Default Package Settings

proposals/0NNN-default-target-settings.md

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+# Default Target Settings
+
+* Proposal: [SE-0NNN](0NNN-default-target-settings.md)
+* Authors: [Matt Massicotte](https://github.com/mattmassicotte)
+* Review Manager: TBD
+* Status: **Awaiting implementation**
+* Implementation: [swiftlang/swift-package-manager#10033](https://github.com/swiftlang/swift-package-manager/pull/10033)
+* Review: ([pitch](https://forums.swift.org/t/default-package-swift-settings/71872))
+
+## Introduction
+
+It is very common for Swift packages to using the same settings flags across all their targets.
+A built-in mechanism to apply these base settings
+offers improved readability and convenience for package manifests.
+
+## Motivation
+
+The default SwiftPM package template generates a manifest with two targets: primary and test.
+There are many examples of packages that go far beyond this default of two,
+but single-target packages are quite rare.
+
+That same default template, as of Swift 6.4,
+also includes the same setting for both of these targets.
+Here's a snippet from the Package.swift file:
+
+```swift
+let package = Package(
+  // ...
+  targets: [
+    .target(
+      name: "MyPackage",
+      swiftSettings: [
+        .enableUpcomingFeature("ApproachableConcurrency"),
+      ],
+    ),
+    .testTarget(
+      name: "MyPackageTests",
+      dependencies: ["MyPackage"],
+      swiftSettings: [
+        .enableUpcomingFeature("ApproachableConcurrency"),
+      ],
+    ),
+  ]
+)
+```
+
+This pattern comes up over and over again across the package ecosystem.
+Selectively adopting upcoming language features across all targets is very common.
+For the prototypical primary-test target pair,
+duplicating one or two settings might not be ideal, but it is feasible.
+For packages with many targets and/or complex manifest files,
+it came become quite challenging to reason about the setting being applied.
+
+A possible solution involves applying a constant array to each target.
+This is sufficient as long as all targets use identical settings.
+But, if a package author does happen to need slightly different settings for even
+one target, additional logic needs to be introduced.
+
+```swift
+let swiftSettings: [SwiftSetting] = [
+  .enableUpcomingFeature("ApproachableConcurrency"),
+]
+
+let package = Package(
+  // ...
+  targets: [
+    .target(
+      name: "MyPackage",
+      swiftSettings: swiftSettings + [.enableUpcomingFeature("Lifetimes")],
+    ),
+    .testTarget(
+      name: "MyPackageTests",
+      dependencies: ["MyPackage"],
+      swiftSettings: swiftSettings,
+    ),
+  ]
+)
+```
+
+And, all of this is just discussing how a uniform list of settings could be applied.
+There are packages that have much more complex requirements.
+Typically, this requires at least some logic within the manifest file.
+
+These existing solutiuons are inconvenient, verbose, and error-prone.
+And because of the subtleties that can arise from compiler behavior differences,
+errors here can be particularly painful.
+
+## Proposed solution
+
+The desired configuration for the vast majority of package authors is the same.
+Begin with a core list of settings that define baseline behaviors,
+along with per-target refinements to that list as needed.
+
+The package manifest API should provide a way to express this directly.
+
+## Detailed design
+
+There are two core components to this change.
+The first is the ability to define and apply a base list of default settings.
+The second is mechanism to control how these defaults apply on a per-target basis.
+
+### Manifest APIs
+
+The `Package` class is extended to define a set of default settings:
+
+```swift
+public final class Package {
+  // ...
+
+  public var defaultSwiftSettings: Set<SwiftSetting>
+  public var defaultCSettings: Set<CSetting>
+  public var defaultCXXSettings: Set<CXXSetting>
+  public var defaultLinkerSettings: Set<LinkerSetting>
+
+  public init(
+    name: String,
+    defaultLocalization: LanguageTag? = nil,
+    platforms: [SupportedPlatform]? = nil,
+    pkgConfig: String? = nil,
+    providers: [SystemPackageProvider]? = nil,
+    products: [Product] = [],
+    traits: Set<Trait> = [],
+    dependencies: [Dependency] = [],
+    targets: [Target] = [],
+    swiftLanguageVersions: [SwiftVersion]? = nil,
+    defaultSwiftSettings: Set<SwiftSetting> = [],
+    cLanguageStandard: CLanguageStandard? = nil,
+    defaultCSettings: Set<CSetting> = [],
+    cxxLanguageStandard: CXXLanguageStandard? = nil,
+    defaultCXXSettings: Set<CXXSetting> = [],
+    defaultLinkerSettings: Set<LinkerSetting> = []
+  )
+}
+```
+
+```swift
+struct SwiftSettings {
+  // ...
