Use availability annotations instead of #if to unify PackageDescription 4.0 and 4.2+

Replace the compile-time conditionals in PackageDescription with availability annotations.  This will let us build a single PackageDescription, which will make the build scripts easier (with the goal of being able to build it entirely from the package itself).

Author: abertelrud

Package.swift

@@ -122,6 +122,7 @@ let package = Package(
             name: "PackageDescription",
             swiftSettings: [
                 .define("PACKAGE_DESCRIPTION_4_2"),
+                .unsafeFlags(["-package-description-version", "5.5"])
             ]),
 
         .target(

Sources/PackageDescription/LanguageStandardSettings.swift

@@ -144,7 +144,6 @@ public enum CXXLanguageStandard: String, Encodable {
     case gnucxx20 = "gnu++20"
 }
 
-#if !PACKAGE_DESCRIPTION_4
 /// The version of the Swift language to use for compiling Swift sources in the package.
 public enum SwiftVersion {
     @available(_PackageDescription, introduced: 4, obsoleted: 5)
@@ -187,4 +186,3 @@ extension SwiftVersion: Encodable {
         try container.encode(value)
     }
 }
-#endif

Sources/PackageDescription/PackageDependency.swift

@@ -147,11 +147,7 @@ extension Package.Dependency {
         url: String,
         _ range: Range<Version>
     ) -> Package.Dependency {
-      #if PACKAGE_DESCRIPTION_4
         return .init(name: nil, url: url, requirement: .rangeItem(range))
-      #else
-        return .init(name: nil, url: url, requirement: ._rangeItem(range))
-      #endif
     }
 
     /// Adds a package dependency starting with a specific minimum version, up to
@@ -172,11 +168,7 @@ extension Package.Dependency {
         url: String,
         _ range: Range<Version>
     ) -> Package.Dependency {
-      #if PACKAGE_DESCRIPTION_4
         return .init(name: name, url: url, requirement: .rangeItem(range))
-      #else
-        return .init(name: name, url: url, requirement: ._rangeItem(range))
-      #endif
     }
 
     /// Adds a package dependency starting with a specific minimum version, going
@@ -202,11 +194,7 @@ extension Package.Dependency {
             upper.major, upper.minor, upper.patch + 1,
             prereleaseIdentifiers: upper.prereleaseIdentifiers,
             buildMetadataIdentifiers: upper.buildMetadataIdentifiers)
-      #if PACKAGE_DESCRIPTION_4
         return .init(name: nil, url: url, requirement: .rangeItem(range.lowerBound..<upperBound))
-      #else
-        return .init(name: nil, url: url, requirement: ._rangeItem(range.lowerBound..<upperBound))
-      #endif
     }
 
     /// Adds a package dependency starting with a specific minimum version, going
@@ -233,14 +221,9 @@ extension Package.Dependency {
             upper.major, upper.minor, upper.patch + 1,
             prereleaseIdentifiers: upper.prereleaseIdentifiers,
             buildMetadataIdentifiers: upper.buildMetadataIdentifiers)
-      #if PACKAGE_DESCRIPTION_4
         return .init(name: name, url: url, requirement: .rangeItem(range.lowerBound..<upperBound))
-      #else
-        return .init(name: name, url: url, requirement: ._rangeItem(range.lowerBound..<upperBound))
-      #endif
     }
 
-  #if !PACKAGE_DESCRIPTION_4
     /// Adds a package dependency to a local package on the filesystem.
     ///
     /// The Swift Package Manager uses the package dependency as-is
@@ -253,7 +236,7 @@ extension Package.Dependency {
     public static func package(
         path: String
     ) -> Package.Dependency {
-        return .init(name: nil, url: path, requirement: ._localPackageItem)
+        return .init(name: nil, url: path, requirement: .localPackageItem)
     }
 
     /// Adds a package dependency to a local package on the filesystem.
@@ -271,9 +254,8 @@ extension Package.Dependency {
         name: String? = nil,
         path: String
     ) -> Package.Dependency {
-        return .init(name: name, url: path, requirement: ._localPackageItem)
+        return .init(name: name, url: path, requirement: .localPackageItem)
     }
-  #endif
 }
 
 // Mark common APIs used by mistake as unavailable to provide better error messages.

