Jump through hoops to avoid adding a new Serialization -> clang::Module dependency.

Author: DougGregor

include/swift/ClangImporter/ClangModule.h

@@ -56,6 +56,11 @@ class ClangModuleUnit final : public LoadedFile {
   /// Returns the Swift module that overlays this Clang module.
   ModuleDecl *getAdapterModule() const;
 
+  /// Retrieve the "exported" name of the module, which is usually the module
+  /// name, but might be the name of the public module through which this
+  /// (private) module is re-exported.
+  std::string getExportedModuleName() const;
+
   virtual bool isSystemModule() const override;
 
   virtual void lookupValue(ModuleDecl::AccessPathTy accessPath,

lib/ClangImporter/ClangImporter.cpp

@@ -2859,6 +2859,13 @@ clang::ASTContext &ClangModuleUnit::getClangASTContext() const {
   return owner.getClangASTContext();
 }
 
+std::string ClangModuleUnit::getExportedModuleName() const {
+  if (clangModule && !clangModule->ExportAsModule.empty())
+    return clangModule->ExportAsModule;
+
+  return getParentModule()->getName().str();
+}
+
 ModuleDecl *ClangModuleUnit::getAdapterModule() const {
   if (!clangModule)
     return nullptr;

lib/Serialization/ModuleFile.cpp

@@ -21,9 +21,9 @@
 #include "swift/AST/USRGeneration.h"
 #include "swift/Basic/Range.h"
 #include "swift/ClangImporter/ClangImporter.h"
+#include "swift/ClangImporter/ClangModule.h"
 #include "swift/Serialization/BCReadingExtras.h"
 #include "swift/Serialization/SerializedModuleLoader.h"
-#include "clang/Basic/Module.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/OnDiskHashTable.h"
@@ -1562,18 +1562,20 @@ void ModuleFile::loadExtensions(NominalTypeDecl *nominal) {
   }
 
   if (nominal->getParent()->isModuleScopeContext()) {
-    Identifier moduleName = nominal->getParentModule()->getName();
+    auto parentModule = nominal->getParentModule();
+    StringRef moduleName = parentModule->getName().str();
 
     // If the originating module is a private module whose interface is
     // re-exported via public module, check the name of the public module.
-    if (auto clangModule
-          = nominal->getParentModule()->findUnderlyingClangModule()) {
-      if (!clangModule->ExportAsModule.empty())
-        moduleName = getContext().getIdentifier(clangModule->ExportAsModule);
+    std::string exportedModuleName;
+    if (auto clangModuleUnit =
+            dyn_cast<ClangModuleUnit>(parentModule->getFiles().front())) {
+      exportedModuleName = clangModuleUnit->getExportedModuleName();
+      moduleName = exportedModuleName;
     }
 
     for (auto item : *iter) {
-      if (item.first != moduleName.str())
+      if (item.first != moduleName)
         continue;
       Expected<Decl *> declOrError = getDeclChecked(item.second);
       if (!declOrError) {