Implement per target swift version selection

Swift Forums pitch thread - https://forums.swift.org/t/pitch-swiftpm-swift-language-version-per-target/71067

Add a new Swift target setting API, similar to `enable{Upcoming, Experimental}Feature`,
to specify a Swift language version that should be used to build the target, if such
version is not specified, fallback to the current language version determination logic.

Resolves: rdar://125732014

Author: xedin

Sources/Build/BuildDescription/SwiftTargetBuildDescription.swift

@@ -477,7 +477,6 @@ package final class SwiftTargetBuildDescription {
     package func compileArguments() throws -> [String] {
         var args = [String]()
         args += try self.buildParameters.targetTripleArgs(for: self.target)
-        args += ["-swift-version", self.swiftVersion.rawValue]
 
         // pass `-v` during verbose builds.
         if self.buildParameters.outputParameters.isVerbose {
@@ -579,6 +578,11 @@ package final class SwiftTargetBuildDescription {
         // Add arguments from declared build settings.
         args += try self.buildSettingsFlags()
 
+        // Fallback to package wide setting if there is no target specific version.
+        if args.firstIndex(of: "-swift-version") == nil {
+            args += ["-swift-version", self.swiftVersion.rawValue]
+        }
+
         // Add the output for the `.swiftinterface`, if requested or if library evolution has been enabled some other
         // way.
         if self.buildParameters.driverParameters.enableParseableModuleInterfaces || args.contains("-enable-library-evolution") {

Sources/PackageDescription/BuildSettings.swift

@@ -387,6 +387,23 @@ public struct SwiftSetting {
         return SwiftSetting(
           name: "interoperabilityMode", value: [mode.rawValue], condition: condition)
     }
+
+    /// Defines a `-swift-version` to pass  to the
+    /// corresponding build tool.
+    ///
+    /// - Since: First available in PackageDescription 6.0.
+    ///
+    /// - Parameters:
+    ///   - version: The Swift language version to use.
+    ///   - condition: A condition that restricts the application of the build setting.
+    @available(_PackageDescription, introduced: 6.0)
+    public static func swiftLanguageVersion(
+      _ version: SwiftVersion,
+      _ condition: BuildSettingCondition? = nil
+    ) -> SwiftSetting {
+        return SwiftSetting(
+            name: "swiftLanguageVersion", value: [.init(describing: version)], condition: condition)
+    }
 }
 
 /// A linker build setting.

Sources/PackageLoading/ManifestJSONParser.swift

@@ -533,6 +533,21 @@ extension TargetBuildSettingDescription.Kind {
             return .enableExperimentalFeature(value)
         case "unsafeFlags":
             return .unsafeFlags(values)
+
+        case "swiftLanguageVersion":
+            guard let rawVersion = values.first else {
+                throw InternalError("invalid (empty) build settings value")
+            }
+
+            if values.count > 1 {
+                throw InternalError("invalid build settings value")
+            }
+
+            guard let version = SwiftLanguageVersion(string: rawVersion) else {
+                throw InternalError("unknown swift language version: \(rawVersion)")
+            }
+
+            return .swiftLanguageVersion(version)
         default:
             throw InternalError("invalid build setting \(name)")
         }

Sources/PackageLoading/PackageBuilder.swift

@@ -1140,6 +1140,17 @@ public final class PackageBuilder {
                 }
 
                 values = ["-enable-experimental-feature", value]
+
+            case .swiftLanguageVersion(let version):
+                switch setting.tool {
+                case .c, .cxx, .linker:
+                    throw InternalError("only Swift supports swift language version")
+
+                case .swift:
+                    decl = .OTHER_SWIFT_FLAGS
+                }
+
+                values = ["-swift-version", version.rawValue]
             }
 
             // Create an assignment for this setting.

Sources/PackageModel/Manifest/TargetBuildSettingDescription.swift

@@ -12,7 +12,6 @@
 
 /// A namespace for target-specific build settings.
 public enum TargetBuildSettingDescription {
-
     /// The tool for which a build setting is declared.
     public enum Tool: String, Codable, Hashable, CaseIterable, Sendable {
         case c
@@ -40,12 +39,15 @@ public enum TargetBuildSettingDescription {
 
         case unsafeFlags([String])
 
+        case swiftLanguageVersion(SwiftLanguageVersion)
+
         public var isUnsafeFlags: Bool {
             switch self {
             case .unsafeFlags(let flags):
                 // If `.unsafeFlags` is used, but doesn't specify any flags, we treat it the same way as not specifying it.
                 return !flags.isEmpty
-            case .headerSearchPath, .define, .linkedLibrary, .linkedFramework, .interoperabilityMode, .enableUpcomingFeature, .enableExperimentalFeature:
+            case .headerSearchPath, .define, .linkedLibrary, .linkedFramework, .interoperabilityMode,
+                 .enableUpcomingFeature, .enableExperimentalFeature, .swiftLanguageVersion:
                 return false
             }
         }

Sources/PackageModel/ManifestSourceGeneration.swift

@@ -525,6 +525,12 @@ fileprivate extension SourceCodeFragment {
                 params.append(SourceCodeFragment(from: condition))
             }
             self.init(enum: setting.kind.name, subnodes: params)
+        case .swiftLanguageVersion(let version):
+            params.append(SourceCodeFragment(from: version))
+            if let condition = setting.condition {
+                params.append(SourceCodeFragment(from: condition))
+            }
+            self.init(enum: setting.kind.name, subnodes: params)
         }
     }
 }
@@ -677,6 +683,8 @@ extension TargetBuildSettingDescription.Kind {
             return "enableUpcomingFeature"
         case .enableExperimentalFeature:
             return "enableExperimentalFeature"
+        case .swiftLanguageVersion:
+            return "swiftLanguageVersion"
         }
     }
 }