Index: include/clang/Lex/PPCallbacks.h
===================================================================
--- include/clang/Lex/PPCallbacks.h
+++ include/clang/Lex/PPCallbacks.h
@@ -132,6 +132,28 @@
                                   SrcMgr::CharacteristicKind FileType) {
   }
 
+  /// Callback invoked whenever a submodule was entered.
+  ///
+  /// \param M The submodule we have entered.
+  ///
+  /// \param ImportLoc The location of import directive token.
+  ///
+  /// \param ForPragma If entering from pragma directive.
+  ///
+  virtual void EnteredSubmodule(Module *M, SourceLocation ImportLoc,
+                                bool ForPragma) { }
+
+  /// Callback invoked whenever a submodule was left.
+  ///
+  /// \param M The submodule we have left.
+  ///
+  /// \param ImportLoc The location of import directive token.
+  ///
+  /// \param ForPragma If entering from pragma directive.
+  ///
+  virtual void LeftSubmodule(Module *M, SourceLocation ImportLoc,
+                             bool ForPragma) { }
+
   /// Callback invoked whenever there was an explicit module-import
   /// syntax.
   ///
@@ -395,6 +417,18 @@
                                Imported, FileType);
   }
 
+  void EnteredSubmodule(Module *M, SourceLocation ImportLoc,
+                        bool ForPragma) override {
+    First->EnteredSubmodule(M, ImportLoc, ForPragma);
+    Second->EnteredSubmodule(M, ImportLoc, ForPragma);
+  }
+
+  void LeftSubmodule(Module *M, SourceLocation ImportLoc,
+                     bool ForPragma) override {
+    First->LeftSubmodule(M, ImportLoc, ForPragma);
+    Second->LeftSubmodule(M, ImportLoc, ForPragma);
+  }
+
   void moduleImport(SourceLocation ImportLoc, ModuleIdPath Path,
                     const Module *Imported) override {
     First->moduleImport(ImportLoc, Path, Imported);
Index: lib/Lex/PPLexerChange.cpp
===================================================================
--- lib/Lex/PPLexerChange.cpp
+++ lib/Lex/PPLexerChange.cpp
@@ -647,6 +647,8 @@
     BuildingSubmoduleStack.push_back(
         BuildingSubmoduleInfo(M, ImportLoc, ForPragma, CurSubmoduleState,
                               PendingModuleMacroNames.size()));
+    if (Callbacks)
+      Callbacks->EnteredSubmodule(M, ImportLoc, ForPragma);
     return;
   }
 
@@ -691,6 +693,9 @@
       BuildingSubmoduleInfo(M, ImportLoc, ForPragma, CurSubmoduleState,
                             PendingModuleMacroNames.size()));
 
+  if (Callbacks)
+    Callbacks->EnteredSubmodule(M, ImportLoc, ForPragma);
+
   // Switch to this submodule as the current submodule.
   CurSubmoduleState = &State;
 
@@ -731,6 +736,10 @@
     // are tracking macro visibility, don't build any, and preserve the list
     // of pending names for the surrounding submodule.
     BuildingSubmoduleStack.pop_back();
+
+    if (Callbacks)
+      Callbacks->LeftSubmodule(LeavingMod, ImportLoc, ForPragma);
+
     makeModuleVisible(LeavingMod, ImportLoc);
     return LeavingMod;
   }
@@ -815,6 +824,9 @@
 
   BuildingSubmoduleStack.pop_back();
 
+  if (Callbacks)
+    Callbacks->LeftSubmodule(LeavingMod, ImportLoc, ForPragma);
+
   // A nested #include makes the included submodule visible.
   makeModuleVisible(LeavingMod, ImportLoc);
   return LeavingMod;