A sketch for an adaptors module

Package.swift

@@ -31,17 +31,23 @@ struct Available {
   var swiftSetting: SwiftSetting {
     #if SYSTEM_ABI_STABLE
     // Use availability matching Darwin API.
-    let availability = self.osAvailability
+    let availabilityType = self.osAvailability
     #else
     // Use availability matching SwiftPM default.
-    let availability = self.sourceAvailability
+    let availabilityType = self.sourceAvailability
     #endif
     return .enableExperimentalFeature(
-      "AvailabilityMacro=\(self.name) \(version):\(availability)")
+      "AvailabilityMacro=\(self.name) \(version):\(availabilityType)")
+  }
+
+  var compatibilitySetting: SwiftSetting {
+    .enableExperimentalFeature(
+      "AvailabilityMacro=\(self.name) \(version):\(osAvailability)"
+    )
   }
 }
 
-let availability: [Available] = [
+let availabilityList: [Available] = [
   Available("0.0.1", "macOS 11.0, iOS 14.0, watchOS 7.0, tvOS 14.0"),
 
   Available("0.0.2", "macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0"),
@@ -68,7 +74,7 @@ let availability: [Available] = [
   Available("99", "macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, visionOS 9999"),
 ]
 
-let swiftSettingsAvailability = availability.map(\.swiftSetting)
+let availabilitySettings = availabilityList.map(\.swiftSetting)
 
 #if SYSTEM_CI
 let swiftSettingsCI: [SwiftSetting] = [
@@ -78,7 +84,7 @@ let swiftSettingsCI: [SwiftSetting] = [
 let swiftSettingsCI: [SwiftSetting] = []
 #endif
 
-let swiftSettings = swiftSettingsAvailability + swiftSettingsCI + [
+let sharedSwiftSettings = swiftSettingsCI + [
   .define(
     "SYSTEM_PACKAGE_DARWIN",
     .when(platforms: [.macOS, .macCatalyst, .iOS, .watchOS, .tvOS, .visionOS])),
@@ -87,6 +93,8 @@ let swiftSettings = swiftSettingsAvailability + swiftSettingsCI + [
   .enableExperimentalFeature("Lifetimes"),
 ]
 
+let swiftSettings = sharedSwiftSettings + availabilitySettings
+
 let cSettings: [CSetting] = [
   .define("_CRT_SECURE_NO_WARNINGS", .when(platforms: [.windows])),
 ]
@@ -135,12 +143,29 @@ let package = Package(
       exclude: filesToExclude,
       cSettings: cSettings,
       swiftSettings: swiftSettings),
+    .target(
+      name: "SystemCompatibilityAdaptors",
+      dependencies: ["SystemPackage"],
+      path: "Sources/SystemCompatibilityAdaptors",
+      exclude: [],
+      cSettings: cSettings,
+      swiftSettings:
+        sharedSwiftSettings + availabilityList.map(\.compatibilitySetting)
+    ),
     .testTarget(
       name: "SystemTests",
       dependencies: ["SystemPackage"],
       exclude: testsToExclude,
       cSettings: cSettings,
       swiftSettings: swiftSettings),
+    .testTarget(
+      name: "SystemCompatibilityAdaptorTests",
+      dependencies: ["SystemCompatibilityAdaptors", "SystemPackage"],
+      exclude: [],
+      cSettings: cSettings,
+      swiftSettings:
+      	sharedSwiftSettings + availabilityList.map(\.compatibilitySetting)
+    ),
   ],
   swiftLanguageVersions: [.v5]
 )

Sources/SystemCompatibilityAdaptors/CompatibilityAdaptors.swift

@@ -0,0 +1,43 @@
+/*
+ This source file is part of the Swift System open source project
+
+ Copyright (c) 2025 - 2026 Apple Inc. and the Swift System project authors
+ Licensed under Apache License v2.0 with Runtime Library Exception
+
+ See https://swift.org/LICENSE.txt for license information
+*/
+
+#if canImport(System) && canImport(SystemPackage)
+import System
+import SystemPackage
+
+extension SystemPackage.FilePath {
+  @available(System 0.0.2, *)
+  public init(converting path: System.FilePath) {
+    self = path.withPlatformString(Self.init(platformString:))
+  }
+}
+
+@available(System 0.0.1, *)
+extension System.FilePath {
+  @available(System 0.0.2, *)
+  public init(converting path: SystemPackage.FilePath) {
+    self = path.withPlatformString(Self.init(platformString:))
+  }
+}
+
+extension SystemPackage.FileDescriptor {
+  @available(System 0.0.1, *)
+  public init(converting descriptor: System.FileDescriptor) {
+    self.init(rawValue: descriptor.rawValue)
+  }
+}
+
+@available(System 0.0.1, *)
+extension System.FileDescriptor {
+  @available(System 0.0.1, *)
+  public init(converting descriptor: SystemPackage.FileDescriptor) {
+    self.init(rawValue: descriptor.rawValue)
+  }
+}
+#endif // canImport(System) && canImport(SystemPackage)

Tests/SystemCompatibilityAdaptorTests/AdaptorTests.swift

@@ -0,0 +1,34 @@
+#if canImport(System) && canImport(SystemPackage)
+
+import Testing
+
+import SystemPackage
+import System
+
+import SystemCompatibilityAdaptors
+
+@Suite("CompatibilityAdaptors")
+private struct CompatibilityAdaptorTests {
+
+  @available(System 0.0.2, *)
+  @Test
+  func filePathAdaptor() throws {
+    let fp = SystemPackage.FilePath("/bar")
+
+    let sfp = System.FilePath(converting: fp)
+
+    // If we had access to the underlying array as API, this would look less silly
+    #expect(fp == SystemPackage.FilePath(converting: sfp))
+  }
+
+  @available(System 0.0.1, *)
+  @Test
+  func fileDescriptorAdaptor() throws {
+    let fd = try SystemPackage.FileDescriptor.standardInput.duplicate()
+
+    let sfd: System.FileDescriptor = System.FileDescriptor(converting: fd)
+
+    #expect(fd.rawValue == sfd.rawValue)
+  }
+}
+#endif // canImport(System) && canImport(SystemPackage)