Dependencies

The Swift toolchain depends on a number of side-by-side checkouts. utils/update-checkout keeps them at compatible revisions; the canonical list is utils/update-checkout/update-checkout-config.json.

Required side-by-side repos

Repo	Purpose
apple/llvm-project	The Swift fork of LLVM. Provides clang, lldb, lld, the LLVM libraries IRGen and the optimizer link against.
apple/swift-cmark	CommonMark parser used for doc-comment markup.
apple/swift-driver	The Swift-implemented driver; the default user-facing front-end.
apple/swift-tools-support-core	Common utilities used by SwiftPM, swift-driver, etc.
apple/swift-package-manager	swift package, swift build, swift test.
apple/swift-syntax	Swift-implemented Syntax tree + parser library.
apple/swift-corelibs-foundation	The Foundation overlay implementation for non-Apple platforms.
apple/swift-corelibs-libdispatch	libdispatch for non-Apple platforms.
apple/swift-corelibs-xctest	The XCTest implementation for non-Apple platforms.
apple/swift-experimental-string-processing	Upstream of the in-tree regex modules.
apple/llbuild	Low-level build engine used by SwiftPM.
apple/yams, apple/swift-collections, and others	Pulled in by SwiftPM.

The full list (with revisions per release branch) is in utils/update-checkout/update-checkout-config.json.

How they fit together

graph TD
    Swift[apple/swift] --> LLVM[apple/llvm-project]
    Swift --> SwiftSyntax[apple/swift-syntax]
    Swift --> SwiftDriver[apple/swift-driver]
    Swift --> SwiftPM[apple/swift-package-manager]
    Swift --> CMark[apple/swift-cmark]
    Swift --> StringProcessing[apple/swift-experimental-string-processing]
    Foundation[apple/swift-corelibs-foundation] --> Swift
    Dispatch[apple/swift-corelibs-libdispatch] --> Swift
    XCTest[apple/swift-corelibs-xctest] --> Swift
    SwiftPM --> Driver[swift-driver]
    SwiftPM --> TSC[swift-tools-support-core]
    SwiftPM --> LLBuild[swift-llbuild]

When you run update-checkout, it clones (or pulls) all of these into the same parent directory. build-script walks them and invokes their build systems in dependency order.

Build-time dependencies

Beyond the side-by-side repos, the build needs:

• A C++17 toolchain (Clang on Linux/macOS, MSVC + clang-cl on Windows).
• Python 3.6+ for the build scripts and gyb.
• CMake 3.x and Ninja.
• Platform-specific bits (libicu, libpython3, sqlite3, swig on Linux; Visual Studio + Windows SDK on Windows).

See docs/HowToGuides/GettingStarted.md for the per-platform list.

Runtime dependencies

A built Swift program depends on:

• The Swift runtime (libswiftCore.dylib / libswiftCore.so / static-linked equivalent).
• The Swift Concurrency overlay (libswift_Concurrency.dylib / .so).
• libdispatch (Apple), or the corelibs version on Linux.
• The platform's C/C++ runtime.

For embedded targets, the runtime is replaced by SwiftDirectRuntime and most of the standard library is unavailable.