Add build setting for enabling C++ interop

This adds a build setting to the package manifest that enables Swift/C++ Interoperability for a given Swift target:
```
.interoperabilityMode(.Cxx, version: "swift-5.9")
```

This relies on the new Swift driver flag for versioned C++ interop (see swiftlang/swift#64088).

rdar://106756067

Author: egorzhdan

Sources/PackageDescription/BuildSettings.swift

@@ -352,6 +352,41 @@ public struct SwiftSetting {
         return SwiftSetting(
             name: "enableExperimentalFeature", value: [name], condition: condition)
     }
+
+    public enum InteroperabilityMode: String {
+        case C
+        case Cxx
+    }
+
+    /// Enable Swift interoperability with a given language.
+    ///
+    /// This is useful for enabling Swift/C++ interoperability for a given
+    /// target.
+    ///
+    /// Enabling C++ interoperability mode might alter the way some existing
+    /// C/Objective-C APIs are imported.
+    ///
+    /// - Since: First available in PackageDescription 5.9.
+    ///
+    /// - Parameters:
+    ///   - mode: The language mode, either C or Cxx.
+    ///   - version: If Cxx language mode is used, the version of Swift/C++
+    /// interoperability, otherwise `nil`.
+    ///   - condition: A condition that restricts the application of the build
+    /// setting.
+    @available(_PackageDescription, introduced: 5.9)
+    public static func interoperabilityMode(
+      _ mode: InteroperabilityMode,
+      version: String? = nil,
+      _ condition: BuildSettingCondition? = nil
+    ) -> SwiftSetting {
+        var values: [String] = [mode.rawValue]
+        if let version = version {
+            values.append(version)
+        }
+        return SwiftSetting(
+          name: "interoperabilityMode", value: values, condition: condition)
+    }
 }
 
 /// A linker build setting.

Sources/PackageLoading/ManifestJSONParser.swift

@@ -675,6 +675,23 @@ extension TargetBuildSettingDescription.Kind {
                 throw InternalError("invalid (empty) build settings value")
             }
             return .linkedFramework(value)
+        case "interoperabilityMode":
+            guard let rawLang = values.first else {
+                throw InternalError("invalid (empty) build settings value")
+            }
+            guard let lang = TargetBuildSettingDescription.InteroperabilityMode(rawValue: rawLang) else {
+                throw InternalError("unknown interoperability mode: \(rawLang)")
+            }
+            if values.count > 2 {
+                throw InternalError("invalid build settings value")
+            }
+            let version: String?
+            if values.count == 2 {
+                version = values[1]
+            } else {
+                version = nil
+            }
+            return .interoperabilityMode(lang, version)
         case "enableUpcomingFeature":
             guard let value = values.first else {
                 throw InternalError("invalid (empty) build settings value")

Sources/PackageLoading/PackageBuilder.swift

@@ -977,6 +977,24 @@ public final class PackageBuilder {
                     decl = .LINK_FRAMEWORKS
                 }
 
+            case .interoperabilityMode(let lang, let version):
+                switch setting.tool {
+                case .c, .cxx, .linker:
+                    throw InternalError("only Swift supports interoperability")
+
+                case .swift:
+                    decl = .OTHER_SWIFT_FLAGS
+                }
+
+                if lang == .Cxx {
+                    guard let version else {
+                        throw InternalError("C++ interoperability requires a version (e.g. 'swift-5.9')")
+                    }
+                    values = ["-cxx-interoperability-mode=\(version)"]
+                } else {
+                    values = []
+                }
+
             case .unsafeFlags(let _values):
                 values = _values
 

Sources/PackageModel/Manifest/TargetBuildSettingDescription.swift

@@ -21,13 +21,20 @@ public enum TargetBuildSettingDescription {
         case linker
     }
 
+    public enum InteroperabilityMode: String, Codable, Equatable, Sendable {
+        case C
+        case Cxx
+    }
+
     /// The kind of the build setting, with associate configuration
     public enum Kind: Codable, Equatable, Sendable {
         case headerSearchPath(String)
         case define(String)
         case linkedLibrary(String)
         case linkedFramework(String)
 
+        case interoperabilityMode(InteroperabilityMode, String?)
+
         case enableUpcomingFeature(String)
         case enableExperimentalFeature(String)
 
@@ -38,7 +45,7 @@ public enum TargetBuildSettingDescription {
             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, .enableUpcomingFeature, .enableExperimentalFeature:
+            case .headerSearchPath, .define, .linkedLibrary, .linkedFramework, .interoperabilityMode, .enableUpcomingFeature, .enableExperimentalFeature:
                 return false
             }
         }

Sources/PackageModel/ManifestSourceGeneration.swift

@@ -514,6 +514,12 @@ fileprivate extension SourceCodeFragment {
                 params.append(SourceCodeFragment(from: condition))
             }
             self.init(enum: setting.kind.name, subnodes: params)
+        case .interoperabilityMode(let lang, let version):
+            params.append(SourceCodeFragment(enum: lang.rawValue))
+            if let version = version {
+                params.append(SourceCodeFragment(key: "version", string: version))
+            }
+            self.init(enum: setting.kind.name, subnodes: params)
         case .unsafeFlags(let values):
             params.append(SourceCodeFragment(strings: values))
             if let condition = setting.condition {
@@ -666,6 +672,8 @@ extension TargetBuildSettingDescription.Kind {
             return "linkedFramework"
         case .unsafeFlags:
             return "unsafeFlags"
+        case .interoperabilityMode:
+            return "interoperabilityMode"
         case .enableUpcomingFeature:
             return "enableUpcomingFeature"
         case .enableExperimentalFeature:

Tests/BuildTests/BuildPlanTests.swift

@@ -3242,6 +3242,8 @@ final class BuildPlanTests: XCTestCase {
                         .init(tool: .swift, kind: .define("RLINUX"), condition: .init(platformNames: ["linux"], config: "release")),
                         .init(tool: .swift, kind: .define("DMACOS"), condition: .init(platformNames: ["macos"], config: "debug")),
                         .init(tool: .swift, kind: .unsafeFlags(["-Isfoo", "-L", "sbar"])),
+                        .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"))
                     ]
@@ -3250,6 +3252,7 @@ final class BuildPlanTests: XCTestCase {
                     name: "exe", dependencies: ["bar"],
                     settings: [
                         .init(tool: .swift, kind: .define("FOO")),
+                        .init(tool: .swift, kind: .interoperabilityMode(.C, nil)),
                         .init(tool: .linker, kind: .linkedLibrary("sqlite3")),
                         .init(tool: .linker, kind: .linkedFramework("CoreData"), condition: .init(platformNames: ["macos"])),
                         .init(tool: .linker, kind: .unsafeFlags(["-Ilfoo", "-L", "lbar"])),
@@ -3308,7 +3311,7 @@ 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", "-enable-upcoming-feature", "BestFeature", .end])
+            XCTAssertMatch(bar, [.anySequence, "-DLINUX", "-Isfoo", "-L", "sbar", "-cxx-interoperability-mode=swift-5.9", "-enable-upcoming-feature", "BestFeature", .end])
 
             let exe = try result.target(for: "exe").swiftTarget().compileArguments()
             XCTAssertMatch(exe, [.anySequence, "-DFOO", .end])
@@ -3324,7 +3327,7 @@ 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", "-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", .end])
 
             let exe = try result.target(for: "exe").swiftTarget().compileArguments()
             XCTAssertMatch(exe, [.anySequence, "-DFOO", .end])