Libraries

The Swift toolchain ships a number of Swift modules alongside the compiler. They divide into:

• The standard library (Swift module) -- the always-imported core types and protocols.
• Concurrency runtime modules -- _Concurrency, Distributed, Synchronization, Observation, Volatile.
• String processing -- _StringProcessing, RegexBuilder, RegexParser.
• Differentiation -- _Differentiation.
• C++ interop -- Cxx, CxxStdlib.
• Embedded -- EmbeddedPlatform and the SwiftDirectRuntime.
• Reflection / remote inspection -- _RemoteInspection, SwiftRemoteMirror.
• Compatibility shims -- CompatibilityOverride, SwiftOnoneSupport, LLVMSupport, SwiftShims.

This section documents the larger ones individually. The smaller modules are summarized below.

Pages

• Standard library
• Concurrency
• String processing and Regex
• Distributed actors
• C++ interop overlay
• Embedded Swift
• Runtimes -- the alternative stdlib build

Smaller modules at a glance

Module	Code	Purpose
Observation	stdlib/public/Observation/	The @Observable macro + runtime support for SwiftUI-style observation.
Synchronization	stdlib/public/Synchronization/	Atomic<T>, Mutex, WordPair etc.
Volatile	stdlib/public/Volatile/	VolatileMappedRegister<T> for memory-mapped IO (embedded).
_Differentiation	stdlib/public/Differentiation/	Runtime support for the differentiable programming feature.
_RemoteInspection	stdlib/public/RemoteInspection/	Reflective enumeration of remote heap objects.
SwiftRemoteMirror	stdlib/public/SwiftRemoteMirror/	The C library used by swift-inspect and Xcode's memory tools.
SwiftOnoneSupport	stdlib/public/SwiftOnoneSupport/	Hand-written specializations to keep -Onone builds fast.
SwiftShims	stdlib/public/SwiftShims/	Headers shared between C++ runtime and Swift stdlib.
LLVMSupport	stdlib/public/LLVMSupport/	A minimal subset of llvm/ADT/ available in the runtime.
CompatibilityOverride	stdlib/public/CompatibilityOverride/	Mechanism for back-deployed runtime fixes ("compat50", etc.).
RegexParser	stdlib/public/RegexParser/	The internal regex literal parser.
Cxx	stdlib/public/Cxx/	The C++ interop overlay (see C++ interop).
Platform overlays	stdlib/public/Platform/	C standard library overlays per platform.
SDK overlays	stdlib/public/ClangOverlays/	Overlays for SDK-bundled C libs.
WinSDK	stdlib/public/Windows/	Windows SDK overlays.
_StringProcessing	stdlib/public/StringProcessing/	Internal regex match engine.
RegexBuilder	stdlib/public/RegexBuilder/	Result-builder DSL for regexes.