[Serialization] Stop pre-loading generic environments.

Make generic environment deserialization lazy, which eliminates a
significant amount of up-front work. Most clients only need the
generic signature, not the full generic environment.

Author: DougGregor

include/swift/Serialization/ModuleFile.h

@@ -84,17 +84,6 @@ class ModuleFile : public LazyMemberLoader {
   /// The number of entities that are currently being deserialized.
   unsigned NumCurrentDeserializingEntities = 0;
 
-  /// Declaration contexts with delayed generic environments, which will be
-  /// completed as a pending action.
-  ///
-  /// This should only be used on the module returned by
-  /// getModuleFileForDelayedActions().
-  ///
-  /// FIXME: This is needed because completing a generic environment can
-  /// require the type checker, which might be gone if we delay generic
-  /// environments too far. It is a hack.
-  std::vector<DeclContext *> DelayedGenericEnvironments;
-
   /// RAII class to be used when deserializing an entity.
   class DeserializingEntityRAII {
     ModuleFile &MF;

lib/Serialization/Deserialization.cpp

@@ -207,12 +207,6 @@ ModuleFile &ModuleFile::getModuleFileForDelayedActions() {
 void ModuleFile::finishPendingActions() {
   assert(&getModuleFileForDelayedActions() == this &&
          "wrong module used for delayed actions");
-  while (!DelayedGenericEnvironments.empty()) {
-    // Force completion of the last generic environment.
-    auto genericEnvDC = DelayedGenericEnvironments.back();
-    DelayedGenericEnvironments.pop_back();
-    (void)genericEnvDC->getGenericEnvironmentOfContext();
-  }
 }
 
 /// Translate from the serialization DefaultArgumentKind enumerators, which are
@@ -922,8 +916,6 @@ void ModuleFile::configureGenericEnvironment(
   // creation.
   if (auto genericSig = sigOrEnv.dyn_cast<GenericSignature *>()) {
     genericDecl->setLazyGenericEnvironment(this, genericSig, envID);
-    ModuleFile &delayedActionFile = getModuleFileForDelayedActions();
-    delayedActionFile.DelayedGenericEnvironments.push_back(genericDecl);
     return;
   }
 

lib/Serialization/ModuleFile.cpp

@@ -1297,11 +1297,7 @@ Status ModuleFile::associateWithFileContext(FileUnit *file,
   return getStatus();
 }
 
-ModuleFile::~ModuleFile() {
-  assert(DelayedGenericEnvironments.empty() &&
-         "either finishPendingActions() was never called, or someone forgot "
-         "to use getModuleFileForDelayedActions()");
-}
+ModuleFile::~ModuleFile() { }
 
 void ModuleFile::lookupValue(DeclName name,
                              SmallVectorImpl<ValueDecl*> &results) {