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0208-package-manager-system-library-targets.md

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Package Manager System Library Targets

Introduction

This proposal introduces a new type of target "system library target", which moves the current system-module packages feature from package to target level.

Motivation

The package manager currently supports "system-module packages" which are intended to adapt a system installed dependency to work with the package manager. However, this feature is only supported on package declarations, which mean libraries that need it often need to create a separate repository containing the system package and refer to it as a dependency.

Our original motivation in forcing system packages to be declared as standalone packages was to encourage the ecosystem to standardize on them, their names, their repository locations, and their owners. In practice, this effort did not work out and it only made the package manager harder to use.

Terminology

Swift N: The tools version this proposal is implemented in.

Proposed solution

We propose adding a new "system library target", which would supply the same metadata needed to adapt system libraries to work with the package manager, but as a target. This would allow packages to embed these targets with the libraries that need them.

We propose to deprecate the legacy system-module package declaration for packages that are only compatible with Swift N or later.

Detailed design

We will add a new factory method for system library target:

extension Target {
    public static func systemLibrary(
        name: String,
        path: String? = nil,
        pkgConfig: String? = nil,
        providers: [SystemPackageProvider]? = nil
    ) -> Target
}

  • This target factory function will only be available if the tools version of the manifest is greater than or equal to Swift N.

  • During dependency resolution, the package manager will emit a deprecation warning if there is a legacy system-module package in the package graph and the tools version of the root package is greater than or equal to Swift N.

  • Currently, the package manage implicitly assumes a dependency on system-module packages when they are included as a dependency. In the new model, for consistency with the existing target/product model, clients of a system package must also specify explicit dependencies from the targets which use the system library.

  • System library targets may be exported from a package as products. To do so, they must be exported via a library product with exactly one member target i.e. the system library target.

Examples

For example, an existing package which defines only a system library adaptor would be described:

// swift-tools-version:N
import PackageDescription

let package = Package(
    name: "CZLib",
    products: [
        .library(name: "CZLib", targets: ["CZLib"]),
    ],
    targets: [
        .systemLibrary(
            name: "CZLib",
            pkgConfig: "zlib",
            providers: [
                .brew(["zlib"]),
                .apt(["zlib"]),
            ]
        )
    ]
)

A similar package which exported a Swift interface for zlib might look like the following example, which is not expressible today without using a separate repository.

// swift-tools-version:N
import PackageDescription

let package = Package(
    name: "ZLib",
    products: [
        .library(name: "ZLib", targets: ["ZLib"]),
    ],
    targets: [
        .target(
            name: "ZLib",
            dependencies: ["CZLib"]),
        .systemLibrary(
            name: "CZLib")
    ]
)

In this case, the system library is not an exported product, and would not be available to other packages.

Impact on existing packages

None.

Alternatives considered

None.