain is Xcode 27 or later; keep Swift 6.3 / Xcode 26 code paths otherwise. Async defer, cancellation shields, and weak let have deeper coverage in the swift-concurrency skill; Swift Testing changes are in swift-testing.

Contents

• Language
• Standard library
• Performance and ownership
• Interop
• Tooling
• Libraries
• Sources

Language

@available(anyAppleOS 27, *)
func useNewAPI() { }

@diagnose(DeprecatedDeclaration, as: ignored)
func legacyBridge() { /* calls deprecated API on purpose */ }

Verify exact @diagnose group names and syntax against the compiler before committing them.

Standard library

• withTaskCancellationShield: inside it, cancellation checks return false, so cleanup can finish. Keep the region short.
• mapKeyedValues: like mapValues, but the closure also receives the key.
• A new file path type based on Swift System; FilePath.stat() and FileDescriptor.stat() (with Stat, FileType, FileMode, and related types) arrive in System.
• Continuation: compile-time check that it is resumed once, with UnsafeContinuation efficiency.

Performance and ownership

• Iterable: for loops borrow elements in batches via spans, supporting noncopyable elements and avoiding reference counting. for prefers Sequence when both are available.
• UniqueBox, UniqueArray: standard library storage for noncopyable values without a fixed size limit.
• Ref, MutableRef: borrowed or mutable access to one value (for example, hoisting a dictionary lookup out of a loop).
• borrow and mutate accessors: in-place read-only or exclusive access without copying.
• Equatable, Comparable, Hashable work with noncopyable types; associated types may be noncopyable or non-escapable.
• @inline(always) (SE-0496) and @specialized(where:) (SE-0460) are covered in swift-attributes-interop.md; pair @inline(always) with final on class methods.
• withTemporaryAllocation uses OutputSpan.

Reach for these only after measuring. Default to ordinary value types and arrays.

Interop

• @c (SE-0495) exposes a Swift function to C and shipped in Swift 6.3; see swift-attributes-interop.md. WWDC26 also shows @implementation, which implements a function already declared in a C header; verify its availability in the toolchain.
• Span bridges to C, and C++20 std::span bridges to Swift Span.
• Swift closures convert to std::function; SWIFT_REFCOUNTED_PTR bridges smart pointers to Swift classes.
• Official Swift SDK for Android; Embedded Swift gains existentials and untyped throws.

Tooling

• Swift Build (the open-source Xcode build system) is the default backend for SwiftPM.
• Xcode 27 removes the ld64 linker; -ld_classic is no longer supported.
• swift test supports --maximum-repetitions with --repeat-until.
• A computed property with both an init accessor and a literal initial value fails to compile when the getter comes first; declare the init accessor first.

Libraries

• Subprocess 1.0: cross-platform process launching, streaming output through AsyncBufferSequence, strings() for line-by-line reads.
• Foundation: ProgressManager for async progress reporting with typed metadata; faster Data operations and URL parsing.

Sources

• https://sosumi.ai/videos/play/wwdc2026/262
• https://developer.apple.com/wwdc26/guides/swift/
• https://sosumi.ai/documentation/xcode-release-notes/xcode-27-release-notes