Fix direct clang cc1 emit-pcm commands with vfs overlay on Windows (#85325)

Explicit module builds currently fail on Windows because
direct-clang-cc1-module-build emit-pcm commands take overlaid system
module map files as inputs but miss the clang VFS overlay. This change
adds the overlay and fixes explicit module builds on Windows.

include/swift/ClangImporter/ClangImporter.h

@@ -75,6 +75,7 @@ namespace dependencies {
 namespace swift {
 enum class ResultConvention : uint8_t;
 class ASTContext;
+class CASOptions;
 class CompilerInvocation;
 class ClangImporterOptions;
 class ClangInheritanceInfo;
@@ -83,6 +84,7 @@ class ClangNode;
 class ConcreteDeclRef;
 class Decl;
 class DeclContext;
+class DiagnosticEngine;
 class EffectiveClangContext;
 class EnumDecl;
 class FuncDecl;
@@ -878,6 +880,25 @@ struct ClangInvocationFileMapping {
   bool requiresBuiltinHeadersInSystemModules;
 };
 
+class ClangInvocationFileMappingContext {
+public:
+  const LangOptions &LangOpts;
+  SearchPathOptions &SearchPathOpts;
+  ClangImporterOptions &ClangImporterOpts;
+  const CASOptions &CASOpts;
+  DiagnosticEngine &Diags;
+
+  ClangInvocationFileMappingContext(
+    const LangOptions &LangOpts, SearchPathOptions &SearchPathOpts,
+    ClangImporterOptions &ClangImporterOpts, const CASOptions &CASOpts,
+    DiagnosticEngine &Diags)
+    : LangOpts(LangOpts), SearchPathOpts(SearchPathOpts),
+      ClangImporterOpts(ClangImporterOpts), CASOpts(CASOpts),
+      Diags(Diags) {}
+
+  ClangInvocationFileMappingContext(const swift::ASTContext &Ctx);
+};
+
 /// On Linux, some platform libraries (glibc, libstdc++) are not modularized.
 /// We inject modulemaps for those libraries into their include directories
 /// to allow using them from Swift.
@@ -885,7 +906,7 @@ struct ClangInvocationFileMapping {
 /// `suppressDiagnostic` prevents us from emitting warning messages when we
 /// are unable to find headers.
 ClangInvocationFileMapping getClangInvocationFileMapping(
-    const ASTContext &ctx,
+    const ClangInvocationFileMappingContext &ctx,
     llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> vfs = nullptr,
     bool suppressDiagnostic = false);
 
@@ -893,7 +914,7 @@ ClangInvocationFileMapping getClangInvocationFileMapping(
 /// primarily to inject modulemaps on platforms with non-modularized
 /// platform libraries.
 ClangInvocationFileMapping applyClangInvocationMapping(
-    const ASTContext &ctx,
+    const ClangInvocationFileMappingContext &ctx,
     llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> baseVFS,
     llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> &fileSystem,
     bool suppressDiagnostics = false);

lib/ClangImporter/ClangIncludePaths.cpp

@@ -202,7 +202,8 @@ ClangImporter::createClangArgs(const ClangImporterOptions &ClangImporterOpts,
 }
 
 static SmallVector<std::pair<std::string, std::string>, 2>
-getLibcFileMapping(const ASTContext &ctx, StringRef modulemapFileName,
+getLibcFileMapping(const ClangInvocationFileMappingContext &ctx,
+                   StringRef modulemapFileName,
                    std::optional<ArrayRef<StringRef>> maybeHeaderFileNames,
                    const llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> &vfs,
                    bool suppressDiagnostic) {
@@ -269,7 +270,8 @@ getLibcFileMapping(const ASTContext &ctx, StringRef modulemapFileName,
 }
 
 static void getLibStdCxxFileMapping(
-    ClangInvocationFileMapping &fileMapping, const ASTContext &ctx,
+    ClangInvocationFileMapping &fileMapping,
+    const ClangInvocationFileMappingContext &ctx,
     const llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> &vfs,
     bool suppressDiagnostic) {
   assert(ctx.LangOpts.EnableCXXInterop &&
@@ -475,7 +477,8 @@ GetPlatformAuxiliaryFile(StringRef Platform, StringRef File,
 }
 
 void GetWindowsFileMappings(
-    ClangInvocationFileMapping &fileMapping, const ASTContext &Context,
+    ClangInvocationFileMapping &fileMapping,
+    const ClangInvocationFileMappingContext &Context,
     const llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> &driverVFS,
     bool &requiresBuiltinHeadersInSystemModules) {
   const llvm::Triple &Triple = Context.LangOpts.Target;
@@ -615,8 +618,14 @@ void GetWindowsFileMappings(
 }
 } // namespace
 
+ClangInvocationFileMappingContext::ClangInvocationFileMappingContext(
+    const swift::ASTContext &Ctx)
+  : ClangInvocationFileMappingContext(Ctx.LangOpts, Ctx.SearchPathOpts,
+        Ctx.ClangImporterOpts, Ctx.CASOpts, Ctx.Diags) {}
+
 ClangInvocationFileMapping swift::getClangInvocationFileMapping(
-  const ASTContext &ctx, llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> vfs,
+  const ClangInvocationFileMappingContext &ctx,
+  llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> vfs,
   bool suppressDiagnostic) {
   ClangInvocationFileMapping result;
   if (!vfs)
@@ -687,10 +696,11 @@ ClangInvocationFileMapping swift::getClangInvocationFileMapping(
   return result;
 }
 
