diff --git a/clang/include/clang/Lex/Preprocessor.h b/clang/include/clang/Lex/Preprocessor.h
--- a/clang/include/clang/Lex/Preprocessor.h
+++ b/clang/include/clang/Lex/Preprocessor.h
@@ -33,6 +33,7 @@
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/FoldingSet.h"
+#include "llvm/ADT/FunctionExtras.h"
 #include "llvm/ADT/None.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/PointerUnion.h"
@@ -48,8 +49,8 @@
 #include <cassert>
 #include <cstddef>
 #include <cstdint>
-#include <memory>
 #include <map>
+#include <memory>
 #include <string>
 #include <utility>
 #include <vector>
@@ -124,6 +125,7 @@
   friend class VAOptDefinitionContext;
   friend class VariadicMacroScopeGuard;
 
+  llvm::unique_function<void(const clang::Token &)> OnToken;
   std::shared_ptr<PreprocessorOptions> PPOpts;
   DiagnosticsEngine        *Diags;
   LangOptions       &LangOpts;
@@ -997,6 +999,12 @@
   }
   /// \}
 
+  /// Register a function that would be called on each token in the final
+  /// expanded token stream.
+  void setTokenWatcher(llvm::unique_function<void(const clang::Token &)> F) {
+    OnToken = std::move(F);
+  }
+
   bool isMacroDefined(StringRef Id) {
     return isMacroDefined(&Identifiers.get(Id));
   }
diff --git a/clang/include/clang/Lex/TokenLexer.h b/clang/include/clang/Lex/TokenLexer.h
--- a/clang/include/clang/Lex/TokenLexer.h
+++ b/clang/include/clang/Lex/TokenLexer.h
@@ -147,6 +147,10 @@
   /// preprocessor directive.
   bool isParsingPreprocessorDirective() const;
 
+  /// Returns true iff the TokenLexer is expanding a macro and not replaying a
+  /// stream of tokens.
+  bool isMacroExpansion() const { return Macro != nullptr; }
+
 private:
   void destroy();
 
diff --git a/clang/lib/Lex/Preprocessor.cpp b/clang/lib/Lex/Preprocessor.cpp
--- a/clang/lib/Lex/Preprocessor.cpp
+++ b/clang/lib/Lex/Preprocessor.cpp
@@ -867,20 +867,27 @@
   // We loop here until a lex function returns a token; this avoids recursion.
   bool ReturnedToken;
   bool IsNewToken = true;
+  // Whether to call the OnToken callback on this token.
+  bool Report;
   do {
     switch (CurLexerKind) {
     case CLK_Lexer:
+      Report = true;
       ReturnedToken = CurLexer->Lex(Result);
       break;
     case CLK_TokenLexer:
+      Report = CurTokenLexer->isMacroExpansion();
       ReturnedToken = CurTokenLexer->Lex(Result);
       break;
     case CLK_CachingLexer:
       CachingLex(Result, IsNewToken);
+      Report = IsNewToken;
       ReturnedToken = true;
       break;
     case CLK_LexAfterModuleImport:
-      ReturnedToken = LexAfterModuleImport(Result);
+      LexAfterModuleImport(Result);
+      Report = true;
+      ReturnedToken = true;
       break;
     }
   } while (!ReturnedToken);
@@ -937,6 +944,8 @@
 
   LastTokenWasAt = Result.is(tok::at);
   --LexLevel;
+  if (OnToken && LexLevel == 0 && Report)
+    OnToken(Result);
 }
 
 /// Lex a header-name token (including one formed from header-name-tokens if