diff --git a/clang/lib/Format/FormatTokenSource.h b/clang/lib/Format/FormatTokenSource.h
--- a/clang/lib/Format/FormatTokenSource.h
+++ b/clang/lib/Format/FormatTokenSource.h
@@ -1,4 +1,3 @@
-
 //===--- FormatTokenSource.h - Format C++ code ------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
@@ -8,7 +7,7 @@
 //===----------------------------------------------------------------------===//
 ///
 /// \file
-/// This file defines the \c TokenSource interface, which provides a token
+/// This file defines the \c FormatTokenSource interface, which provides a token
 /// stream as well as the ability to manipulate the token stream.
 ///
 //===----------------------------------------------------------------------===//
@@ -24,6 +23,10 @@
 namespace clang {
 namespace format {
 
+// Navigate a token stream.
+//
+// Enables traversal of a token stream, resetting the position in a token
+// stream, as well as inserting new tokens.
 class FormatTokenSource {
 public:
   virtual ~FormatTokenSource() {}
@@ -50,11 +53,25 @@
   // Resets the token stream to the state it was in when getPosition() returned
   // Position, and return the token at that position in the stream.
   virtual FormatToken *setPosition(unsigned Position) = 0;
+
+  // Insert the given tokens before the current position.
+  // Returns the first token in \c Tokens.
+  // The next returned token will be the second token in \c Tokens.
+  // Requires the last token in Tokens to be EOF; once the EOF token is reached,
+  // the next token will be the last token returned by getNextToken();
+  //
+  // For example, given the token sequence 'a1 a2':
+  // getNextToken() -> a1
+  // insertTokens('b1 b2') -> b1
+  // getNextToken() -> b2
+  // getNextToken() -> a1
+  // getNextToken() -> a2
+  virtual FormatToken *insertTokens(ArrayRef<FormatToken *> Tokens) = 0;
 };
 
 class LLVM_GSL_POINTER IndexedTokenSource : public FormatTokenSource {
 public:
-  IndexedTokenSource(ArrayRef<FormatToken *> Tokens)
+  IndexedTokenSource(SmallVectorImpl<FormatToken *> &Tokens)
       : Tokens(Tokens), Position(-1) {}
 
   FormatToken *getNextToken() override {
@@ -65,7 +82,7 @@
       });
       return Tokens[Position];
     }
-    ++Position;
+    Position = next(Position);
     LLVM_DEBUG({
       llvm::dbgs() << "Next ";
       dbgToken(Position);
@@ -80,10 +97,10 @@
   FormatToken *peekNextToken(bool SkipComment = false) override {
     if (isEOF())
       return Tokens[Position];
-    int Next = Position + 1;
+    int Next = next(Position);
     if (SkipComment)
       while (Tokens[Next]->is(tok::comment))
-        ++Next;
+        Next = next(Next);
     LLVM_DEBUG({
       llvm::dbgs() << "Peeking ";
       dbgToken(Next);
@@ -107,9 +124,40 @@
     return Tokens[Position];
   }
 
+  FormatToken *insertTokens(ArrayRef<FormatToken *> New) override {
+    assert(Position != -1);
+    assert((*New.rbegin())->Tok.is(tok::eof));
+    LLVM_DEBUG(llvm::dbgs() << "Inserting:\n");
+    int Next = Tokens.size();
+    Tokens.append(New.begin(), New.end());
+    LLVM_DEBUG({
+      for (int I = Next, E = Tokens.size(); I != E; ++I)
+        dbgToken(I, "  ");
+      llvm::dbgs() << "  Jump from: " << (Tokens.size() - 1) << " -> "
+                   << Position << "\n";
+    });
+    Jumps[Tokens.size() - 1] = Position;
+    Position = Next;
+    LLVM_DEBUG({
+      llvm::dbgs() << "At inserted token ";
+      dbgToken(Position);
+    });
+    return Tokens[Position];
+  }
+
   void reset() { Position = -1; }
 
 private:
+  int next(int Current) {
+    int Next = Current + 1;
+    auto it = Jumps.find(Next);
+    if (it != Jumps.end()) {
+      Next = it->second;
+      assert(Jumps.find(Next) == Jumps.end());
+    }
+    return Next;
+  }
+
   void dbgToken(int Position, llvm::StringRef Indent = "") {
     FormatToken *Tok = Tokens[Position];
     llvm::dbgs() << Indent << "[" << Position
@@ -117,8 +165,12 @@
                  << ", Macro: " << !!Tok->MacroCtx << "\n";
   }
 
