Add PlatformConformance in tests to ensure platform specifc types follow a uniform shape

Author: iCharlesHu

Package.swift

@@ -6,7 +6,9 @@ import PackageDescription
 var dep: [Package.Dependency] = [
     .package(
         url: "https://github.com/apple/swift-system",
-        from: "1.5.0"
+        // Temporarily pin to 1.5.0 because 1.6.0 has a breaking change for Ubuntu Focal
+        // https://github.com/apple/swift-system/issues/237
+        exact: "1.5.0"
     )
 ]
 #if !os(Windows)

Tests/SubprocessTests/PlatformConformance.swift

@@ -0,0 +1,40 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+@testable import Subprocess
+
+#if canImport(Darwin)
+import Darwin
+#elseif canImport(Bionic)
+import Bionic
+#elseif canImport(Glibc)
+import Glibc
+#elseif canImport(Musl)
+import Musl
+#elseif canImport(WinSDK)
+import WinSDK
+#endif
+
+/// This file defines protocols for platform-specific structs
+/// and adds retroactive conformances to them to ensure they all
+/// conform to a uniform shape. We opted to keep these protocols
+/// in the test target as opposed to making them public APIs
+/// because we don't directly use them in public APIs.
+
+protocol ProcessIdentifierProtocol: Sendable, Hashable, CustomStringConvertible, CustomDebugStringConvertible {
+    #if os(Windows)
+    var value: DWORD { get }
+    #else
+    var value: pid_t { get }
+    #endif
+}
+
+extension ProcessIdentifier : ProcessIdentifierProtocol {}