Modernise swiftpm package manifest and enable development with Swift 4 manifest API #721
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In Swift 3.1, swiftpm added support for evaluating environment variables in the package manifest. This enables us to remove the duplicated `.Package.swift` file, making it much easier to maintain the manifest.
Allows development using the swift 4 tools, allowing features like test-only dependencies and `swift run`.
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My primary goal with this PR was to try and find a configuration that would allow us to start using the Swift 4 version of the swiftpm package manifest. Why is this useful?
swift run*Previously I thought that adopting Swift 4 package manager features would be a breaking change, but I just learned that it's possible to provide version-specific
Package.swiftfiles. So what this PR does is add a newPackage@swift-4.swiftmanifest, that swiftpm happily picks up when using the Swift 4 compiler.However there's a bit of an issue here: we already have to maintain both
Package.swiftand.Package.test.swiftin order to define test-only dependencies, so adding this new manifest means that we're mainting 3 different manifest files. In order to resolve this, I've updated our minimum swift version to 3.1, which gives us the ability to read environment variables inside the package manifest. In this way, you can enable test dependencies under Swift 3 by setting theSWIFT_PACKAGE_TEST_Quickvariable to a truthy value. (I learned this technique from ReactiveSwift's package manifest.) This removes the need to maintain.Package.test.swift.One more note — the swiftpm docs explicitly recommend against using the version-specific manifest feature to support the latest release of Swift:
While Swift 4 is still in beta, I believe it's safe to use the version-specific format. In fact, I tried the alternative of providing
Package.swiftandPackage@swift-3.swift, and it unfortunately breaks the ability to use the Swift 3 package manager for development (importantly, it doesn't break anything for consumers of Quick, but the development loss is not ideal). Once Swift 4 is out of beta, I think we should rename the Swift 4 manifest to be the default, and provide aPackage@swift-3.swiftfile for backwards compatibility until we decide to drop Swift 3 support altogether.* The new
swift runcommand allows packages to easily ship command line tools, which users can run just by sayingswift run tool. I'm pretty excited about this, and am currently exploring the possibility ofQuickshipping a tool to make various automation tasks easier.