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Gazelle Plugin for Swift and Swift Package Rules for Bazel

Build

This repository contains a Gazelle plugin and Bazel repository rules that can be used to download, build, and consume Swift packages. The rules in this repository build the external Swift packages using rules_swift and native C/C++ rulesets making the Swift package products and targets available as Bazel targets.

This repository is designed to fully replace rules_spm and provide utilities to ease Swift development inside a Bazel workspace.

Table of Contents

Documentation

Prerequisites

Mac OS

Be sure to install Xcode.

Linux

You will need to install Swift. Make sure that running swift --version works properly.

Don't forget that rules_swift expects the use of clang. Hence, you will need to specify CC=clang before running Bazel.

Finally, help rules_swift and rules_swift_package_manager find the Swift toolchain by ensuring that a PATH that includes the Swift binary is available in the Bazel actions.

cat >>local.bazelrc <<EOF
build --action_env=PATH
EOF

This approach is necessary to successfully execute the examples on an Ubuntu runner using Github actions. See the CI GitHub workflow for more details.

Quickstart

The following provides a quick introduction on how to set up and use the features in this repository. These instructions assume that you are using Bazel modules to load your external dependencies. If you are using Bazel's legacy external dependency management, we recommend using Bazel's hybrid mode, then follow the steps in this quickstart guide.

Also, check out the examples for more information.

1. Enable bzlmod

This repository supports bzlmod.

common --enable_bzlmod

2. Configure your MODULE.bazel to use rules_swift_package_manager.

Add a dependency on rules_swift_package_manager.

bazel_dep(name = "rules_swift_package_manager", version = "0.43.0")

In addition, add the following to load the external dependencies described in your Package.swift and Package.resolved files.

swift_deps = use_extension(
    "@rules_swift_package_manager//:extensions.bzl",
    "swift_deps",
)
swift_deps.from_package(
    resolved = "//:Package.resolved",
    swift = "//:Package.swift",
)
use_repo(
    swift_deps,
    "swift_deps_info",  # This is generated by the ruleset.
    # The name of the Swift package repositories will be added to this declaration in step 4 after
    # running `bazel mod tidy`.
    # NOTE: The name of the Bazel external repository for a Swift package is `swiftpkg_xxx` where
    # `xxx` is the Swift package identity, lowercase, with punctuation replaced by `hyphen`. For
    # example, the repository name for apple/swift-nio is `swiftpkg_swift_nio`.
)

You will also need to add a dependency on rules_swift.

NOTE: Some Swift package manager features (e.g., resources) use rules from rules_apple. It is a dependency for rules_swift_package_manager. However, you do not need to declare it unless you use any of the rules in your project.

(Optional) Use swift_package repository for updating packages

The swift_deps module extension will by default generate a swift_package repository which can be used to execute swift package commands. This is useful if you'd like to control the flags and behavior of swift package, as well as for using the correct swift binary according to the Bazel configured toolchain.

For example, to resolve the Package.swift file:

bazel run @swift_package//:resolve

To update packages to their latest supported version:

bazel run @swift_package//:update

Both targets support passing arguments as well, so for example, you could update a single package:

bazel run @swift_package//:update -- MyPackage

These targets will update the Package.resolved file defined in swift_deps.from_package. The targets come with default flags applied to enable the best Bazel compatibility, if you wish to configure it further, you can do so with configure_swift_package:

# MODULE.bazel

swift_deps.configure_swift_package(
    build_path = "spm-build",
    cache_path = "spm-cache",
    dependency_caching = "false",
    manifest_cache = "none",
    manifest_caching = "false",
)

If you do not want to use the swift_package repository you can disable it in the swift_deps.from_package call:

swift_deps.from_package(
    declare_swift_package = False,  # <=== Disable the `swift_package` repository
    resolved = "//:Package.resolved",
    swift = "//:Package.swift",
)

(Optional) Enable swift_deps_info generation for the Gazelle plugin

If you will be using the Gazelle plugin for Swift, you will need to enable the generation of the swift_deps_info repository by enabling declare_swift_deps_info.

swift_deps.from_package(
    declare_swift_deps_info = True, # <=== Enable swift_deps_info generation for the Gazelle plugin
    resolved = "//:Package.resolved",
    swift = "//:Package.swift",
)

You will also need to add a dependency on Gazelle.

