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;
@@ -911,6 +913,14 @@
   }
   /// \}
 
+  /// Register a function that would be called on each token seen by the
+  /// preprocessor. This is a very low-level hook, the produced token stream is
+  /// tied to the internals of the preprocessor so interpreting result of the
+  /// callback is hard.
+  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/lib/Lex/Preprocessor.cpp b/clang/lib/Lex/Preprocessor.cpp
--- a/clang/lib/Lex/Preprocessor.cpp
+++ b/clang/lib/Lex/Preprocessor.cpp
@@ -864,6 +864,7 @@
 void Preprocessor::Lex(Token &Result) {
   // We loop here until a lex function returns a token; this avoids recursion.
   bool ReturnedToken;
+  bool Cached = false;
   do {
     switch (CurLexerKind) {
     case CLK_Lexer:
@@ -875,6 +876,7 @@
     case CLK_CachingLexer:
       CachingLex(Result);
       ReturnedToken = true;
+      Cached = true;
       break;
     case CLK_LexAfterModuleImport:
       LexAfterModuleImport(Result);
@@ -893,6 +895,8 @@
   }
 
   LastTokenWasAt = Result.is(tok::at);
+  if (OnToken && ReturnedToken && !Cached)
+    OnToken(Result);
 }
 
 /// Lex a header-name token (including one formed from header-name-tokens if