+
+  public static func inherited() -> SwiftSettings {
+    // ...
+  }
+}
+
+struct CSettings {
+  // ...
+
+  public static func inherited() -> CSettings {
+    // ...
+  }
+}
+
+struct CXXSettings {
+  // ...
+
+  public static func inherited() -> CXXSettings {
+    // ...
+  }
+}
+
+struct LinkerSettings {
+  // ...
+
+  public static func inherited() -> LinkerSettings {
+    // ...
+  }
+}
+```
+
+With these changes in place, the default package template could look like this:
+
+```swift
+let package = Package(
+  // ...
+  targets: [
+    .target(
+      name: "MyPackage"
+    ),
+    .testTarget(
+      name: "MyPackageTests",
+      dependencies: ["MyPackage"]
+    ),
+  ],
+  defaultSwiftSettings: [
+    .enableUpcomingFeature("ApproachableConcurrency"),
+  ]
+)
+```
+
+### Settings Inheritance
+
+It is important that it be possible to control defaults on a per-target basis.
+This is supported with a new `inherited` placeholder setting.
+When setting are evaluated, this placeholder is substituted with the corresponding default values.
+
+Here are four possible target configurations that demonstrate the functionality.
+
+```swift
+let package = Package(
+  // ...
+  targets: [
+    .target(
+      name: "A",
+    ),
+    .target(
+      name: "B",
+      swiftSettings: [
+        .inherited(),
+      ]
+    ),
+    .target(
+      name: "C",
+      swiftSettings: [
+      ]
+    ),
+    .target(
+      name: "D",
+      swiftSettings: [
+        .enableExperimentalFeature("Lifetimes"),
+        .inherited(),
+      ]
+    ),
+  ],
+  defaultSwiftSettings: [
+    .defaultIsolation(MainActor.self),
+  ]
+)
+```
+
+- Target `A`: `swiftSettings` is omitted, so defaults apply
+- Target `B`: explicitly opts into inheriting the defaults
+- Target `C`: defines settings without inheriting, no defaults are applied
+- Target `D`: defines settings that control the order of the inheritance
+
+The behavior is identical for the `cSettings`, `cxxSettings`, and `linkerSettings` properties.
+
+For compatibility with conditional compilation,
+empty default settings arrays are accepted and do not have any special meaning.
+
+This inheritance mechanism matches the existing behaivor of the settings definition APIs.
+This means that duplicates and invalid combinations are perimitted.
+This situations are handled either by later stages of package validation or by the build tools themselves.
+In many cases, this results in "last entry wins" semantics.
+
+### Restrictions
+
+Default settings can have conditions, just like regular target settings.
+Supporting and resolving this correctly represents considerable additional complexity.
+For now, the `inherited` placeholder setting it self does not accept conditions.
+
+## Source compatibility
+
+This is a purely additive change and is fully compatible with existing manifest files.
+
+It is worth noting that there is now a semantic difference between
+a target omitting a settings array and an explicit empty array.
+However, because this difference only matters when defaults are present,
+it will not have any impact on existing package manifests.
+
+## ABI compatibility
+
+This change does not have any effect on ABI.
+
+## Implications on adoption
+
+This change impacts manifest authors, but should have no effects at all on package consumers.
+Authors will be able to adopt default settings freely without concern for compatibility.
+
+## Future directions
+
+Conditional inheritance could also be something that package authors find useful.
+It would be possible to add support for this in an API-compatible way.
+
+## Alternatives considered
+
+An earlier version of this proposal suggested an automatic,
+predefined merging strategy without the `inherited` placeholder.
+
+With some settings, such as `defaultIsolation`,
+the results of a merge seem quite unambiguous.
+But, this is not the case for all values, and the merging logic can be involved.
+The `inherited` mechanism is both intuitive and more powerful.
+
+## Acknowledgments
+
+Max Desiatov provided some much-appreciated general guidance that helped get this idea off the ground. Boris Buegling, Tony Allevato, Owen Voorhees, and Allen Humphreys all provided great feedback on the concept of inheritance.