Disable Skip with a package trait

[dfabulich]

https://docs.swift.org/swiftpm/documentation/packagemanagerdocs/packagetraits/

Xcode 26.4 supports package traits in the Package Dependencies view. It might be a nicer way to enable/disable Skip.

[marcprux]

It would be great if this could work. One big shortcoming of package traits is that you can only disable target dependencies with them, and not package dependencies or plugins. So the skip plugin would still be downloaded, need to be trusted, and run on the module even if Skip is disabled. Merely dropping the dependencies like skip-ui isn't much of a win by itself.

Ideally traits could be used as a replacement for the SKIP_ZERO environment. So rather than the (somewhat hacky) method we currently use at https://github.com/skiptools/skipapp-hello/blob/main/Package.swift, an app that wants to enable/disable Skip might look like:

// swift-tools-version: 6.1
import PackageDescription

let package = Package(
    name: "skipapp-hello",
    products: [
        .library(name: "HelloSkip", type: .dynamic, targets: ["HelloSkip"]),
    ],
    traits: [
        .trait(name: "SKIP", description: "Enable Skip for the app"),
        .default(enabledTraits: ["SKIP"])
    ],
    dependencies: [
        .package(url: "https://source.skip.tools/skip.git", from: "1.0.0", condition: .when(traits: ["SKIP"])),
        .package(url: "https://source.skip.tools/skip-ui.git", from: "1.0.0", condition: .when(traits: ["SKIP"]))
    ],
    targets: [
        .target(name: "HelloSkip", dependencies: [
            .product(name: "SkipUI", package: "skip-ui", condition: .when(traits: ["SKIP"]))
        ],
        plugins: [
            .plugin(name: "skipstone", package: "skip", condition: .when(traits: ["SKIP"]))
        ])
    ]
)

Flipping off "SKIP" would completely eliminate Skip from the project, just like the SKIP_ZERO environment variable does today.

[marcprux]

Another area where traits have the potential to be useful is for various Skip libraries that might be useful for iOS developers who have not (yet) chosen to use Skip. For example, SkipSQL is a capable little ORM that would be nice to offer to the non-Skip community without all the baggage (plugins + dependencies) that come along with Skip, and traits might offer a way forward for this scenario.