-  ArrayRef<FormatToken *> Tokens;
+  SmallVectorImpl<FormatToken *> &Tokens;
   int Position;
+
+  // Maps from position a to position b, so that when we reach a, the token
+  // stream continues at position b instead.
+  std::map<int, int> Jumps;
 };
 
 class ScopedMacroState : public FormatTokenSource {
@@ -175,6 +227,10 @@
     return Token;
   }
 
+  FormatToken *insertTokens(ArrayRef<FormatToken *> Tokens) override {
+    assert(false && "Cannot insert tokens while parsing a macro.");
+  }
+
 private:
   bool eof() {
     return Token && Token->HasUnescapedNewline &&
diff --git a/clang/lib/Format/UnwrappedLineParser.h b/clang/lib/Format/UnwrappedLineParser.h
--- a/clang/lib/Format/UnwrappedLineParser.h
+++ b/clang/lib/Format/UnwrappedLineParser.h
@@ -89,7 +89,8 @@
 public:
   UnwrappedLineParser(const FormatStyle &Style,
                       const AdditionalKeywords &Keywords,
-                      unsigned FirstStartColumn, ArrayRef<FormatToken *> Tokens,
+                      unsigned FirstStartColumn,
+                      SmallVectorImpl<FormatToken *> &Tokens,
                       UnwrappedLineConsumer &Callback);
 
   void parse();
@@ -283,7 +284,9 @@
   // FIXME: This is a temporary measure until we have reworked the ownership
   // of the format tokens. The goal is to have the actual tokens created and
   // owned outside of and handed into the UnwrappedLineParser.
-  ArrayRef<FormatToken *> AllTokens;
+  // FIXME: The above fixme doesn't work if we need to create tokens while
+  // parsing.
+  SmallVectorImpl<FormatToken *> &AllTokens;
 
   // Keeps a stack of the states of nested control statements (true if the
   // statement contains more than some predefined number of nested statements).
diff --git a/clang/lib/Format/UnwrappedLineParser.cpp b/clang/lib/Format/UnwrappedLineParser.cpp
--- a/clang/lib/Format/UnwrappedLineParser.cpp
+++ b/clang/lib/Format/UnwrappedLineParser.cpp
@@ -146,7 +146,7 @@
 UnwrappedLineParser::UnwrappedLineParser(const FormatStyle &Style,
                                          const AdditionalKeywords &Keywords,
                                          unsigned FirstStartColumn,
-                                         ArrayRef<FormatToken *> Tokens,
+                                         SmallVectorImpl<FormatToken *> &Tokens,
                                          UnwrappedLineConsumer &Callback)
     : Line(new UnwrappedLine), MustBreakBeforeNextToken(false),
       CurrentLines(&Lines), Style(Style), Keywords(Keywords),
diff --git a/clang/unittests/Format/FormatTokenSourceTest.cpp b/clang/unittests/Format/FormatTokenSourceTest.cpp
--- a/clang/unittests/Format/FormatTokenSourceTest.cpp
+++ b/clang/unittests/Format/FormatTokenSourceTest.cpp
@@ -30,6 +30,12 @@
     FormatToken *Tok = FormatTok;                                              \
     EXPECT_EQ((Tok)->Tok.getKind(), Kind) << *(Tok);                           \
   } while (false);
+#define EXPECT_TOKEN_ID(FormatTok, Name)                                       \
+  do {                                                                         \
+    FormatToken *Tok = FormatTok;                                              \
+    EXPECT_EQ((Tok)->Tok.getKind(), tok::identifier) << *(Tok);                \
+    EXPECT_EQ((Tok)->TokenText, Name) << *(Tok);                               \
+  } while (false);
 
 TEST_F(IndexedTokenSourceTest, EmptyInput) {
   TokenList Tokens = lex("");
@@ -60,6 +66,8 @@
   EXPECT_TOKEN_KIND(Source.peekNextToken(), tok::eof);
   EXPECT_TOKEN_KIND(Source.getNextToken(), tok::eof);
   EXPECT_TOKEN_KIND(Source.getPreviousToken(), tok::semi);
+  EXPECT_TOKEN_KIND(Source.getNextToken(), tok::eof);
+  EXPECT_TOKEN_KIND(Source.getPreviousToken(), tok::semi);
 }
 