3. Create a minimal Package.swift file.

Create a minimal Package.swift file that only contains the external dependencies that are directly used by your Bazel workspace.

// swift-tools-version: 5.7

import PackageDescription

let package = Package(
    name: "my-project",
    dependencies: [
        // Replace these entries with your dependencies.
        .package(url: "https://github.com/apple/swift-argument-parser", from: "1.2.0"),
        .package(url: "https://github.com/apple/swift-log", from: "1.4.4"),
    ]
)

The name of the package can be whatever you like. It is required for the manifest, but it is not used by rules_swift_package_manager. If your project is published and consumed as a Swift package, feel free to populate the rest of the manifest so that your package works properly by Swift package manager. Just note that the Swift Gazelle plugin does not use the manifest to generate Bazel build files, at this time.

4. Run swift package update

This will invoke Swift Package Manager and resolve all dependencies resulting in creation of Package.resolved file.

5. Run bazel mod tidy.

This will update your MODULE.bazel with the correct use_repo declaration.

6. Add Gazelle targets to BUILD.bazel at the root of your workspace.

Add the following to the BUILD.bazel file at the root of your workspace.

load("@gazelle//:def.bzl", "gazelle", "gazelle_binary")

# Ignore the `.build` folder that is created by running Swift package manager
# commands. Be sure to configure your source control to ignore it, as well.
# (i.e., add it to your `.gitignore`).
# NOTE: Swift package manager is not used to build any of the external packages.
# gazelle:exclude .build

# This declaration builds a Gazelle binary that incorporates all of the Gazelle
# plugins for the languages that you use in your workspace. In this example, we
# are only listing the Gazelle plugin for Swift from rules_swift_package_manager.
gazelle_binary(
    name = "gazelle_bin",
    languages = [
        "@rules_swift_package_manager//gazelle",
    ],
)

# This target updates the Bazel build files for your project. Run this target
# whenever you add or remove source files from your project. The
# `swift_deps_info` repository is generated by `rules_swift_package_manager`. It
# creates a target, `@swift_deps_info//:swift_deps_index`, that generates a JSON
# file which maps Swift module names to their respective Bazel target.
gazelle(
    name = "update_build_files",
    data = [
        "@swift_deps_info//:swift_deps_index",
    ],
    extra_args = [
        "-swift_dependency_index=$(location @swift_deps_info//:swift_deps_index)",
    ],
    gazelle = ":gazelle_bin",
)

7. Create or update Bazel build files for your project.

Generate/update the Bazel build files for your project by running the following:

bazel run //:update_build_files

8. Build and test your project.

Build and test your project.

bazel test //...

9. Check in Package.swift, Package.resolved, and MODULE.bazel.

  • The Package.swift file is used by rules_swift_package_manager to generate information about your project's dependencies.
  • The Package.resolved file specifies that exact versions of the downloaded dependencies that were identified.
  • The MODULE.bazel contains the declarations for your external dependencies.

10. Start coding

You are ready to start coding.

Tips and Tricks

The following are a few tips to consider as you work with your repository:

  • When you add or remove source files, run bazel run //:update_build_files. This will create/update the Bazel build files in your project. It is designed to be fast and unobtrusive.
  • If things do not appear to be working properly, run the following:
    • bazel run //:update_build_files
  • Do yourself a favor and create a Bazel target (e.g., //:tidy) that runs your repository maintenance targets (e.g., //:update_build_files, formatting utilities) in the proper order. If you are looking for an easy way to set this up, check out the //:tidy declaration in this repository and the documentation for the tidy macro.
  • Are you trying to use a Swift package and it just won't build under Bazel? If you can figure out how to fix it, you can patch the Swift package. Check out our document on patching Swift packages.