[CAS] Workaround the missing file system dependency from clang importer

include/swift/ClangImporter/ClangImporter.h

@@ -409,6 +409,11 @@ public:
   /// Reads the original source file name from PCH.
   std::string getOriginalSourceFile(StringRef PCHFilename);
 
+  /// Add clang dependency file names.
+  ///
+  /// \param files The list of file to append dependencies to.
+  void addClangInvovcationDependencies(std::vector<std::string> &files);
+
   /// Makes a temporary replica of the ClangImporter's CompilerInstance, reads a
   /// module map into the replica and emits a PCM file for one of the modules it
   /// declares. Delegates to clang for everything except construction of the

lib/AST/ModuleDependencies.cpp

@@ -446,6 +446,11 @@ void SwiftDependencyScanningService::setupCachingDependencyScanningService(
     }
   }
 
+  // Fetch some dependency files from clang importer.
+  auto clangImporter = static_cast<ClangImporter *>(
+      Instance.getASTContext().getClangModuleLoader());
+  clangImporter->addClangInvovcationDependencies(CommonDependencyFiles);
+
   auto CachingFS =
       llvm::cas::createCachingOnDiskFileSystem(Instance.getObjectStore());
   if (!CachingFS) {

lib/ClangImporter/ClangImporter.cpp

@@ -939,6 +939,27 @@ std::string ClangImporter::getOriginalSourceFile(StringRef PCHFilename) {
       Impl.Instance->getPCHContainerReader(), Impl.Instance->getDiagnostics());
 }
 
+void ClangImporter::addClangInvovcationDependencies(
+    std::vector<std::string> &files) {
+  auto addFiles = [&files](const auto &F) {
+    files.insert(files.end(), F.begin(), F.end());
+  };
+  auto &invocation = *Impl.Invocation;
+  // FIXME: Add file dependencies that are not accounted. The long term solution
+  // is to do a dependency scanning for clang importer and use that directly.
+  SmallVector<std::string, 4> HeaderMapFileNames;
+  Impl.Instance->getPreprocessor().getHeaderSearchInfo().getHeaderMapFileNames(
+      HeaderMapFileNames);
+  addFiles(HeaderMapFileNames);
+  addFiles(invocation.getHeaderSearchOpts().VFSOverlayFiles);
+  // FIXME: Should not depend on working directory. Build system/swift driver
+  // should not pass working directory here but if that option is passed,
+  // repect that and add that into CASFS.
+  auto CWD = invocation.getFileSystemOpts().WorkingDir;
+  if (!CWD.empty())
+    files.push_back(CWD);
+}
+
 Optional<std::string>
 ClangImporter::getPCHFilename(const ClangImporterOptions &ImporterOptions,
                               StringRef SwiftPCHHash, bool &isExplicit) {