Swift Rules for Bazel

This repository contains rules for Bazel that can be used to build Swift libraries, tests, and executables for macOS and Linux.

To build applications for all of Apple's platforms (macOS, iOS, tvOS, and watchOS), they can be combined with the Apple Rules.

If you run into any problems with these rules, please file an issue!

Reference Documentation

Click here for the reference documentation for the rules and other definitions in this repository.

Compatibility

Please refer to the release notes for a given release to see which version of Bazel it is compatible with.

Quick Setup

1. Install Swift

Before getting started, make sure that you have a Swift toolchain installed.

Apple users: Install Xcode. If this is your first time installing it, make sure to open it once after installing so that the command line tools are correctly configured.

Linux users: Follow the instructions on the Swift download page to download and install the appropriate Swift toolchain for your platform. Take care to ensure that you have all of Swift's dependencies installed (such as ICU, Clang, and so forth), and also ensure that the Swift compiler is available on your system path.

2. Configure your workspace

Add the following to your WORKSPACE file to add the external repositories, replacing the urls and sha256 attributes with the values from the release you wish to depend on:

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

http_archive(
    name = "build_bazel_rules_swift",
    sha256 = "8407fa0fd04a7ce1d6bb95e90b216404466f809eda459c23cb57b5fa1ef9d639",
    url = "https://github.com/bazelbuild/rules_swift/releases/download/0.21.0/rules_swift.0.21.0.tar.gz",
)

load(
    "@build_bazel_rules_swift//swift:repositories.bzl",
    "swift_rules_dependencies",
)

swift_rules_dependencies()

load(
    "@build_bazel_rules_swift//swift:extras.bzl",
    "swift_rules_extra_dependencies",
)

swift_rules_extra_dependencies()

The swift_rules_dependencies macro creates a toolchain appropriate for your platform (either by locating an installation of Xcode on macOS, or looking for swiftc on the system path on Linux).

3. Additional configuration (Linux only)

The swift_binary and swift_test rules expect to use clang as the driver for linking, and they query the Bazel C++ API and CROSSTOOL to determine which arguments should be passed to the linker. By default, the C++ toolchain used by Bazel is gcc, so Swift users on Linux need to override this by setting the environment variable CC=clang when invoking Bazel.

This step is not necessary for macOS users because the Xcode toolchain always uses clang.

Building with Custom Toolchains

macOS hosts: You can build with a custom toolchain installed in /Library/Developer/Toolchains instead of Xcode's default. To do so, pass the following flag to Bazel:

--define=SWIFT_CUSTOM_TOOLCHAIN=toolchain.id

where toolchain.id is the value of the CFBundleIdentifier key in the toolchain's Info.plist file.

To list the available toolchains and their bundle identifiers, you can run:

bazel run @build_bazel_rules_swift//tools/dump_toolchains

Linux hosts: At this time, Bazel uses whichever swift executable is encountered first on your PATH.

Supporting remote builds

To make remote builds work correctly with debugging and general reproducibility see this doc

Future Work

• Support for building and linking to shared libraries (.dylib/.so) written in Swift.
• Interoperability with Swift Package Manager.
• Migration to the Bazel platforms/toolchains APIs.
• Support for multiple toolchains, and support for non-Xcode toolchains on macOS.
• Automatically download a Linux toolchain from swift.org if one is not already installed.

Acknowledgments

We gratefully acknowledge the following external packages that rules_swift depends on:

• Apple Support for Bazel (Google)
• Bazel Skylib (Google)
• JSON for Modern C++ (Niels Lohmann)
• Protocol Buffers (Google)
• Swift gRPC (Google)
• Swift Protobuf (Apple)
• zlib (Jean-loup Gailly and Mark Adler)