diff --git a/clang/lib/Tooling/Syntax/Tokens.cpp b/clang/lib/Tooling/Syntax/Tokens.cpp
new file mode 100644
index 0000000..f291f18
--- /dev/null
+++ b/clang/lib/Tooling/Syntax/Tokens.cpp
@@ -0,0 +1,509 @@
+//===- Tokens.cpp - collect tokens from preprocessing ---------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+#include "clang/Tooling/Syntax/Tokens.h"
+
+#include "clang/Basic/Diagnostic.h"
+#include "clang/Basic/IdentifierTable.h"
+#include "clang/Basic/LLVM.h"
+#include "clang/Basic/LangOptions.h"
+#include "clang/Basic/SourceLocation.h"
+#include "clang/Basic/SourceManager.h"
+#include "clang/Basic/TokenKinds.h"
+#include "clang/Lex/Preprocessor.h"
+#include "clang/Lex/Token.h"
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/None.h"
+#include "llvm/ADT/Optional.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/FormatVariadic.h"
+#include "llvm/Support/raw_ostream.h"
+#include <algorithm>
+#include <cassert>
+#include <iterator>
+#include <string>
+#include <utility>
+#include <vector>
+
+using namespace clang;
+using namespace clang::syntax;
+
+syntax::Token::Token(const clang::Token &T)
+    : Token(T.getLocation(), T.getLength(), T.getKind()) {
+  assert(!T.isAnnotation());
+}
+
+llvm::StringRef syntax::Token::text(const SourceManager &SM) const {
+  bool Invalid = false;
+  const char *Start = SM.getCharacterData(location(), &Invalid);
+  assert(!Invalid);
+  return llvm::StringRef(Start, length());
+}
+
+FileRange syntax::Token::range(const SourceManager &SM) const {
+  assert(location().isFileID() && "must be a spelled token");
+  FileID File;
+  unsigned StartOffset;
+  std::tie(File, StartOffset) = SM.getDecomposedLoc(location());
+  return FileRange(File, StartOffset, StartOffset + length());
+}
+
+FileRange syntax::Token::range(const SourceManager &SM,
+                               const syntax::Token &First,
+                               const syntax::Token &Last) {
+  auto F = First.range(SM);
+  auto L = Last.range(SM);
+  assert(F.file() == L.file() && "tokens from different files");
+  assert(F.endOffset() <= L.beginOffset() && "wrong order of tokens");
+  return FileRange(F.file(), F.beginOffset(), L.endOffset());
+}
+
+llvm::raw_ostream &syntax::operator<<(llvm::raw_ostream &OS, const Token &T) {
+  return OS << T.str();
+}
+
+FileRange::FileRange(FileID File, unsigned BeginOffset, unsigned EndOffset)
+    : File(File), Begin(BeginOffset), End(EndOffset) {
+      assert(File.isValid());
+      assert(BeginOffset <= EndOffset);
+}
+
+FileRange::FileRange(const SourceManager &SM, SourceLocation BeginLoc,
+                     unsigned Length) {
+  assert(BeginLoc.isValid());
+  assert(BeginLoc.isFileID());
+
+  std::tie(File, Begin) = SM.getDecomposedLoc(BeginLoc);
+  End = Begin + Length;
+}
+FileRange::FileRange(const SourceManager &SM, SourceLocation BeginLoc,
+                     SourceLocation EndLoc) {
+  assert(BeginLoc.isValid());
+  assert(BeginLoc.isFileID());
+  assert(EndLoc.isValid());
+  assert(EndLoc.isFileID());
+  assert(SM.getFileID(BeginLoc) == SM.getFileID(EndLoc));
+  assert(SM.getFileOffset(BeginLoc) <= SM.getFileOffset(EndLoc));
+
+  std::tie(File, Begin) = SM.getDecomposedLoc(BeginLoc);
+  End = SM.getFileOffset(EndLoc);
+}
+
+llvm::raw_ostream &syntax::operator<<(llvm::raw_ostream &OS,
+                                      const FileRange &R) {
+  return OS << llvm::formatv("FileRange(file = {0}, offsets = {1}-{2})",
+                             R.file().getHashValue(), R.beginOffset(),
+                             R.endOffset());
+}
+
+llvm::StringRef FileRange::text(const SourceManager &SM) const {
+  bool Invalid = false;
+  StringRef Text = SM.getBufferData(File, &Invalid);
+  if (Invalid)
+    return "";
+  assert(Begin <= Text.size());
+  assert(End <= Text.size());
+  return Text.substr(Begin, length());
+}
+
+std::pair<const syntax::Token *, const TokenBuffer::Mapping *>
+TokenBuffer::spelledForExpandedToken(const syntax::Token *Expanded) const {
+  assert(Expanded);
+  assert(ExpandedTokens.data() <= Expanded &&
+         Expanded < ExpandedTokens.data() + ExpandedTokens.size());
+
+  auto FileIt = Files.find(
+      SourceMgr->getFileID(SourceMgr->getExpansionLoc(Expanded->location())));
+  assert(FileIt != Files.end() && "no file for an expanded token");
+
+  const MarkedFile &File = FileIt->second;
+
+  unsigned ExpandedIndex = Expanded - ExpandedTokens.data();
+  // Find the first mapping that produced tokens after \p Expanded.
