[clang][modules] Enable built-in modules for the upcoming Apple releases (llvm#102239)

The upcoming Apple SDK releases will support the clang built-in headers
being in the clang built-in modules: stop passing
-fbuiltin-headers-in-system-modules for those SDK versions.

Author: ian-twilightcoder

clang/lib/Driver/ToolChains/Darwin.cpp

@@ -2923,22 +2923,47 @@ bool Darwin::isAlignedAllocationUnavailable() const {
   return TargetVersion < alignedAllocMinVersion(OS);
 }
 
-static bool sdkSupportsBuiltinModules(const Darwin::DarwinPlatformKind &TargetPlatform, const std::optional<DarwinSDKInfo> &SDKInfo) {
+static bool sdkSupportsBuiltinModules(
+    const Darwin::DarwinPlatformKind &TargetPlatform,
+    const Darwin::DarwinEnvironmentKind &TargetEnvironment,
+    const std::optional<DarwinSDKInfo> &SDKInfo) {
+  switch (TargetEnvironment) {
+  case Darwin::NativeEnvironment:
+  case Darwin::Simulator:
+  case Darwin::MacCatalyst:
+    // Standard xnu/Mach/Darwin based environments
+    // depend on the SDK version.
+    break;
+  default:
+    // All other environments support builtin modules from the start.
+    return true;
+  }
+
   if (!SDKInfo)
+    // If there is no SDK info, assume this is building against a
+    // pre-SDK version of macOS (i.e. before Mac OS X 10.4). Those
+    // don't support modules anyway, but the headers definitely
+    // don't support builtin modules either. It might also be some
+    // kind of degenerate build environment, err on the side of
+    // the old behavior which is to not use builtin modules.
     return false;
 
   VersionTuple SDKVersion = SDKInfo->getVersion();
   switch (TargetPlatform) {
+  // Existing SDKs added support for builtin modules in the fall
+  // 2024 major releases.
   case Darwin::MacOS:
-    return SDKVersion >= VersionTuple(99U);
+    return SDKVersion >= VersionTuple(15U);
   case Darwin::IPhoneOS:
-    return SDKVersion >= VersionTuple(99U);
+    return SDKVersion >= VersionTuple(18U);
   case Darwin::TvOS:
-    return SDKVersion >= VersionTuple(99U);
+    return SDKVersion >= VersionTuple(18U);
   case Darwin::WatchOS:
-    return SDKVersion >= VersionTuple(99U);
+    return SDKVersion >= VersionTuple(11U);
   case Darwin::XROS:
-    return SDKVersion >= VersionTuple(99U);
+    return SDKVersion >= VersionTuple(2U);
+
+  // New SDKs support builtin modules from the start.
   default:
     return true;
   }
@@ -3030,7 +3055,7 @@ void Darwin::addClangTargetOptions(
   // i.e. when the builtin stdint.h is in the Darwin module too, the cycle
   // goes away. Note that -fbuiltin-headers-in-system-modules does nothing
   // to fix the same problem with C++ headers, and is generally fragile.
-  if (!sdkSupportsBuiltinModules(TargetPlatform, SDKInfo))
+  if (!sdkSupportsBuiltinModules(TargetPlatform, TargetEnvironment, SDKInfo))
     CC1Args.push_back("-fbuiltin-headers-in-system-modules");
 
   if (!DriverArgs.hasArgNoClaim(options::OPT_fdefine_target_os_macros,

clang/test/Driver/Inputs/DriverKit23.0.sdk/SDKSettings.json

@@ -0,0 +1 @@
+{"Version":"23.0", "MaximumDeploymentTarget": "23.0.99"}

clang/test/Driver/Inputs/MacOSX99.0.sdk/SDKSettings.json renamed to clang/test/Driver/Inputs/MacOSX15.0.sdk/SDKSettings.json

@@ -6,6 +6,7 @@
 // RUN: %clang -isysroot %S/Inputs/iPhoneOS13.0.sdk -target arm64-apple-ios13.0 -### %s 2>&1 | FileCheck %s
 // CHECK: -fbuiltin-headers-in-system-modules
 
-// RUN: %clang -isysroot %S/Inputs/MacOSX99.0.sdk -target x86_64-apple-macos98.0 -### %s 2>&1 | FileCheck --check-prefix=CHECK_FUTURE %s
-// RUN: %clang -isysroot %S/Inputs/MacOSX99.0.sdk -target x86_64-apple-macos99.0 -### %s 2>&1 | FileCheck --check-prefix=CHECK_FUTURE %s
+// RUN: %clang -isysroot %S/Inputs/MacOSX15.0.sdk -target x86_64-apple-macos14.0 -### %s 2>&1 | FileCheck --check-prefix=CHECK_FUTURE %s
+// RUN: %clang -isysroot %S/Inputs/MacOSX15.0.sdk -target x86_64-apple-macos15.0 -### %s 2>&1 | FileCheck --check-prefix=CHECK_FUTURE %s
+// RUN: %clang -isysroot %S/Inputs/DriverKit23.0.sdk -target arm64-apple-driverkit23.0 -### %s 2>&1 | FileCheck --check-prefix=CHECK_FUTURE %s
 // CHECK_FUTURE-NOT: -fbuiltin-headers-in-system-modules