[Clang importer] Eliminate retain cycle involving the Clang importer implementation.

The Clang importer implementation held on to a Clang instance, which
held a list of module file extensions, which included the Clang
importer implementation... break the cycle by separating out the
module file extension from the Clang importer. Fixes SR-562 /
rdar://problem/24225173.

Author: DougGregor

lib/ClangImporter/ClangImporter.cpp

@@ -239,11 +239,10 @@ ClangImporter::ClangImporter(ASTContext &ctx,
   : ClangModuleLoader(tracker),
     Impl(*new Implementation(ctx, clangImporterOpts))
 {
-  Impl.Retain();
 }
 
 ClangImporter::~ClangImporter() {
-  Impl.Release();
+  delete &Impl;
 }
 
 void ClangImporter::setTypeResolver(LazyResolver &resolver) {
@@ -548,7 +547,8 @@ ClangImporter::create(ASTContext &ctx,
                          sourceBuffer.release());
 
   // Install a Clang module file extension to build Swift name lookup tables.
-  invocation->getFrontendOpts().ModuleFileExtensions.push_back(&importer->Impl);
+  invocation->getFrontendOpts().ModuleFileExtensions.push_back(
+    new Implementation::SwiftNameLookupExtension(importer->Impl));
 
   // Create a compiler instance.
   auto PCHContainerOperations =
@@ -4106,7 +4106,8 @@ void ClangImporter::getMangledName(raw_ostream &os,
 // ---------------------------------------------------------------------------
 
 clang::ModuleFileExtensionMetadata
-ClangImporter::Implementation::getExtensionMetadata() const {
+ClangImporter::Implementation::SwiftNameLookupExtension::
+getExtensionMetadata() const {
   clang::ModuleFileExtensionMetadata metadata;
   metadata.BlockName = "swift.lookup";
   metadata.MajorVersion = SWIFT_LOOKUP_TABLE_VERSION_MAJOR;
@@ -4115,18 +4116,20 @@ ClangImporter::Implementation::getExtensionMetadata() const {
   return metadata;
 }
 
-llvm::hash_code ClangImporter::Implementation::hashExtension(
-                  llvm::hash_code code) const {
+llvm::hash_code
+ClangImporter::Implementation::SwiftNameLookupExtension::hashExtension(
+   llvm::hash_code code) const {
   return llvm::hash_combine(code, StringRef("swift.lookup"),
                             SWIFT_LOOKUP_TABLE_VERSION_MAJOR,
                             SWIFT_LOOKUP_TABLE_VERSION_MINOR,
-                            OmitNeedlessWords,
-                            InferDefaultArguments);
+                            Impl.OmitNeedlessWords,
+                            Impl.InferDefaultArguments);
 }
 
 std::unique_ptr<clang::ModuleFileExtensionWriter>
-ClangImporter::Implementation::createExtensionWriter(clang::ASTWriter &writer) {
-  // Local function to populate the lookup table.
+ClangImporter::Implementation::SwiftNameLookupExtension::createExtensionWriter(
+    clang::ASTWriter &writer) {
+    // Local function to populate the lookup table.
   auto populateTable = [this](clang::Sema &sema, SwiftLookupTable &table) {
     for (auto decl
            : sema.Context.getTranslationUnitDecl()->noload_decls()) {
@@ -4138,19 +4141,19 @@ ClangImporter::Implementation::createExtensionWriter(clang::ASTWriter &writer) {
       if (!named) continue;
 
       // Add this entry to the lookup table.
-      addEntryToLookupTable(sema, table, named);
+      Impl.addEntryToLookupTable(sema, table, named);
     }
 
     // Add macros to the lookup table.
-    addMacrosToLookupTable(sema.Context, sema.getPreprocessor(), table);
+    Impl.addMacrosToLookupTable(sema.Context, sema.getPreprocessor(), table);
   };
 
   return std::unique_ptr<clang::ModuleFileExtensionWriter>(
            new SwiftLookupTableWriter(this, populateTable, writer));
 }
 
 std::unique_ptr<clang::ModuleFileExtensionReader>
-ClangImporter::Implementation::createExtensionReader(
+ClangImporter::Implementation::SwiftNameLookupExtension::createExtensionReader(
   const clang::ModuleFileExtensionMetadata &metadata,
   clang::ASTReader &reader,
   clang::serialization::ModuleFile &mod,
@@ -4163,13 +4166,13 @@ ClangImporter::Implementation::createExtensionReader(
   assert(metadata.MinorVersion == SWIFT_LOOKUP_TABLE_VERSION_MINOR);
 
