Swiftpack.co - Package - apple/swift-driver

Swift Compiler Driver

Swift's compiler driver is a program that coordinates the compilation of Swift source code into various compiled results: executables, libraries, object files, Swift modules and interfaces, etc. It is the program one invokes from the command line to build Swift code (i.e., swift or swiftc) and is often invoked on the developer's behalf by a build system such as the Swift Package Manager (SwiftPM) or Xcode's build system.

The swift-driver project is a new implementation of the Swift compiler driver that is intended to replace the existing driver with a more extensible, maintainable, and robust code base. The specific goals of this project include:

  • A maintainable, robust, and flexible Swift code base
  • Library-based architecture that allows better integration with build tools
  • Leverage existing Swift build technologies (SwiftPM, llbuild)
  • A platform for experimenting with more efficient build models for Swift, including compile servers and unifying build graphs across different driver invocations

Getting Started

Note: Currently, swift-driver is only compatible with trunk development snapshots from swift.org.

The preferred way to build swift-driver is to use the Swift package manager:

$ swift build

To use swift-driver in place of the existing Swift driver, create a symbolic link from swift and swiftc to swift-driver:

ln -s /path/to/built/swift-driver $SOME_PATH/swift
ln -s /path/to/built/swift-driver $SOME_PATH/swiftc

Swift packages can be built with the new Swift driver by overriding SWIFT_EXEC to refer to the swiftc symbolic link created above and SWIFT_DRIVER_SWIFT_FRONTEND_EXEC to refer to the original swift-frontend, e.g.,


Similarly, one can use the new Swift driver within Xcode by adding a custom build setting (usually at the project level) named SWIFT_EXEC that refers to $SOME_PATH/swiftc and adding -driver-use-frontend-path $TOOLCHAIN_DIR/usr/bin/swiftc to Other Swift Flags.

Building with CMake

swift-driver can also be built with CMake, which is suggested for environments where the Swift Package Manager is not yet available. Doing so requires several dependencies to be built first, all with CMake:

  • (Non-Apple platforms only) swift-corelibs-foundation
  • llbuild configure CMake with -DLLBUILD_SUPPORT_BINDINGS="Swift" and -DCMAKE_OSX_ARCHITECTURES=x86_64 (If building on Intel) when building
    cmake -B <llbuild-build-dir> -G Ninja <llbuild-source-dir> -DLLBUILD_SUPPORT_BINDINGS="Swift" -DCMAKE_OSX_ARCHITECTURES=x86_64
  • swift-argument-parser
  • Yams

Once those dependencies have built, build swift-driver itself:

cmake -B <swift-driver-build-dir> -G Ninja <swift-driver-source-dir> -DTSC_DIR=<swift-tools-support-core-build-dir>/cmake/modules -DLLBuild_DIR=<llbuild-build-dir>/cmake/modules -DYams_DIR=<yamls-build-dir>/cmake/modules -DArgumentParser_DIR=<swift-argument-parser-build-dir>
cmake --build <swift-driver-build-dir>

Developing swift-driver

The new Swift driver is a work in progress, and there are numerous places for anyone with an interest to contribute! This section covers testing, miscellaneous development tips and tricks, and a rough development plan showing what work still needs to be done.

Driver Documentation

For a conceptual overview of the driver, see The Swift Driver, Compilation Model, and Command-Line Experience. To learn more about the internals, see Driver Design & Internals and Parseable Driver Output.


Test using command-line SwiftPM or Xcode.

$ swift test --parallel

Integration tests are costly to run and are disabled by default. Enable them using SWIFT_DRIVER_ENABLE_INTEGRATION_TESTS environment variable. In Xcode, you can set this variable in the scheme's test action.


Some integration tests run the lit test suites in a Swift working copy. To enable these, clone Swift and its dependencies and build them with build-script, then set both SWIFT_DRIVER_ENABLE_INTEGRATION_TESTS and SWIFT_DRIVER_LIT_DIR, either in your Xcode scheme or on the command line:

  SWIFT_DRIVER_LIT_DIR=/path/to/build/Ninja-ReleaseAssert/swift-.../test-... \
  swift test -c release --parallel

Testing against swift compiler trunk

swift-driver Continuous Integration runs against the most recent Trunk Development snapshot published at swift.org/download.

When developing patches that have complex interactions with the underlying swift compiler frontend, it may be prudent to ensure that swift-driver tests also pass against the current tip-of-trunk swift. To do so, create an empty pull request against github.com/apple/swift and perform cross-repository testing against your swift-driver pull request #, for example:

@swift-ci smoke test

@swift-ci cross-repository testing facilities are described here.

