diff --git a/clang/include/clang/Tooling/Refactoring/Transformer.h b/clang/include/clang/Tooling/Refactoring/Transformer.h
new file mode 100644
--- /dev/null
+++ b/clang/include/clang/Tooling/Refactoring/Transformer.h
@@ -0,0 +1,249 @@
+//===--- Transformer.h - Clang source-rewriting library ---------*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+///
+///  \file
+///  Defines a library supporting the concise specification of clang-based
+///  source-to-source transformations.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLING_REFACTOR_TRANSFORMER_H_
+#define LLVM_CLANG_TOOLING_REFACTOR_TRANSFORMER_H_
+
+#include "NodeId.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
+#include "clang/ASTMatchers/ASTMatchersInternal.h"
+#include "clang/Tooling/Refactoring/AtomicChange.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/Support/Error.h"
+#include <deque>
+#include <functional>
+#include <string>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+namespace clang {
+namespace tooling {
+/// Determines the part of the AST node to replace.  We support this to work
+/// around the fact that the AST does not differentiate various syntactic
+/// elements into their own nodes, so users can specify them relative to a node,
+/// instead.
+enum class NodePart {
+  /// The node itself.
+  Node,
+  /// Given a \c MemberExpr, selects the member's token.
+  Member,
+  /// Given a \c NamedDecl or \c CxxCtorInitializer, selects that token of the
+  /// relevant name, not including qualifiers.
+  Name,
+};
+
+using TextGenerator =
+    std::function<std::string(const ast_matchers::MatchFinder::MatchResult &)>;
+
+/// Description of a source-code transformation.
+//
+// A *rewrite rule* describes a transformation of source code. It has the
+// following components:
+//
+// * Matcher: the pattern term, expressed as clang matchers (with Transformer
+//   extensions).
+//
+// * Target: the source code impacted by the rule. This identifies an AST node,
+//   or part thereof (\c TargetPart), whose source range indicates the extent of
+//   the replacement applied by the replacement term.  By default, the extent is
+//   the node matched by the pattern term (\c NodePart::Node). Target's are
+//   typed (\c TargetKind), which guides the determination of the node extent
+//   and might, in the future, statically constrain the set of eligible
+//   NodeParts for a given node.
+//
+// * Replacement: a function that produces a replacement string for the target,
+//   based on the match result.
+//
+// * Explanation: explanation of the rewrite.
+//
+// Rules have an additional, implicit, component: the parameters. These are
+// portions of the pattern which are left unspecified, yet named so that we can
+// reference them in the replacement term.  The structure of parameters can be
+// partially or even fully specified, in which case they serve just to identify
+// matched nodes for later reference rather than abstract over portions of the
+// AST.  However, in all cases, we refer to named portions of the pattern as
+// parameters.
+//
+// Parameters can be declared explicitly using the NodeId type and its
+// derivatives or left implicit by using the native support for binding ids in
+// the clang matchers.
+//
+// RewriteRule is constructed in a "fluent" style, by creating a builder and
+// chaining setters of individual components.  We provide ref-qualified
+// overloads of the setters to avoid an unnecessary copy when a RewriteRule is
+// initialized from a temporary (which we expected to be the common case).
+// \code
+//   RewriteRule R = buildRule(functionDecl(...)).replaceWith(...);
+// \endcode
+class RewriteRule {
+  // Id used as the default target of each match.
+  static constexpr char RootId[] = "___root___";
+
+  // Supports any (top-level node) matcher type.
+  ast_matchers::internal::DynTypedMatcher Matcher;
+  // The (bound) id of the node whose source will be replaced.  This id should
+  // never be the empty string.
+  std::string Target;
+  ast_type_traits::ASTNodeKind TargetKind;
+  NodePart TargetPart;
+  TextGenerator Replacement;
+  TextGenerator Explanation;
+
+  static ast_matchers::internal::DynTypedMatcher
+  makeMatcher(ast_matchers::internal::DynTypedMatcher M) {
+    M.setAllowBind(true);
+    // RewriteRule guarantees that the node described by the matcher will always
+    // be accessible as `matchedNode()`, so we bind it here. `tryBind` is
+    // guaranteed to succeed, because `AllowBind` was set to true.
+    return *M.tryBind(matchedNode());
+  }
+
+public:
+  RewriteRule(ast_matchers::internal::DynTypedMatcher M)
+      : Matcher(makeMatcher(std::move(M))), Target(RootId),
+        TargetKind(Matcher.getSupportedKind()), TargetPart(NodePart::Node) {
+    Matcher.setAllowBind(true);
+  }
+
+  // The bound id of the node corresponding to the matcher.
