[swift-evolution] [Draft] Test-Only Package Dependencies and Targets

Robert Widmann devteam.codafi at gmail.com
Tue Jan 24 16:32:53 CST 2017

Hello Swift Community,

Harlan Haskins and I have been working on libraries <https://github.com/trill-lang> to make interacting with LLVM and Clang’s APIs more elegant with native Swift interfaces.  While writing up the packages we realized the package manager wouldn’t allow us to specify testing targets and test-only dependencies.  To rectify that, I have attached a draft proposal for adding test-only targets and dependency fields to the Swift Package manager.  This proposal can also be read in gist form <https://gist.github.com/CodaFi/6bd83e5315c7d30aeaf4154ed3b03a38>.


~Robert Widmann

Test-Only Package Dependencies and Targets

Proposal: SE-NNNN <https://gist.github.com/CodaFi/NNNN-filename.md>
Authors: Harlan Haskins <https://github.com/harlanhaskins>, Robert Widmann <https://github.com/codafi>
Review Manager: TBD
Status: Awaiting review

This proposal reinstates Swift package manager’s ability to fetch dependencies and build targets scoped exclusively to the testing module(s) of a given package.

Swift-evolution thread: Discussion thread topic for that proposal <https://lists.swift.org/pipermail/swift-evolution/>

Soon after SE-0019 <https://github.com/apple/swift-evolution/blob/master/proposals/0019-package-manager-testing.md#test-only-dependencies> identified the need for richer test-only dependencies and targets, a decision was made to remove the package manager’s fledgling ability to treat certain dependencies as test-only. This has led to a myriad of clever-but-needlessly-complex workarounds ([1] <https://github.com/ReactiveCocoa/ReactiveSwift/blob/master/.travis.yml#L85>, [2] <https://github.com/ReactiveX/RxSwift/blob/master/Package.swift#L3>, [3] <https://github.com/Quick/Quick/blob/master/.Package.test.swift>) on the part of 3rd parties to recover the feature themselves. In addition, the Swift community has come up with a number of their own frameworks to augment functionality in XCTest but depending on these external testing frameworks is brittle and difficult to get right.

 <https://gist.github.com/CodaFi/6bd83e5315c7d30aeaf4154ed3b03a38#proposed-solution>Proposed solution

We propose the re-introduction of the testDependencies parameter in Package Manifests to support external test-only dependencies. To support local test-only targets we also propose the introduction of the testTargets parameter and an extension of the existing swift test command to support individual invocation of these targets.

 <https://gist.github.com/CodaFi/6bd83e5315c7d30aeaf4154ed3b03a38#detailed-design>Detailed design

The behavior of the new testDependencies parameter mirrors that of the existing dependencies parameter with one important difference: fetched dependencies are only built to support package-defined test targets as part of an invocation of swift test.

import PackageDescription

let package = Package(
    name: "Foo",
    targets: [
        Target(name: "Foo")
    dependencies: [
        .Package(url: "https://github.com/org/ana.git", versions: Version(1,0,0)...Version(1,9,9)),
    testDependencies: [
        .Package(url: "https://github.com/org/anism.git", versions: Version(1,0,0)...Version(1,9,9)),
Similarly, the behavior of the testTargets field mirrors that of the existing targets field but defines a set of targets that are only built during an invocation of swift test. Importantly, a target defined in testTargets may reference a target defined in targets but not vice-versa. Should that behavior be needed, the test target should be promoted to a “full” target.

import PackageDescription

let package = Package(
    name: "SwiftPM",
    targets: [
            name: "PackageDescription",
            dependencies: []),

        // MARK: Support libraries

            /** Cross-platform access to bare `libc` functionality. */
            name: "libc",
            dependencies: []),
            /** “Swifty” POSIX functions from libc */
            name: "POSIX",
            dependencies: ["libc"]),
            /** Basic support library */
            name: "Basic",
            dependencies: ["libc", "POSIX"]),

        /* Omitted for Brevity */
     testTargets: [
            name: "BasicPerformanceTests",
            dependencies: ["Basic"]),
       /* Omitted for Brevity */
Finally, with well-defined test targets in hand, we propose swift test be amended to support individual test execution.

We propose the following syntax to execute all tests of all known test targets.

$ swift test
To run a set of specific test cases, reference the module-defining test target and the specific name of a subclass of XCTestCase:

$ swift test TestModule.TestCase
To run an individual test case, reference the module-defining test target, the name of the test case subclass, and the name of the test:

$ swift test TestModule.TestCase.exampleTest
 <https://gist.github.com/CodaFi/6bd83e5315c7d30aeaf4154ed3b03a38#impact-on-existing-code>Impact on Existing Code

As this change is purely additive there will be no impact on existing code and no impact on existing packages.

 <https://gist.github.com/CodaFi/6bd83e5315c7d30aeaf4154ed3b03a38#alternatives-considered>Alternatives considered

Other names for the parameters to be added to the package manifest are possible.

The new support for executing specific tests could be exposed behind a flag.
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