+  auto It = llvm::bsearch(File.Mappings, [&](const Mapping &M) {
+    return ExpandedIndex < M.BeginExpanded;
+  });
+  // Our token could only be produced by the previous mapping.
+  if (It == File.Mappings.begin()) {
+    // No previous mapping, no need to modify offsets.
+    return {&File.SpelledTokens[ExpandedIndex - File.BeginExpanded], nullptr};
+  }
+  --It; // 'It' now points to last mapping that started before our token.
+
+  // Check if the token is part of the mapping.
+  if (ExpandedIndex < It->EndExpanded)
+    return {&File.SpelledTokens[It->BeginSpelled], /*Mapping*/ &*It};
+
+  // Not part of the mapping, use the index from previous mapping to compute the
+  // corresponding spelled token.
+  return {
+      &File.SpelledTokens[It->EndSpelled + (ExpandedIndex - It->EndExpanded)],
+      /*Mapping*/ nullptr};
+}
+
+llvm::ArrayRef<syntax::Token> TokenBuffer::spelledTokens(FileID FID) const {
+  auto It = Files.find(FID);
+  assert(It != Files.end());
+  return It->second.SpelledTokens;
+}
+
+std::string TokenBuffer::Mapping::str() const {
+  return llvm::formatv("spelled tokens: [{0},{1}), expanded tokens: [{2},{3})",
+                       BeginSpelled, EndSpelled, BeginExpanded, EndExpanded);
+}
+
+llvm::Optional<llvm::ArrayRef<syntax::Token>>
+TokenBuffer::spelledForExpanded(llvm::ArrayRef<syntax::Token> Expanded) const {
+  // Mapping an empty range is ambiguous in case of empty mappings at either end
+  // of the range, bail out in that case.
+  if (Expanded.empty())
+    return llvm::None;
+
+  // FIXME: also allow changes uniquely mapping to macro arguments.
+
+  const syntax::Token *BeginSpelled;
+  const Mapping *BeginMapping;
+  std::tie(BeginSpelled, BeginMapping) =
+      spelledForExpandedToken(&Expanded.front());
+
+  const syntax::Token *LastSpelled;
+  const Mapping *LastMapping;
+  std::tie(LastSpelled, LastMapping) =
+      spelledForExpandedToken(&Expanded.back());
+
+  FileID FID = SourceMgr->getFileID(BeginSpelled->location());
+  // FIXME: Handle multi-file changes by trying to map onto a common root.
+  if (FID != SourceMgr->getFileID(LastSpelled->location()))
+    return llvm::None;
+
+  const MarkedFile &File = Files.find(FID)->second;
+
+  // Do not allow changes that cross macro expansion boundaries.
+  unsigned BeginExpanded = Expanded.begin() - ExpandedTokens.data();
+  unsigned EndExpanded = Expanded.end() - ExpandedTokens.data();
+  if (BeginMapping && BeginMapping->BeginExpanded < BeginExpanded)
+    return llvm::None;
+  if (LastMapping && EndExpanded < LastMapping->EndExpanded)
+    return llvm::None;
+  // All is good, return the result.