 TEST_F(IndexedTokenSourceTest, ResetPosition) {
@@ -73,6 +81,62 @@
   EXPECT_TOKEN_KIND(Source.setPosition(Position), tok::kw_int);
 }
 
+TEST_F(IndexedTokenSourceTest, InsertTokens) {
+  TokenList TokensA = lex("A1 A2");
+  TokenList TokensB = lex("B1 B2");
+  IndexedTokenSource Source(TokensA);
+  EXPECT_TOKEN_ID(Source.getNextToken(), "A1");
+  EXPECT_TOKEN_ID(Source.insertTokens(TokensB), "B1");
+  EXPECT_TOKEN_ID(Source.getNextToken(), "B2");
+  EXPECT_TOKEN_ID(Source.getNextToken(), "A1");
+  EXPECT_TOKEN_ID(Source.getNextToken(), "A2");
+}
+
+TEST_F(IndexedTokenSourceTest, InsertTokensAtEOF) {
+  TokenList TokensA = lex("A1");
+  TokenList TokensB = lex("B1 B2");
+  IndexedTokenSource Source(TokensA);
+  EXPECT_TOKEN_ID(Source.getNextToken(), "A1");
+  EXPECT_TOKEN_KIND(Source.getNextToken(), tok::eof);
+  EXPECT_TOKEN_ID(Source.insertTokens(TokensB), "B1");
+  EXPECT_TOKEN_ID(Source.getNextToken(), "B2");
+  EXPECT_TOKEN_KIND(Source.getNextToken(), tok::eof);
+}
+
+TEST_F(IndexedTokenSourceTest, InsertTokensRecursive) {
+  TokenList TokensA = lex("A1");
+  TokenList TokensB = lex("B1");
+  TokenList TokensC = lex("C1");
+  TokenList TokensD = lex("D1");
+  IndexedTokenSource Source(TokensA);
+  EXPECT_TOKEN_ID(Source.getNextToken(), "A1");
+  // A1
+  EXPECT_TOKEN_ID(Source.insertTokens(TokensB), "B1");
+  // B1 A1
+  EXPECT_TOKEN_ID(Source.insertTokens(TokensC), "C1");
+  // C1 B1 A1
+  EXPECT_TOKEN_ID(Source.insertTokens(TokensD), "D1");
+  // D1 C1 B1 A1
+  EXPECT_TOKEN_ID(Source.getNextToken(), "C1");
+  EXPECT_TOKEN_ID(Source.getNextToken(), "B1");
+  EXPECT_TOKEN_ID(Source.getNextToken(), "A1");
+}
+
+TEST_F(IndexedTokenSourceTest, InsertTokensRecursiveAtEndOfSequence) {
+  TokenList TokensA = lex("A1");
+  TokenList TokensB = lex("B1");
+  TokenList TokensC = lex("C1");
+  TokenList TokensD = lex("D1");
+  IndexedTokenSource Source(TokensA);
+  EXPECT_TOKEN_ID(Source.getNextToken(), "A1");
+  EXPECT_TOKEN_ID(Source.insertTokens(TokensB), "B1");
+  EXPECT_TOKEN_ID(Source.getNextToken(), "A1");
+  EXPECT_TOKEN_ID(Source.insertTokens(TokensC), "C1");
+  EXPECT_TOKEN_ID(Source.getNextToken(), "A1");
+  EXPECT_TOKEN_ID(Source.insertTokens(TokensD), "D1");
+  EXPECT_TOKEN_ID(Source.getNextToken(), "A1");
+}
+
 } // namespace
 } // namespace format
 } // namespace clang
diff --git a/clang/unittests/Format/TestLexer.h b/clang/unittests/Format/TestLexer.h
--- a/clang/unittests/Format/TestLexer.h
+++ b/clang/unittests/Format/TestLexer.h
@@ -71,7 +71,8 @@
 
   TokenList annotate(llvm::StringRef Code) {
     FormatTokenLexer Lex = getNewLexer(Code);
-    auto Tokens = Lex.lex();
+    auto Toks = Lex.lex();
+    SmallVector<FormatToken *, 10> Tokens(Toks.begin(), Toks.end());
     UnwrappedLineParser Parser(Style, Lex.getKeywords(), 0, Tokens, *this);
     Parser.parse();
     TokenAnnotator Annotator(Style, Lex.getKeywords());