[ModuleInterface] <rdar://46081260> Fallback behaviour and testing env vars.

include/swift/Serialization/SerializedModuleLoader.h

@@ -20,6 +20,13 @@
 namespace swift {
 class ModuleFile;
 
+enum class ModuleLoadingMode {
+  PreferParseable,
+  PreferSerialized,
+  OnlyParseable,
+  OnlySerialized
+};
+
 /// Common functionality shared between \c SerializedModuleLoader and
 /// \c ParseableInterfaceModuleLoader.
 class SerializedModuleLoaderBase : public ModuleLoader {
@@ -32,7 +39,9 @@ class SerializedModuleLoaderBase : public ModuleLoader {
 protected:
   llvm::StringMap<std::unique_ptr<llvm::MemoryBuffer>> MemoryBuffers;
   ASTContext &Ctx;
-  SerializedModuleLoaderBase(ASTContext &ctx, DependencyTracker *tracker);
+  ModuleLoadingMode LoadMode;
+  SerializedModuleLoaderBase(ASTContext &ctx, DependencyTracker *tracker,
+                             ModuleLoadingMode LoadMode);
 
   using AccessPathElem = std::pair<Identifier, SourceLoc>;
   bool findModule(AccessPathElem moduleID,
@@ -100,8 +109,9 @@ public:
 /// Imports serialized Swift modules into an ASTContext.
 class SerializedModuleLoader : public SerializedModuleLoaderBase {
 
-  SerializedModuleLoader(ASTContext &ctx, DependencyTracker *tracker)
-    : SerializedModuleLoaderBase(ctx, tracker)
+  SerializedModuleLoader(ASTContext &ctx, DependencyTracker *tracker,
+                         ModuleLoadingMode loadMode)
+    : SerializedModuleLoaderBase(ctx, tracker, loadMode)
   {}
 
 public:
@@ -120,9 +130,10 @@ public:
   /// Create a new importer that can load serialized Swift modules
   /// into the given ASTContext.
   static std::unique_ptr<SerializedModuleLoader>
-  create(ASTContext &ctx, DependencyTracker *tracker = nullptr) {
+  create(ASTContext &ctx, DependencyTracker *tracker = nullptr,
+         ModuleLoadingMode loadMode = ModuleLoadingMode::PreferSerialized) {
     return std::unique_ptr<SerializedModuleLoader>{
-      new SerializedModuleLoader(ctx, tracker)
+      new SerializedModuleLoader(ctx, tracker, loadMode)
     };
   }
 };