+  return llvm::makeArrayRef(
+      BeginMapping ? File.SpelledTokens.data() + BeginMapping->BeginSpelled
+                   : BeginSpelled,
+      LastMapping ? File.SpelledTokens.data() + LastMapping->EndSpelled
+                  : LastSpelled + 1);
+}
+
+std::vector<syntax::Token> syntax::tokenize(FileID FID, const SourceManager &SM,
+                                            const LangOptions &LO) {
+  std::vector<syntax::Token> Tokens;
+  IdentifierTable Identifiers(LO);
+  auto AddToken = [&](clang::Token T) {
+    // Fill the proper token kind for keywords, etc.
+    if (T.getKind() == tok::raw_identifier && !T.needsCleaning() &&
+        !T.hasUCN()) { // FIXME: support needsCleaning and hasUCN cases.
+      clang::IdentifierInfo &II = Identifiers.get(T.getRawIdentifier());
+      T.setIdentifierInfo(&II);
+      T.setKind(II.getTokenID());
+    }
+    Tokens.push_back(syntax::Token(T));
+  };
+
+  Lexer L(FID, SM.getBuffer(FID), SM, LO);
+
+  clang::Token T;
+  while (!L.LexFromRawLexer(T))
+    AddToken(T);
+  // 'eof' is only the last token if the input is null-terminated. Never store
+  // it, for consistency.
+  if (T.getKind() != tok::eof)
+    AddToken(T);
+  return Tokens;
+}
+
+/// Fills in the TokenBuffer by tracing the run of a preprocessor. The
+/// implementation tracks the tokens, macro expansions and directives coming
+/// from the preprocessor and:
+/// - for each token, figures out if it is a part of an expanded token stream,
+///   spelled token stream or both. Stores the tokens appropriately.
+/// - records mappings from the spelled to expanded token ranges, e.g. for macro
+///   expansions.
+/// FIXME: also properly record:
+///          - #include directives,
+///          - #pragma, #line and other PP directives,
+///          - skipped pp regions,
+///          - ...
+
+TokenCollector::TokenCollector(Preprocessor &PP) : PP(PP) {
+  // Collect the expanded token stream during preprocessing.
+  PP.setTokenWatcher([this](const clang::Token &T) {
+    if (T.isAnnotation())
+      return;
+    DEBUG_WITH_TYPE("collect-tokens", llvm::dbgs()
+                                          << "Token: "
+                                          << syntax::Token(T).dumpForTests(
+                                                 this->PP.getSourceManager())
+                                          << "\n"
+
+    );
+    Expanded.push_back(syntax::Token(T));
+  });
+}
+
+/// Builds mappings and spelled tokens in the TokenBuffer based on the expanded
+/// token stream.
+class TokenCollector::Builder {
+public:
+  Builder(std::vector<syntax::Token> Expanded, const SourceManager &SM,
+          const LangOptions &LangOpts)
+      : Result(SM), SM(SM), LangOpts(LangOpts) {
+    Result.ExpandedTokens = std::move(Expanded);
+  }
+
+  TokenBuffer build() && {
+    buildSpelledTokens();
+
+    // Walk over expanded tokens and spelled tokens in parallel, building the
+    // mappings between those using source locations.
+
+    // The 'eof' token is special, it is not part of spelled token stream. We
+    // handle it separately at the end.
+    assert(!Result.ExpandedTokens.empty());
+    assert(Result.ExpandedTokens.back().kind() == tok::eof);
+    for (unsigned I = 0; I < Result.ExpandedTokens.size() - 1; ++I) {
+      // (!) I might be updated by the following call.
+      processExpandedToken(I);
+    }
+
+    // 'eof' not handled in the loop, do it here.
+    assert(SM.getMainFileID() ==
+           SM.getFileID(Result.ExpandedTokens.back().location()));
+    fillGapUntil(Result.Files[SM.getMainFileID()],
+                 Result.ExpandedTokens.back().location(),
+                 Result.ExpandedTokens.size() - 1);
+    Result.Files[SM.getMainFileID()].EndExpanded = Result.ExpandedTokens.size();
+
+    // Some files might have unaccounted spelled tokens at the end, add an empty
+    // mapping for those as they did not have expanded counterparts.
+    fillGapsAtEndOfFiles();
+
+    return std::move(Result);
+  }
+
+private:
+  /// Process the next token in an expanded stream and move corresponding
+  /// spelled tokens, record any mapping if needed.
+  /// (!) \p I will be updated if this had to skip tokens, e.g. for macros.
