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Design for Swift Bazel

This document provides a high-level description for the design of the rules and utilities provided by the Swift Bazel repository.

Table of Contents

Goals

  1. Generate Bazel build files for Swift source files in a Bazel workspace.
  2. Allow a Bazel workspace to download and use external Swift packages in a Bazel workspace.

Implementation

The implementation in this repository is separated into two parts:

  1. A Gazelle plugin.
  2. Bazel repository rules: swift_package and local_swift_package.

The Gazelle plugin generates Bazel build and Starlark files for your project. The swift_package repository rule generates Bazel build files for the external Swift packages.

Gazelle Plugin

The Gazelle plugin is implemented in Go. It has two modes of operation: update-repos and update.

The update-repos mode

  1. Resolves the direct and transitive dependencies for the project using a Package.swift file.
  2. Writes a Package.resolved file.
  3. Writes a swift_deps_index.json file.
  4. Writes swift_package declarations for the direct and transitive dependencies.

The update mode

  1. Reads the swift_deps_index.json file.
  2. Inspects the project looking for Swift source files.
  3. Identifies the Bazel packages that should contain Swift declarations.
  4. Writes the Swift declarations to Bazel build files.

Repository Rules: swift_package and local_swift_package

The swift_package repository rule downloads a Swift package and generates the Bazel build files that will build the Swift targets and products.

The local_swift_package repository rule references a Swift package directory on disk much like Bazel's local_repository rule. Like swift_package, it too generates Bazel build files for the Swift package.

The repository rules are implemented using Bazel Starlark.