   // Check whether we already have an entry in the set of lookup tables.
-  auto &entry = LookupTables[mod.ModuleName];
+  auto &entry = Impl.LookupTables[mod.ModuleName];
   if (entry) return nullptr;
 
   // Local function used to remove this entry when the reader goes away.
   std::string moduleName = mod.ModuleName;
   auto onRemove = [this, moduleName]() {
-    LookupTables.erase(moduleName);
+    Impl.LookupTables.erase(moduleName);
   };
 
   // Create the reader.

lib/ClangImporter/ImporterImpl.h

@@ -220,10 +220,31 @@ using api_notes::FactoryAsInitKind;
 
 /// \brief Implementation of the Clang importer.
 class LLVM_LIBRARY_VISIBILITY ClangImporter::Implementation 
-  : public LazyMemberLoader, public clang::ModuleFileExtension
+  : public LazyMemberLoader
 {
   friend class ClangImporter;
 
+  class SwiftNameLookupExtension : public clang::ModuleFileExtension {
+    Implementation &Impl;
+
+  public:
+    SwiftNameLookupExtension(Implementation &impl) : Impl(impl) { }
+
+    clang::ModuleFileExtensionMetadata getExtensionMetadata() const override;
+    llvm::hash_code hashExtension(llvm::hash_code code) const override;
+
+    std::unique_ptr<clang::ModuleFileExtensionWriter>
+    createExtensionWriter(clang::ASTWriter &writer) override;
+
+    std::unique_ptr<clang::ModuleFileExtensionReader>
+    createExtensionReader(const clang::ModuleFileExtensionMetadata &metadata,
+                          clang::ASTReader &reader,
+                          clang::serialization::ModuleFile &mod,
+                          const llvm::BitstreamCursor &stream) override;
+
+  };
+  friend class SwiftNameLookupExtension;
+
 public:
   /// \brief Describes how a particular C enumeration type will be imported
   /// into Swift. All of the possibilities have the same storage
@@ -261,6 +282,16 @@ class LLVM_LIBRARY_VISIBILITY ClangImporter::Implementation
     "<bridging-header-import>";
 
 private:
+  /// The Swift lookup table for the bridging header.
+  SwiftLookupTable BridgingHeaderLookupTable;
+
+  /// The Swift lookup tables, per module.
+  ///
+  /// Annoyingly, we list this table early so that it gets torn down after
+  /// the underlying Clang instances that reference it
+  /// (through the Swift name lookup module file extension).
+  llvm::StringMap<std::unique_ptr<SwiftLookupTable>> LookupTables;
+
   /// \brief A count of the number of load module operations.
   /// FIXME: Horrible, horrible hack for \c loadModule().
   unsigned ImportCounter = 0;
@@ -292,12 +323,6 @@ class LLVM_LIBRARY_VISIBILITY ClangImporter::Implementation
   /// if type checking has begun.
   llvm::PointerIntPair<LazyResolver *, 1, bool> typeResolver;
 
-  /// The Swift lookup table for the bridging header.
-  SwiftLookupTable BridgingHeaderLookupTable;
-
-  /// The Swift lookup tables, per module.
-  llvm::StringMap<std::unique_ptr<SwiftLookupTable>> LookupTables;
-
 public:
   /// \brief Mapping of already-imported declarations.
   llvm::DenseMap<const clang::Decl *, Decl *> ImportedDecls;
@@ -1283,20 +1308,6 @@ class LLVM_LIBRARY_VISIBILITY ClangImporter::Implementation
     return D;
   }
 
-  // Module file extension overrides
-
-  clang::ModuleFileExtensionMetadata getExtensionMetadata() const override;
-  llvm::hash_code hashExtension(llvm::hash_code code) const override;
-
-  std::unique_ptr<clang::ModuleFileExtensionWriter>
-  createExtensionWriter(clang::ASTWriter &writer) override;
-
-  std::unique_ptr<clang::ModuleFileExtensionReader>
-  createExtensionReader(const clang::ModuleFileExtensionMetadata &metadata,
-                        clang::ASTReader &reader,
-                        clang::serialization::ModuleFile &mod,
-                        const llvm::BitstreamCursor &stream) override;
-
   /// Find the lookup table that corresponds to the given Clang module.
   ///
   /// \param clangModule The module, or null to indicate that we're talking