[SYS-0010] A Win32 Namespace and Win32Error for Swift System

Sources/System/CMakeLists.txt

@@ -80,6 +80,9 @@ target_sources(SystemPackage PRIVATE
   Internals/RawBuffer.swift
   Internals/Syscalls.swift
   Internals/WindowsSyscallAdapters.swift)
+target_sources(SystemPackage PRIVATE
+  Win32/Win32.swift
+  Win32/Win32Error.swift)
 target_link_libraries(SystemPackage PUBLIC
   CSystem)
 

Sources/System/ErrnoWindows.swift

@@ -1,7 +1,7 @@
 /*
  This source file is part of the Swift System open source project
 
- Copyright (c) 2024 Apple Inc. and the Swift System project authors
+ Copyright (c) 2024 - 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
@@ -17,4 +17,17 @@ extension Errno {
   }
 }
 
+@available(System 199, *)
+extension Errno {
+  /// Creates the closest POSIX ``Errno`` for a Windows system error.
+  ///
+  /// This mapping is lossy. It approximates the C runtime's `_dosmaperr`,
+  /// folding Windows' several thousand system error codes onto a small set
+  /// of ``Errno`` values. Unrecognized codes become ``Errno/invalidArgument``.
+  /// Prefer handling ``Win32Error`` directly.
+  public init(approximating error: Win32Error) {
+    self.init(windowsError: error.rawValue)
+  }
+}
+
 #endif

Sources/System/Win32/Win32.swift

@@ -0,0 +1,28 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift System open source project
+//
+// Copyright (c) 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 os(Windows)
+/// A namespace for Win32 APIs on Windows.
+///
+/// The types and functions nested here wrap the Win32 base API, the
+/// documented, ABI-stable system interface for desktop Windows.
+@frozen
+@available(System 199, *)
+public enum Win32 {}
+#elseif SYSTEM_PACKAGE
+// Package builds on other platforms get unavailable stubs for `Win32` and
+// `Win32Error`. Non-package builds omit them to avoid exporting ABI symbols.
+@available(*, unavailable, message: "Win32 APIs are only available on Windows.")
+public enum Win32 {}
+
+@available(*, unavailable, message: "Win32 APIs are only available on Windows.")
+public struct Win32Error: Error {}
+#endif