+  void processExpandedToken(unsigned &I) {
+    auto L = Result.ExpandedTokens[I].location();
+    if (L.isMacroID()) {
+      processMacroExpansion(SM.getExpansionRange(L), I);
+      return;
+    }
+    if (L.isFileID()) {
+      auto FID = SM.getFileID(L);
+      TokenBuffer::MarkedFile &File = Result.Files[FID];
+
+      fillGapUntil(File, L, I);
+
+      // Skip the token.
+      assert(File.SpelledTokens[NextSpelled[FID]].location() == L &&
+             "no corresponding token in the spelled stream");
+      ++NextSpelled[FID];
+      return;
+    }
+  }
+
+  /// Skipped expanded and spelled tokens of a macro expansion that covers \p
+  /// SpelledRange. Add a corresponding mapping.
+  /// (!) \p I will be the index of the last token in an expansion after this
+  /// function returns.
+  void processMacroExpansion(CharSourceRange SpelledRange, unsigned &I) {
+    auto FID = SM.getFileID(SpelledRange.getBegin());
+    assert(FID == SM.getFileID(SpelledRange.getEnd()));
+    TokenBuffer::MarkedFile &File = Result.Files[FID];
+
+    fillGapUntil(File, SpelledRange.getBegin(), I);
+
+    TokenBuffer::Mapping M;
+    // Skip the spelled macro tokens.
+    std::tie(M.BeginSpelled, M.EndSpelled) =
+        consumeSpelledUntil(File, SpelledRange.getEnd().getLocWithOffset(1));
+    // Skip all expanded tokens from the same macro expansion.
+    M.BeginExpanded = I;
+    for (; I + 1 < Result.ExpandedTokens.size(); ++I) {
+      auto NextL = Result.ExpandedTokens[I + 1].location();
+      if (!NextL.isMacroID() ||
+          SM.getExpansionLoc(NextL) != SpelledRange.getBegin())
+        break;
+    }
+    M.EndExpanded = I + 1;
+
+    // Add a resulting mapping.
+    File.Mappings.push_back(M);
+  }
+
+  /// Initializes TokenBuffer::Files and fills spelled tokens and expanded
+  /// ranges for each of the files.
+  void buildSpelledTokens() {
+    for (unsigned I = 0; I < Result.ExpandedTokens.size(); ++I) {
+      auto FID =
+          SM.getFileID(SM.getExpansionLoc(Result.ExpandedTokens[I].location()));
+      auto It = Result.Files.try_emplace(FID);
+      TokenBuffer::MarkedFile &File = It.first->second;
+
+      File.EndExpanded = I + 1;
+      if (!It.second)
+        continue; // we have seen this file before.
+
+      // This is the first time we see this file.
+      File.BeginExpanded = I;
+      File.SpelledTokens = tokenize(FID, SM, LangOpts);
+    }
+  }
+
+  /// Consumed spelled tokens until location L is reached (token starting at L
+  /// is not included). Returns the indicies of the consumed range.
+  std::pair</*Begin*/ unsigned, /*End*/ unsigned>
+  consumeSpelledUntil(TokenBuffer::MarkedFile &File, SourceLocation L) {
+    assert(L.isFileID());
+    FileID FID;
+    unsigned Offset;
+    std::tie(FID, Offset) = SM.getDecomposedLoc(L);
+
+    // (!) we update the index in-place.
+    unsigned &SpelledI = NextSpelled[FID];
+    unsigned Before = SpelledI;
+    for (; SpelledI < File.SpelledTokens.size() &&
+           SM.getFileOffset(File.SpelledTokens[SpelledI].location()) < Offset;
+         ++SpelledI) {
+    }
+    return std::make_pair(Before, /*After*/ SpelledI);
+  };
+
+  /// Consumes spelled tokens until location \p L is reached and adds a mapping
+  /// covering the consumed tokens. The mapping will point to an empty expanded
+  /// range at position \p ExpandedIndex.
+  void fillGapUntil(TokenBuffer::MarkedFile &File, SourceLocation L,
+                    unsigned ExpandedIndex) {
+    unsigned BeginSpelledGap, EndSpelledGap;
+    std::tie(BeginSpelledGap, EndSpelledGap) = consumeSpelledUntil(File, L);
+    if (BeginSpelledGap == EndSpelledGap)
+      return; // No gap.