Sources/PackageDescription/PackageDescription.swift

@@ -122,26 +122,14 @@ public final class Package {
         /// dependency requirements; you should remove commit-based dependency
         /// requirements before publishing a version of your package.
         public enum Requirement {
-          #if PACKAGE_DESCRIPTION_4
             case exactItem(Version)
             case rangeItem(Range<Version>)
             case revisionItem(String)
             case branchItem(String)
             case localPackageItem
-          #else
-            case _exactItem(Version)
-            case _rangeItem(Range<Version>)
-            case _revisionItem(String)
-            case _branchItem(String)
-            case _localPackageItem
-          #endif
 
             var isLocalPackage: Bool {
-              #if PACKAGE_DESCRIPTION_4
                 if case .localPackageItem = self { return true }
-              #else
-                if case ._localPackageItem = self { return true }
-              #endif
                 return false
             }
         }
@@ -167,14 +155,12 @@ public final class Package {
     /// The name of the Swift package.
     public var name: String
 
-  #if !PACKAGE_DESCRIPTION_4
     /// The list of supported platforms with a custom deployment target.
     @available(_PackageDescription, introduced: 5)
     public var platforms: [SupportedPlatform]? {
         get { return _platforms }
         set { _platforms = newValue }
     }
-  #endif
     private var _platforms: [SupportedPlatform]?
 
     /// The default localization for resources.
@@ -203,27 +189,21 @@ public final class Package {
     /// The list of package dependencies.
     public var dependencies: [Dependency]
 
-  #if PACKAGE_DESCRIPTION_4
-    /// The list of Swift versions that this package is compatible with.
-    public var swiftLanguageVersions: [Int]?
-  #else
     /// The list of Swift versions that this package is compatible with.
     public var swiftLanguageVersions: [SwiftVersion]?
-  #endif
 
     /// The C language standard to use for all C targets in this package.
     public var cLanguageStandard: CLanguageStandard?
 
     /// The C++ language standard to use for all C++ targets in this package.
     public var cxxLanguageStandard: CXXLanguageStandard?
 
-  #if PACKAGE_DESCRIPTION_4
     /// Initializes a Swift package with configuration options you provide.
     ///
     /// - Parameters:
     ///     - name: The name of the Swift package, or `nil`, if you want the Swift Package Manager to deduce the
     ///           name from the package’s Git URL.
-    ///     - pkgConfig: The name to use for C modules. If present, the Swift 
+    ///     - pkgConfig: The name to use for C modules. If present, the Swift
     ///           Package Manager searches for a `<name>.pc` file to get the
     ///           additional flags required for a system target.
     ///     - providers: The package providers for a system package.
@@ -233,6 +213,7 @@ public final class Package {
     ///     - swiftLanguageVersions: The list of Swift versions that this package is compatible with.
     ///     - cLanguageStandard: The C language standard to use for all C targets in this package.
     ///     - cxxLanguageStandard: The C++ language standard to use for all C++ targets in this package.
+    @available(_PackageDescription, obsoleted: 4.2)
     public init(
         name: String,
         pkgConfig: String? = nil,
@@ -250,12 +231,12 @@ public final class Package {
         self.products = products
         self.dependencies = dependencies
         self.targets = targets
-        self.swiftLanguageVersions = swiftLanguageVersions
+        self.swiftLanguageVersions = swiftLanguageVersions.map{ $0.map{ .version("\($0)") } }
         self.cLanguageStandard = cLanguageStandard
         self.cxxLanguageStandard = cxxLanguageStandard
         registerExitHandler()
     }
-  #else
+
     /// Initializes a Swift package with configuration options you provide.
     ///
     /// - Parameters:
@@ -378,7 +359,6 @@ public final class Package {
         self.cxxLanguageStandard = cxxLanguageStandard
         registerExitHandler()
     }
-  #endif
 
     private func registerExitHandler() {
         // Add a custom exit handler to cause the package's JSON representation
@@ -457,26 +437,18 @@ extension LanguageTag: CustomStringConvertible {
 /// The system package providers used in this Swift package.
 public enum SystemPackageProvider {
 
-  #if PACKAGE_DESCRIPTION_4
     case brewItem([String])
     case aptItem([String])
-  #else
-    case _brewItem([String])
-    case _aptItem([String])
-    case _yumItem([String])
-  #endif
+    @available(_PackageDescription, introduced: 5.3)
+    case yumItem([String])
 
     /// Creates a system package provider with a list of installable packages
     /// for users of the HomeBrew package manager on macOS.
     ///
     /// - Parameters:
     ///     - packages: The list of package names.
     public static func brew(_ packages: [String]) -> SystemPackageProvider {
-      #if PACKAGE_DESCRIPTION_4
         return .brewItem(packages)
-      #else
-        return ._brewItem(packages)
-      #endif
     }
 
     /// Creates a system package provider with a list of installable packages
@@ -485,26 +457,18 @@ public enum SystemPackageProvider {
     /// - Parameters:
     ///     - packages: The list of package names.
     public static func apt(_ packages: [String]) -> SystemPackageProvider {
-      #if PACKAGE_DESCRIPTION_4
         return .aptItem(packages)
-      #else
-        return ._aptItem(packages)
-      #endif
     }
 
