Lift nested type lookup fast-pathing up to FileUnit.

We use this to avoid circularity issues in serialization; we'd like to
extend that to the Clang importer. This is only necessary because we
can't look up a single member at a time, but it can still fix issues
in the short term.

This commit should have no effect on functionality.

include/swift/AST/Module.h

@@ -555,6 +555,18 @@ public:
     return nullptr;
   }
 
+  /// Directly look for a nested type declared within this module inside the
+  /// given nominal type (including any extensions).
+  ///
+  /// This is a fast-path hack to avoid circular dependencies in deserialization
+  /// and the Clang importer.
+  ///
+  /// Private and fileprivate types should not be returned by this lookup.
+  virtual TypeDecl *lookupNestedType(Identifier name,
+                                     const NominalTypeDecl *parent) const {
+    return nullptr;
+  }
+
   /// Find ValueDecls in the module and pass them to the given consumer object.
   ///
   /// This does a simple local lookup, not recursively looking through imports.

include/swift/Serialization/ModuleFile.h

@@ -603,7 +603,7 @@ public:
 
   /// Searches the module's nested type decls table for the given member of
   /// the given type.
-  TypeDecl *lookupNestedType(Identifier name, const ValueDecl *parent);
+  TypeDecl *lookupNestedType(Identifier name, const NominalTypeDecl *parent);
 
   /// Searches the module's operators for one with the given name and fixity.
   ///

include/swift/Serialization/SerializedModuleLoader.h

@@ -132,6 +132,10 @@ public:
 
   virtual TypeDecl *lookupLocalType(StringRef MangledName) const override;
 
+  virtual TypeDecl *
+  lookupNestedType(Identifier name,
+                   const NominalTypeDecl *parent) const override;
+
   virtual OperatorDecl *lookupOperator(Identifier name,
                                        DeclKind fixity) const override;
 

lib/Serialization/Deserialization.cpp

@@ -1319,9 +1319,7 @@ ModuleFile::resolveCrossReference(ModuleDecl *baseModule, uint32_t pathLen) {
                                                   &blobData);
     switch (recordID) {
     case XREF_TYPE_PATH_PIECE: {
-      if (values.size() == 1) {
+      if (values.size() == 1 && isa<NominalTypeDecl>(values.front())) {
-        ModuleDecl *module = values.front()->getModuleContext();
-
         // Fast path for nested types that avoids deserializing all
         // members of the parent type.
         IdentifierID IID;
@@ -1334,29 +1332,23 @@ ModuleFile::resolveCrossReference(ModuleDecl *baseModule, uint32_t pathLen) {
           "If you're seeing a crash here, try passing "
             "-Xfrontend -disable-serialization-nested-type-lookup-table"};
 
+        auto *baseType = cast<NominalTypeDecl>(values.front());
         TypeDecl *nestedType = nullptr;
         if (onlyInNominal) {
           // Only look in the file containing the type itself.
           const DeclContext *dc = values.front()->getDeclContext();
-          auto *serializedFile =
+          auto *containingFile =
-            dyn_cast<SerializedASTFile>(dc->getModuleScopeContext());
+            dyn_cast<FileUnit>(dc->getModuleScopeContext());
-          if (serializedFile) {
+          if (containingFile) {
-            nestedType =
+            nestedType = containingFile->lookupNestedType(memberName, baseType);
-              serializedFile->File.lookupNestedType(memberName,
-                                                    values.front());
           }
         } else {
           // Fault in extensions, then ask every serialized AST in the module.
-          (void)cast<NominalTypeDecl>(values.front())->getExtensions();
+          (void)baseType->getExtensions();
-          for (FileUnit *file : module->getFiles()) {
+          for (FileUnit *file : baseType->getModuleContext()->getFiles()) {
             if (file == getFile())
               continue;
-            auto *serializedFile = dyn_cast<SerializedASTFile>(file);
+            nestedType = file->lookupNestedType(memberName, baseType);
-            if (!serializedFile)
-              continue;
-            nestedType =
-              serializedFile->File.lookupNestedType(memberName,
-                                                    values.front());
             if (nestedType)
               break;
           }

lib/Serialization/ModuleFile.cpp

@@ -1363,7 +1363,7 @@ TypeDecl *ModuleFile::lookupLocalType(StringRef MangledName) {
 }
 
 TypeDecl *ModuleFile::lookupNestedType(Identifier name,
-                                       const ValueDecl *parent) {
+                                       const NominalTypeDecl *parent) {
   PrettyStackTraceModuleFile stackEntry(*this);
 
   if (!NestedTypeDecls)

lib/Serialization/SerializedModuleLoader.cpp

@@ -525,6 +525,12 @@ TypeDecl *SerializedASTFile::lookupLocalType(llvm::StringRef MangledName) const{
   return File.lookupLocalType(MangledName);
 }
 
+TypeDecl *
+SerializedASTFile::lookupNestedType(Identifier name,
+                                    const NominalTypeDecl *parent) const {
+  return File.lookupNestedType(name, parent);
+}
+
 OperatorDecl *SerializedASTFile::lookupOperator(Identifier name,
                                                 DeclKind fixity) const {
   return File.lookupOperator(name, fixity);