+    TokenBuffer::Mapping M;
+    M.BeginSpelled = BeginSpelledGap;
+    M.EndSpelled = EndSpelledGap;
+    M.BeginExpanded = M.EndExpanded = ExpandedIndex;
+    File.Mappings.push_back(M);
+  };
+
+  /// Adds empty mappings for unconsumed spelled tokens at the end of each file.
+  void fillGapsAtEndOfFiles() {
+    for (auto &F : Result.Files) {
+      unsigned Next = NextSpelled[F.first];
+      if (F.second.SpelledTokens.size() == Next)
+        continue; // All spelled tokens are accounted for.
+
+      // Record a mapping for the gap at the end of the spelled tokens.
+      TokenBuffer::Mapping M;
+      M.BeginSpelled = Next;
+      M.EndSpelled = F.second.SpelledTokens.size();
+      M.BeginExpanded = F.second.EndExpanded;
+      M.EndExpanded = F.second.EndExpanded;
+
+      F.second.Mappings.push_back(M);
+    }
+  }
+
+  TokenBuffer Result;
+  /// For each file, a position of the next spelled token we will consume.
+  llvm::DenseMap<FileID, unsigned> NextSpelled;
+  const SourceManager &SM;
+  const LangOptions &LangOpts;
+};
+
+TokenBuffer TokenCollector::consume() && {
+  PP.setTokenWatcher(nullptr);
+  return Builder(std::move(Expanded), PP.getSourceManager(), PP.getLangOpts())
+      .build();
+}
+
+std::string syntax::Token::str() const {
+  return llvm::formatv("Token({0}, length = {1})", tok::getTokenName(kind()),
+                       length());
+}
+
+std::string syntax::Token::dumpForTests(const SourceManager &SM) const {
+  return llvm::formatv("{0}   {1}", tok::getTokenName(kind()), text(SM));
+}
+
+std::string TokenBuffer::dumpForTests() const {
+  auto PrintToken = [this](const syntax::Token &T) -> std::string {
+    if (T.kind() == tok::eof)
+      return "<eof>";
+    return T.text(*SourceMgr);
+  };
+
+  auto DumpTokens = [this, &PrintToken](llvm::raw_ostream &OS,
+                                        llvm::ArrayRef<syntax::Token> Tokens) {
+    if (Tokens.size() == 1) {
+      assert(Tokens[0].kind() == tok::eof);
+      OS << "<empty>";
+      return;
+    }
+    OS << Tokens[0].text(*SourceMgr);
+    for (unsigned I = 1; I < Tokens.size(); ++I) {
+      if (Tokens[I].kind() == tok::eof)
+        continue;
+      OS << " " << PrintToken(Tokens[I]);
+    }
+  };
+
+  std::string Dump;
+  llvm::raw_string_ostream OS(Dump);
+
+  OS << "expanded tokens:\n"
+     << "  ";
+  DumpTokens(OS, ExpandedTokens);
+  OS << "\n";
+
+  std::vector<FileID> Keys;
+  for (auto F : Files)
+    Keys.push_back(F.first);
+  llvm::sort(Keys);
+
+  for (FileID ID : Keys) {
+    const MarkedFile &File = Files.find(ID)->second;
+    auto *Entry = SourceMgr->getFileEntryForID(ID);
+    if (!Entry)
+      continue; // Skip builtin files.
+    OS << llvm::formatv("file '{0}'\n", Entry->getName())
+       << "  spelled tokens:\n"
+       << "    ";
+    DumpTokens(OS, File.SpelledTokens);
+    OS << "\n";
+
+    if (File.Mappings.empty()) {
+      OS << "  no mappings.\n";
+      continue;
+    }
+    OS << "  mappings:\n";
+    for (auto &M : File.Mappings) {
+      OS << llvm::formatv(
+          "    ['{0}'_{1}, '{2}'_{3}) => ['{4}'_{5}, '{6}'_{7})\n",
+          PrintToken(File.SpelledTokens[M.BeginSpelled]), M.BeginSpelled,
+          M.EndSpelled == File.SpelledTokens.size()
+              ? "<eof>"
+              : PrintToken(File.SpelledTokens[M.EndSpelled]),
+          M.EndSpelled, PrintToken(ExpandedTokens[M.BeginExpanded]),
+          M.BeginExpanded, PrintToken(ExpandedTokens[M.EndExpanded]),
+          M.EndExpanded);
+    }
+  }
+  return OS.str();
+}