-#if PACKAGE_DESCRIPTION_4
-// yum is not supported
-#else
     /// Creates a system package provider with a list of installable packages
     /// for users of the yum package manager on Red Hat Enterprise Linux or CentOS.
     ///
     /// - Parameters:
     ///     - packages: The list of package names.
     @available(_PackageDescription, introduced: 5.3)
     public static func yum(_ packages: [String]) -> SystemPackageProvider {
-        return ._yumItem(packages)
+        return .yumItem(packages)
     }
-#endif
 }
 
 // MARK: Package JSON serialization
@@ -528,26 +492,19 @@ extension Package: Encodable {
         var container = encoder.container(keyedBy: CodingKeys.self)
         try container.encode(name, forKey: .name)
 
-      #if !PACKAGE_DESCRIPTION_4
         if let defaultLocalization = _defaultLocalization {
             try container.encode(defaultLocalization.tag, forKey: .defaultLocalization)
         }
         if let platforms = self._platforms {
             try container.encode(platforms, forKey: .platforms)
         }
-      #endif
 
         try container.encode(pkgConfig, forKey: .pkgConfig)
         try container.encode(providers, forKey: .providers)
         try container.encode(products, forKey: .products)
         try container.encode(dependencies, forKey: .dependencies)
         try container.encode(targets, forKey: .targets)
-      #if PACKAGE_DESCRIPTION_4
-        let slv = swiftLanguageVersions?.map({ String($0) })
-        try container.encode(slv, forKey: .swiftLanguageVersions)
-      #else
         try container.encode(swiftLanguageVersions, forKey: .swiftLanguageVersions)
-      #endif
         try container.encode(cLanguageStandard, forKey: .cLanguageStandard)
         try container.encode(cxxLanguageStandard, forKey: .cxxLanguageStandard)
     }
@@ -567,28 +524,17 @@ extension SystemPackageProvider: Encodable {
 
     public func encode(to encoder: Encoder) throws {
         var container = encoder.container(keyedBy: CodingKeys.self)
-      #if PACKAGE_DESCRIPTION_4
         switch self {
         case .brewItem(let packages):
             try container.encode(Name.brew, forKey: .name)
             try container.encode(packages, forKey: .values)
         case .aptItem(let packages):
             try container.encode(Name.apt, forKey: .name)
             try container.encode(packages, forKey: .values)
-        }
-      #else
-        switch self {
-        case ._brewItem(let packages):
-            try container.encode(Name.brew, forKey: .name)
-            try container.encode(packages, forKey: .values)
-        case ._aptItem(let packages):
-            try container.encode(Name.apt, forKey: .name)
-            try container.encode(packages, forKey: .values)
-        case ._yumItem(let packages):
+        case .yumItem(let packages):
             try container.encode(Name.yum, forKey: .name)
             try container.encode(packages, forKey: .values)
         }
-      #endif
     }
 }
 
@@ -626,36 +572,21 @@ extension Target.Dependency: Encodable {
 
     public func encode(to encoder: Encoder) throws {
         var container = encoder.container(keyedBy: CodingKeys.self)
-      #if PACKAGE_DESCRIPTION_4
-        switch self {
-        case .targetItem(let name):
-            try container.encode(Kind.target, forKey: .type)
-            try container.encode(name, forKey: .name)
-        case .productItem(let name, let package):
-            try container.encode(Kind.product, forKey: .type)
-            try container.encode(name, forKey: .name)
-            try container.encode(package, forKey: .package)
-        case .byNameItem(let name):
-            try container.encode(Kind.byName, forKey: .type)
-            try container.encode(name, forKey: .name)
-        }
-      #else
         switch self {
-        case ._targetItem(let name, let condition):
+        case .targetItem(let name, let condition):
             try container.encode(Kind.target, forKey: .type)
             try container.encode(name, forKey: .name)
             try container.encode(condition, forKey: .condition)
-        case ._productItem(let name, let package, let condition):
+        case .productItem(let name, let package, let condition):
             try container.encode(Kind.product, forKey: .type)
             try container.encode(name, forKey: .name)
             try container.encode(package, forKey: .package)
             try container.encode(condition, forKey: .condition)
-        case ._byNameItem(let name, let condition):
+        case .byNameItem(let name, let condition):
             try container.encode(Kind.byName, forKey: .type)
             try container.encode(name, forKey: .name)
             try container.encode(condition, forKey: .condition)
         }
-      #endif
     }
 }