[cherry-pick stable/20230725] [Modules] Make clang modules for the C standard library headers

Differential Revision: https://reviews.llvm.org/D159064
rdar://105819340

Author: git apple-llvm automerger

clang/lib/Basic/Module.cpp

@@ -308,8 +308,7 @@ bool Module::directlyUses(const Module *Requested) {
   // Anyone is allowed to use our builtin stdarg.h and stddef.h and their
   // accompanying modules.
   if (Requested->getTopLevelModuleName() == "_Builtin_stdarg" ||
-      Requested->getTopLevelModuleName() == "_Builtin_stddef" ||
-      (!Requested->Parent && Requested->Name == "_Builtin_stddef_max_align_t"))
+      Requested->getTopLevelModuleName() == "_Builtin_stddef")
     return true;
 
   if (NoUndeclaredIncludes)

clang/lib/Headers/__stddef_max_align_t.h

@@ -1,4 +1,4 @@
-/*===---- __stddef_max_align_t.h - Definition of max_align_t for modules ---===
+/*===---- __stddef_max_align_t.h - Definition of max_align_t ---------------===
  *
  * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
  * See https://llvm.org/LICENSE.txt for license information.

clang/lib/Headers/__stddef_null.h

@@ -7,7 +7,15 @@
  *===-----------------------------------------------------------------------===
  */
 
+#if !defined(NULL) || !__has_feature(modules)
+
+/* linux/stddef.h will define NULL to 0. glibc (and other) headers then define
+ * __need_NULL and rely on stddef.h to redefine NULL to the correct value again.
+ * Modules don't support redefining macros like that, but support that pattern
+ * in the non-modules case.
+ */
 #undef NULL
+
 #ifdef __cplusplus
 #if !defined(__MINGW32__) && !defined(_MSC_VER)
 #define NULL __null
@@ -17,3 +25,5 @@
 #else
 #define NULL ((void *)0)
 #endif
+
+#endif

clang/lib/Headers/__stddef_nullptr_t.h

@@ -7,19 +7,20 @@
  *===-----------------------------------------------------------------------===
  */
 
-#if !defined(_NULLPTR_T) || __has_feature(modules)
-/* Always define nullptr_t when modules are available. */
-#if !__has_feature(modules)
+#ifndef _NULLPTR_T
 #define _NULLPTR_T
-#endif
+
 #ifdef __cplusplus
 #if defined(_MSC_EXTENSIONS) && defined(_NATIVE_NULLPTR_SUPPORTED)
 namespace std {
 typedef decltype(nullptr) nullptr_t;
 }
 using ::std::nullptr_t;
 #endif
-#else
+/* FIXME: This is using the placeholder dates Clang produces for these macros
+   in C2x mode; switch to the correct values once they've been published. */
+#elif defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202000L
 typedef typeof(nullptr) nullptr_t;
 #endif
+
 #endif

clang/lib/Headers/__stddef_offsetof.h

@@ -7,7 +7,6 @@
  *===-----------------------------------------------------------------------===
  */
 
-#if !defined(offsetof) || __has_feature(modules)
-/* Always define offsetof when modules are available. */
+#ifndef offsetof
 #define offsetof(t, d) __builtin_offsetof(t, d)
 #endif

clang/lib/Headers/__stddef_ptrdiff_t.h

@@ -7,10 +7,9 @@
  *===-----------------------------------------------------------------------===
  */
 
-#if !defined(_PTRDIFF_T) || __has_feature(modules)
-/* Always define ptrdiff_t when modules are available. */
-#if !__has_feature(modules)
+#ifndef _PTRDIFF_T
 #define _PTRDIFF_T
-#endif
+
 typedef __PTRDIFF_TYPE__ ptrdiff_t;
+
 #endif

clang/lib/Headers/__stddef_rsize_t.h

@@ -7,10 +7,9 @@
  *===-----------------------------------------------------------------------===
  */
 
-#if !defined(_RSIZE_T) || __has_feature(modules)
-/* Always define rsize_t when modules are available. */
-#if !__has_feature(modules)
+#ifndef _RSIZE_T
 #define _RSIZE_T
-#endif
+
 typedef __SIZE_TYPE__ rsize_t;
+
 #endif

clang/lib/Headers/__stddef_size_t.h

@@ -7,10 +7,9 @@
  *===-----------------------------------------------------------------------===
  */
 
-#if !defined(_SIZE_T) || __has_feature(modules)
-/* Always define size_t when modules are available. */
-#if !__has_feature(modules)
+#ifndef _SIZE_T
 #define _SIZE_T
-#endif
+
 typedef __SIZE_TYPE__ size_t;
+
 #endif

clang/lib/Headers/__stddef_unreachable.h

@@ -7,7 +7,6 @@
  *===-----------------------------------------------------------------------===
  */
 
-#if !defined(unreachable) || __has_feature(modules)
-/* Always define unreachable when modules are available. */
+#ifndef unreachable
 #define unreachable() __builtin_unreachable()
 #endif

clang/lib/Headers/__stddef_wchar_t.h

@@ -8,14 +8,16 @@
  */
 
 #if !defined(__cplusplus) || (defined(_MSC_VER) && !_NATIVE_WCHAR_T_DEFINED)
-/* Always define wchar_t when modules are available. */
-#if !defined(_WCHAR_T) || __has_feature(modules)
-#if !__has_feature(modules)
+
+#ifndef _WCHAR_T
 #define _WCHAR_T
-#if defined(_MSC_EXTENSIONS)
+
+#ifdef _MSC_EXTENSIONS
 #define _WCHAR_T_DEFINED
 #endif
-#endif
+
 typedef __WCHAR_TYPE__ wchar_t;
+
 #endif
+
 #endif