Add build setting for enabling the builtin module

Sources/PackageDescription/BuildSettings.swift

@@ -387,6 +387,12 @@ public struct SwiftSetting {
         return SwiftSetting(
           name: "interoperabilityMode", value: values, condition: condition)
     }
+
+    @available(_PackageDescription, introduced: 5.9)
+    @_spi(FrontendOptions)
+    public static var enableBuiltinModule: SwiftSetting {
+        SwiftSetting(name: "enableBuiltinModule", value: [], condition: nil)
+    }
 }
 
 /// A linker build setting.

Sources/PackageLoading/ManifestJSONParser.swift

@@ -704,6 +704,8 @@ extension TargetBuildSettingDescription.Kind {
             return .enableExperimentalFeature(value)
         case "unsafeFlags":
             return .unsafeFlags(values)
+        case "enableBuiltinModule":
+            return .enableBuiltinModule
         default:
             throw InternalError("invalid build setting \(name)")
         }

Sources/PackageLoading/PackageBuilder.swift

@@ -1038,6 +1038,18 @@ public final class PackageBuilder {
                 }
 
                 values = ["-enable-experimental-feature", value]
+
+            case .enableBuiltinModule:
+                switch setting.tool {
+                case .c, .cxx, .linker:
+                    throw InternalError(
+                        "only Swift supports enabling the builtin module")
+
+                case .swift:
+                    decl = .OTHER_SWIFT_FLAGS
+                }
+
+                values = ["-enable-builtin-module"]
             }
 
             // Create an assignment for this setting.

Sources/PackageModel/Manifest/TargetBuildSettingDescription.swift

@@ -40,12 +40,14 @@ public enum TargetBuildSettingDescription {
 
         case unsafeFlags([String])
 
+        case enableBuiltinModule
+
         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, .enableBuiltinModule:
                 return false
             }
         }

Sources/PackageModel/ManifestSourceGeneration.swift

@@ -526,6 +526,8 @@ fileprivate extension SourceCodeFragment {
                 params.append(SourceCodeFragment(from: condition))
             }
             self.init(enum: setting.kind.name, subnodes: params)
+        case .enableBuiltinModule:
+            self.init(enum: setting.kind.name)
         }
     }
 }
@@ -678,6 +680,8 @@ extension TargetBuildSettingDescription.Kind {
             return "enableUpcomingFeature"
         case .enableExperimentalFeature:
             return "enableExperimentalFeature"
+        case .enableBuiltinModule:
+            return "enableBuiltinModule"
         }
     }
 }

Tests/BuildTests/BuildPlanTests.swift

@@ -3245,7 +3245,8 @@ final class BuildPlanTests: XCTestCase {
                         .init(tool: .swift, kind: .interoperabilityMode(.Cxx, "swift-5.9"), condition: .init(platformNames: ["linux"])),
                         .init(tool: .swift, kind: .interoperabilityMode(.Cxx, "swift-6.0"), condition: .init(platformNames: ["macos"])),
                         .init(tool: .swift, kind: .enableUpcomingFeature("BestFeature")),
-                        .init(tool: .swift, kind: .enableUpcomingFeature("WorstFeature"), condition: .init(platformNames: ["macos"], config: "debug"))
+                        .init(tool: .swift, kind: .enableUpcomingFeature("WorstFeature"), condition: .init(platformNames: ["macos"], config: "debug")),
+                        .init(tool: .swift, kind: .enableBuiltinModule)
                     ]
                 ),
                 try TargetDescription(
@@ -3311,7 +3312,7 @@ final class BuildPlanTests: XCTestCase {
             XCTAssertMatch(cbar, [.anySequence, "-DCCC=2", "-I\(A.appending(components: "Sources", "cbar", "Sources", "headers"))", "-I\(A.appending(components: "Sources", "cbar", "Sources", "cppheaders"))", "-Icfoo", "-L", "cbar", "-Icxxfoo", "-L", "cxxbar", .end])
 
             let bar = try result.target(for: "bar").swiftTarget().compileArguments()
-            XCTAssertMatch(bar, [.anySequence, "-DLINUX", "-Isfoo", "-L", "sbar", "-cxx-interoperability-mode=swift-5.9", "-enable-upcoming-feature", "BestFeature", .end])
+            XCTAssertMatch(bar, [.anySequence, "-DLINUX", "-Isfoo", "-L", "sbar", "-cxx-interoperability-mode=swift-5.9", "-enable-upcoming-feature", "BestFeature", "-enable-builtin-module", .end])
 
             let exe = try result.target(for: "exe").swiftTarget().compileArguments()
             XCTAssertMatch(exe, [.anySequence, "-DFOO", .end])
@@ -3327,7 +3328,7 @@ final class BuildPlanTests: XCTestCase {
             XCTAssertMatch(cbar, [.anySequence, "-DCCC=2", "-I\(A.appending(components: "Sources", "cbar", "Sources", "headers"))", "-I\(A.appending(components: "Sources", "cbar", "Sources", "cppheaders"))", "-Icfoo", "-L", "cbar", "-Icxxfoo", "-L", "cxxbar", .end])
 
             let bar = try result.target(for: "bar").swiftTarget().compileArguments()
-            XCTAssertMatch(bar, [.anySequence, "-DDMACOS", "-Isfoo", "-L", "sbar", "-cxx-interoperability-mode=swift-6.0", "-enable-upcoming-feature", "BestFeature", "-enable-upcoming-feature", "WorstFeature", .end])
+            XCTAssertMatch(bar, [.anySequence, "-DDMACOS", "-Isfoo", "-L", "sbar", "-cxx-interoperability-mode=swift-6.0", "-enable-upcoming-feature", "BestFeature", "-enable-upcoming-feature", "WorstFeature", "-enable-builtin-module", .end])
 
             let exe = try result.target(for: "exe").swiftTarget().compileArguments()
             XCTAssertMatch(exe, [.anySequence, "-DFOO", .end])