Instead of importing platform SDK modules, just use C shims

Author: rintaro

Sources/SwiftCompilerPluginMessageHandling/JSON/JSONDecoding.swift

@@ -11,21 +11,9 @@
 //===----------------------------------------------------------------------===//
 
 #if swift(>=6.0)
-#if canImport(Darwin)
-private import Darwin
-#elseif canImport(Glibc)
-private import Glibc
-#elseif canImport(ucrt)
-private import ucrt
-#endif
+private import _SwiftSyntaxCShims
 #else
-#if canImport(Darwin)
-import Darwin
-#elseif canImport(Glibc)
-import Glibc
-#elseif canImport(ucrt)
-import ucrt
-#endif
+@_implementationOnly import _SwiftSyntaxCShims
 #endif
 
 func decodeFromJSON<T: Decodable>(json: UnsafeBufferPointer<UInt8>) throws -> T {

Sources/SwiftCompilerPluginMessageHandling/StandardIOMessageConnection.swift

@@ -12,22 +12,8 @@
 
 #if swift(>=6.0)
 private import _SwiftSyntaxCShims
-#if canImport(Darwin)
-private import Darwin
-#elseif canImport(Glibc)
-private import Glibc
-#elseif canImport(ucrt)
-private import ucrt
-#endif
 #else
-import _SwiftSyntaxCShims
-#if canImport(Darwin)
-import Darwin
-#elseif canImport(Glibc)
-import Glibc
-#elseif canImport(ucrt)
-import ucrt
-#endif
+@_implementationOnly import _SwiftSyntaxCShims
 #endif
 
 #if canImport(ucrt)

Sources/SwiftSyntax/SyntaxArena.swift

@@ -10,7 +10,11 @@
 //
 //===----------------------------------------------------------------------===//
 
-import _SwiftSyntaxCShims
+#if swift(>=6.0)
+private import _SwiftSyntaxCShims
+#else
+@_implementationOnly import _SwiftSyntaxCShims
+#endif
 
 /// A syntax arena owns the memory for all syntax nodes within it.
 ///

Sources/SwiftSyntax/SyntaxText.swift

@@ -11,25 +11,9 @@
 //===----------------------------------------------------------------------===//
 
 #if swift(>=6.0)
-#if canImport(Darwin)
-private import Darwin
-#elseif canImport(Glibc)
-private import Glibc
-#elseif canImport(Musl)
-private import Musl
-#elseif canImport(ucrt)
-private import ucrt
-#endif
+private import _SwiftSyntaxCShims
 #else
-#if canImport(Darwin)
-import Darwin
-#elseif canImport(Glibc)
-import Glibc
-#elseif canImport(Musl)
-import Musl
-#elseif canImport(ucrt)
-import ucrt
-#endif
+@_implementationOnly import _SwiftSyntaxCShims
 #endif
 
 /// Represent a string.

Sources/_SwiftSyntaxCShims/include/module.modulemap

@@ -1,4 +1,5 @@
 module _SwiftSyntaxCShims {
+  header "swiftsyntax/_includes.h"
   header "swiftsyntax/AtomicBool.h"
   header "swiftsyntax/errno.h"
   header "swiftsyntax/stdio.h"

Sources/_SwiftSyntaxCShims/include/swiftsyntax/_includes.h

@@ -0,0 +1,35 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2024 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
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+//
+// This file is for, instead of importing platform modules in Swift file, like:
+//
+//   #if canImport(Darwin)
+//   import Darwin
+//   #elseif canImport(Glibc)
+//   import Glibc
+//   ...
+//
+// Just include them here using C facilities, so that Swift module can just:
+//
+//   import _SwiftSyntaxCShims
+//
+//===----------------------------------------------------------------------===//
+
+#if defined(_WIN32)
+#include <io.h>
+#elif defined(__unix__) || defined(__APPLE__)
+#include <unistd.h>
+#endif
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>