[swift-evolution] [SwiftPM] Proposal: Add support for test-only dependencies

[thislooksfun]

Does anyone else have any thoughts on this, or should I go ahead an assemble a PR?

-thislooksfun (tlf)

> On Dec 26, 2016, at 1:56 PM, thislooksfun <thislooksfun at repbot.org> wrote:
> 
> (I think this is the right place for this suggestion, but please let me know if I'm mistaken)
> 
> There is currently no supported way to have some dependencies only used for testing (`swift test`), and the workarounds for it, while workable, are unnecessary crude.
> 
> Currently, what a lot of projects are doing is defining a `.Package.test.swift` file that is then copied over `Package.swift` when tests are to be run (on external CI, for example). While this works, it adds a extra step and another point of failure (if you forget to add a new dependency to both `Package` files, for example.
> 
> What I propose is a new section of `Package.swift`, labelled either `testDependencies` or `devDependencies`, and the modules referenced within are only loaded/compiled when running `swift test`
> 
> // Package.swift
> import PackageDescription
> let package = Package(
>     name: "Project",
>     targets: [
>       Target(name: "BoxioDebug", dependencies: ["Core"]),
>     ],
>     dependencies: [
> 	//Same as before
>     ],
>     testDependencies: [
>         .Package(url: "https://github.com/FooBar/PackageOnlyNeededForTesting.git <https://github.com/FooBar/PackageOnlyNeededForTesting.git>", majorVersion: 1),
>     ]
> )
> 
> This solves having to manually switch out Package files or `.Package` statements inside one Package file when trying to run tests, and keeps all dependencies neatly organized in one file.
> 
> This change is purely additive and optional, no existing code needs to change for this to be added.
> 
> A real world example is a dependency on Quick (or another such testing framework), that itself uses XCTest, and thus crashes when being run outside of `swift test` (add Quick to Package.swift, run `swift build` and run the compiled binary). With the new `test[dev]Dependencies`, running `swift build` wouldn't even see Quick, and thus would compile and run correctly, but `swift test` would both see and use Quick for running the test suite.
> 
> 
> Other solutions considered:
> Another potential solution is to only compile the dependencies that were actually used when running `swift build`, since that seems to be the cause of the above problem.
> 
> 
> -thislooksfun (tlf)
> 

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