Testing in Xcode with custom toolchain

After the toolchain is installed, Xcode needs to be told to use it. This can mean two things, building the driver with the toolchain and telling the driver to use the toolchain when running.

Building with the toolchain is easy, set the toolchain in Xcode: Menu Bar > Xcode > Toolchains > select your toolchain

Running the driver requires setting the TOOLCHAINS environment variable. This tells xcrun which toolchain to use (on darwin xcrun is used to find tools). This variable is the name of the toolchain and not the path (ex: Swift Development Snapshot). Important note: xcrun lookup is lower priority than the SWIFT_EXEC_*_EXEC family of environment variables, the tools directory, and any tools in the same directory as the driver (This includes a driver installed in a toolchain). Even though TOOLCHAINS is not highest priority it's a convenient way to run the xctest suite using a custom toolchain.

Preparing a Linux docker for debug

When developing on macOS without quick access to a Linux machine, using a Linux Docker is often helpful when debugging.

To get a docker up and running to the following:

  • Install Docker for Mac.
  • Get the newest swift docker image docker pull swift.
  • Run the following command to start a docker
$ docker run -v /path/to/swift-driver:/home/swift-driver \
  --cap-add=SYS_PTRACE --security-opt seccomp=unconfined \
  --security-opt apparmor=unconfined -it swift:latest bash
  • Install dependencies by running
$ apt-get update
$ apt-get install libsqlite3-dev
$ apt-get install libncurses-dev
  • You can now go to /home/swift-driver and run swift test --parallel to run your tests.

Rebuilding Options.swift

Options.swift, which contains the complete set of options that can be parsed by the driver, is automatically generated from the option tables in the Swift compiler. If you need to regenerate Options.swift, you will need to build the Swift compiler and then build makeOptions program with a -I that allows the generated Options.inc to be found, e.g.:

$ swift build -Xcc -I/path/to/build/Ninja-ReleaseAssert/swift-.../include --product makeOptions

Then, run makeOptions and redirect the output to overwrite Options.swift:

$ .build/path/to/makeOptions > Sources/SwiftOptions/Options.swift

Development Plan

The goal of the new Swift driver is to provide a drop-in replacement for the existing driver, which means that there is a fixed initial feature set to implement before the existing Swift driver can be deprecated and removed. The development plan below covers that feature set, as well as describing a number of tasks that can improve the Swift driver---from code cleanups, to improving testing, implementing missing features, and integrating with existing systems.

  • Code and documentation quality
    • Search for FIXME: or TODO:: there are lots of little things to improve!
    • Improve documentation of how to incorporate the driver into your own builds
    • Add useful descriptions to any Error thrown within the library
  • Option parsing
    • Look for complete "coverage" of the options in Options.swift. Is every option there checked somewhere in the driver?
    • Find a better way to describe aliases for options. Can they be of some other type OptionAlias so we can't make the mistake of (e.g.) asking for an alias option when we're translating options?
    • Diagnose unused options on the command line
    • Typo correction for misspelled option names
    • Find a better way than makeOptions.cpp to translate the command-line options from Swift's repository into Options.swift.
  • Platform support
    • Teach the DarwinToolchain to also handle iOS, tvOS, watchOS
    • Fill out the GenericUnixToolchain toolchain to get it working
    • Implement a WindowsToolchain
    • Implement proper tokenization for response files
  • Compilation modes
    • Batch mode
    • Whole-module-optimization mode
    • REPL mode
    • Immediate mode
  • Features
    • Precompiled bridging headers
    • Support embedding of bitcode
    • Incremental compilation
    • Parseable output, as used by SwiftPM
    • Response files
    • Input and primary input file lists
    • Complete OutputFileMap implementation to handle all file types uniformly
  • Testing
    • Build stuff with SwiftPM or Xcode or your favorite build system, using swift-driver. Were the results identical? What changed?
    • Shim in swift-driver so it can run the Swift repository's driver test suite.
    • Investigate differences in the test results for the Swift repository's driver test suite (above) between the existing and new driver.
    • Port interesting tests from the Swift repository's driver test suite over to XCTest
    • Fuzz the command-line options to try to crash the Swift driver itself
  • Integration
    • Teach the Swift compiler's build-script to build swift-driver.
    • Building on the above, teach the Swift compiler's build-toolchain to install swift-driver as the primary driver so we can test full toolchains with the new driver