-ClangInvocationFileMapping swift::applyClangInvocationMapping(const ASTContext &ctx,
-                                        llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> baseVFS,
-                                        llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> &fileSystem,
-                                        bool suppressDiagnostics) {
+ClangInvocationFileMapping swift::applyClangInvocationMapping(
+    const ClangInvocationFileMappingContext &ctx,
+    llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> baseVFS,
+    llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> &fileSystem,
+    bool suppressDiagnostics) {
   if (ctx.CASOpts.HasImmutableFileSystem)
     return ClangInvocationFileMapping();
 
@@ -719,13 +729,9 @@ ClangInvocationFileMapping swift::applyClangInvocationMapping(const ASTContext &
                      << "' with the following contents:\n";
         llvm::errs() << file.second << "\n";
       }
-      auto contents = ctx.Allocate<char>(file.second.size() + 1);
-      std::copy(file.second.begin(), file.second.end(), contents.begin());
-      // null terminate the buffer.
-      contents[contents.size() - 1] = '\0';
+      // Note MemoryBuffer is guaranteeed to be null-terminated.
       overridenVFS->addFile(file.first, 0,
-                            llvm::MemoryBuffer::getMemBuffer(StringRef(
-                                contents.begin(), contents.size() - 1)));
+                            llvm::MemoryBuffer::getMemBufferCopy(file.second));
     }
     llvm::IntrusiveRefCntPtr<llvm::vfs::OverlayFileSystem> overlayVFS =
         new llvm::vfs::OverlayFileSystem(fileSystem);

lib/DependencyScan/ModuleDependencyScanner.cpp

@@ -1992,6 +1992,14 @@ ModuleDependencyInfo ModuleDependencyScanner::bridgeClangModuleDependency(
     }
   }
 
+  // Pass the -sdk flag to make the system header VFS overlay finable
+  // for the -direct-clang-cc1-module-build emit-pcm command on Windows.
+  StringRef SDKPath = ScanASTContext.SearchPathOpts.getSDKPath();
+  if (!SDKPath.empty()) {
+    swiftArgs.push_back("-sdk");
+    swiftArgs.push_back(SDKPath.str());
+  }
+
   // Add args reported by the scanner.
   auto clangArgs = invocation.getCC1CommandLine();
   llvm::for_each(clangArgs, addClangArg);

lib/Frontend/Frontend.cpp

@@ -711,6 +711,25 @@ bool CompilerInstance::setUpVirtualFileSystemOverlays() {
         new llvm::vfs::OverlayFileSystem(MemFS);
     OverlayVFS->pushOverlay(SourceMgr.getFileSystem());
     SourceMgr.setFileSystem(std::move(OverlayVFS));
+  } else {
+    // For non-caching -direct-clang-cc1-module-build emit-pcm build,
+    // setup the clang VFS so it can find system modulemap files
+    // (like vcruntime.modulemap) as an input file.
+    if (Invocation.getClangImporterOptions().DirectClangCC1ModuleBuild &&
+        Invocation.getFrontendOptions().RequestedAction ==
+            FrontendOptions::ActionType::EmitPCM) {
+      llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS =
+          SourceMgr.getFileSystem();
+      ClangInvocationFileMappingContext Context(
+          Invocation.getLangOptions(), Invocation.getSearchPathOptions(),
+          Invocation.getClangImporterOptions(), Invocation.getCASOptions(),
+          Diagnostics);
+      ClangInvocationFileMapping FileMapping = applyClangInvocationMapping(
+          Context, nullptr, VFS, /*suppressDiagnostic=*/false);
+      if (!FileMapping.redirectedFiles.empty()) {
+        SourceMgr.setFileSystem(std::move(VFS));
+      }
+    }
   }
 
   auto ExpectedOverlay =

test/ScanDependencies/Inputs/WinSDK/usr/share/vcruntime.modulemap

@@ -0,0 +1,4 @@
+module SAL [system] {
+  header "sal.h"
+  export *
+}

test/ScanDependencies/direct-clang-cc1-emit-pcm-vfs-overlay.swift

@@ -0,0 +1,13 @@
+// REQUIRES: OS=windows-msvc
+
+// Test that the -direct-clang-cc1-module-build is able to create a module from the VC runtime with an overlaid modulemap file
+
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: %swift_frontend_plain -target %target-triple -module-cache-path %t/clang-module-cache -scan-dependencies -module-name Test -sdk %S/Inputs/WinSDK %t/Test.swift -o %t/deps.json
+// RUN: %{python} %S/../CAS/Inputs/BuildCommandExtractor.py %t/deps.json clang:SAL > %t/SAL.cmd
+// RUN: %swift_frontend_plain @%t/SAL.cmd
+
+//--- Test.swift
+import SAL