Fix direct clang cc1 emit-pcm commands with vfs overlay on Windows

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.

Author: hjyamauchi

include/swift/Frontend/Frontend.h

@@ -736,6 +736,8 @@ class CompilerInstance {
   /// Retrieve the printing path for bridging header.
   std::string getBridgingHeaderPath() const;
 
+  void setUpVFSForDirectClangCC1EmitPCM();
+
 private:
   /// Set up the file system by loading and validating all VFS overlay YAML
   /// files. If the process of validating VFS files failed, or the overlay

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

@@ -559,6 +559,23 @@ void CompilerInstance::setupCachingDiagnosticsProcessorIfNeeded() {
   CDP->startDiagnosticCapture();
 }
 
+void CompilerInstance::setUpVFSForDirectClangCC1EmitPCM() {
+  // Set up the VFS overlay so that the -direct-clang-cc1-module-build
+  // emit-pcm command can find a system modulemap file (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();
+    ClangInvocationFileMapping FileMapping = applyClangInvocationMapping(
+        *Context, nullptr, VFS, /*suppressDiagnostic=*/false);
+    if (!FileMapping.redirectedFiles.empty()) {
+      SourceMgr.setFileSystem(std::move(VFS));
+    }
+  }
+}
+
 bool CompilerInstance::setup(const CompilerInvocation &Invoke,
                              std::string &Error, ArrayRef<const char *> Args) {
   Invocation = Invoke;
@@ -582,13 +599,17 @@ bool CompilerInstance::setup(const CompilerInvocation &Invoke,
 
   assert(Lexer::isIdentifier(Invocation.getModuleName()));
 
-  if (setUpInputs()) {
-    Error = "Setting up inputs failed";
+  if (setUpASTContextIfNeeded()) {
+    Error = "Setting up ASTContext failed";
     return true;
   }
 
-  if (setUpASTContextIfNeeded()) {
-    Error = "Setting up ASTContext failed";
+  // ASTContext is needed to set up the VFS overlay, which is needed
+  // to allow inputs from the VFS overlay are set up.
+  setUpVFSForDirectClangCC1EmitPCM();
+
+  if (setUpInputs()) {
+    Error = "Setting up inputs failed";
     return true;
   }
 

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 +1,6 @@
+// REQUIRES: OS=windows-msvc
+
+// RUN: %empty-directory(%t)
+
+// Test that the -direct-clang-cc1-module-build is able to create a module from the VC runtime with an overlaid modulemap file
+// RUN: %swift_frontend_plain -frontend -emit-pcm -module-name SAL -o %t/SAL.pcm -direct-clang-cc1-module-build "%vctoolsinstalldir\include\module.modulemap" -sdk %S/Inputs/WinSDK -Xcc -fsystem-module -Xcc -emit-module -Xcc -isystem -Xcc "%vctoolsinstalldir\include" -Xcc -cc1 -Xcc -fmodules -Xcc -fmodules-cache-path=%t/module-cache

test/lit.cfg

@@ -857,6 +857,10 @@ config.substitutions.append( ('%clang',
 
 config.substitutions.append(('%existing-swift-features', shlex.quote(json.dumps(list(config.existing_swift_features)))))
 
+if kIsWindows:
+    VCToolsInstallDir = os.path.normpath(os.getenv("VCToolsInstallDir"))
+    config.substitutions.append(('%vctoolsinstalldir', VCToolsInstallDir))
+
 ###
 
 def disallow(execName):