+  static llvm::StringRef matchedNode() { return RootId; }
+
+  void setTarget(StringRef S) { Target = S; }
+  void setTargetKind(ast_type_traits::ASTNodeKind K) { TargetKind = K; }
+  void setTargetPart(NodePart P) { TargetPart = P; }
+  void setReplacement(TextGenerator T) { Replacement = std::move(T); }
+  void setExplanation(TextGenerator T) { Explanation = std::move(T); }
+
+  const ast_matchers::internal::DynTypedMatcher &matcher() const {
+    return Matcher;
+  }
+  llvm::StringRef target() const { return Target; }
+  ast_type_traits::ASTNodeKind targetKind() const { return TargetKind; }
+  NodePart targetPart() const { return TargetPart; }
+
+  std::string
+  replacement(const ast_matchers::MatchFinder::MatchResult &R) const {
+    return Replacement(R);
+  }
+
+  std::string
+  explanation(const ast_matchers::MatchFinder::MatchResult &R) const {
+    return Explanation(R);
+  }
+};
+
+/// A fluent, builder class for \c RewriteRule.  See comments on \c RewriteRule,
+/// above.
+class RewriteRuleBuilder {
+  RewriteRule Rule;
+
+public:
+  RewriteRuleBuilder(ast_matchers::internal::DynTypedMatcher M)
+      : Rule(std::move(M)) {}
+
+  /// (Implicit) "build" operator to build a RewriteRule from this builder.
+  operator RewriteRule() && { return std::move(Rule); }
+
+  // Sets the target kind based on a clang AST node type.
+  template <typename T> RewriteRuleBuilder &&as() &&;
+
+  template <typename T>
+  RewriteRuleBuilder &&change(const TypedNodeId<T> &Target,
+                              NodePart Part = NodePart::Node) &&;
+
+  RewriteRuleBuilder &&replaceWith(TextGenerator Replacement) &&;
+  RewriteRuleBuilder &&replaceWith(std::string Replacement) && {
+    return std::move(
+        std::move(*this).replaceWith(text(std::move(Replacement))));
+  }
+
+  RewriteRuleBuilder &&because(TextGenerator Explanation) &&;
+  RewriteRuleBuilder &&because(std::string Explanation) && {
+    return std::move(std::move(*this).because(text(std::move(Explanation))));
+  }
+
+private:
+  // Wraps a string as a TextGenerator.
+  static TextGenerator text(std::string M) {
+    return [M](const ast_matchers::MatchFinder::MatchResult &) { return M; };
+   }
+};
+
+/// Convenience factory functions for starting construction of a \c RewriteRule.
+inline RewriteRuleBuilder buildRule(ast_matchers::internal::DynTypedMatcher M) {
+  return RewriteRuleBuilder(std::move(M));
+}
+template <typename T>
+RewriteRuleBuilder buildRule(ast_matchers::internal::Matcher<T> M) {
+  return RewriteRuleBuilder(M);
+}
+
+template <typename T> RewriteRuleBuilder &&RewriteRuleBuilder::as() && {
+  Rule.setTargetKind(ast_type_traits::ASTNodeKind::getFromNodeKind<T>());
+  return std::move(*this);
+}
+
+template <typename T>
+RewriteRuleBuilder &&RewriteRuleBuilder::change(const TypedNodeId<T> &TargetId,
+                                                NodePart Part) && {
+  Rule.setTarget(TargetId.id());
+  Rule.setTargetKind(ast_type_traits::ASTNodeKind::getFromNodeKind<T>());
+  Rule.setTargetPart(Part);
+  return std::move(*this);
+}
+
+/// A source "transformation," represented by a character range in the source to
+/// be replaced and a corresponding replacement string.
+struct Transformation {
+  CharSourceRange Range;
+  std::string Replacement;
+};
+
+/// Attempts to apply a rule to a match.  Fails if the match is not eligible for
+/// rewriting or, for example, if any invariants are violated relating to bound
+/// nodes in the match.
+Expected<Transformation>
+applyRewriteRule(const RewriteRule &Rule,
+                 const ast_matchers::MatchFinder::MatchResult &Match);
+
+/// Handles the matcher and callback registration for a single rewrite rule, as
+/// defined by the arguments of the constructor.
+class Transformer : public ast_matchers::MatchFinder::MatchCallback {
+public:
+  using ChangeConsumer =
+      std::function<void(const clang::tooling::AtomicChange &Change)>;
+
+  /// \param Consumer Receives each successful rewrites as an \c AtomicChange.
+  Transformer(RewriteRule Rule, ChangeConsumer Consumer)
+      : Rule(std::move(Rule)), Consumer(std::move(Consumer)) {}
+
+  /// N.B. Passes `this` pointer to `MatchFinder`.  So, this object should not
+  /// be moved after this call.
+  void registerMatchers(ast_matchers::MatchFinder *MatchFinder);
+
+  /// Not called directly by users -- called by the framework, via base class
+  /// pointer.
+  void run(const ast_matchers::MatchFinder::MatchResult &Result) override;
+
+private:
+  RewriteRule Rule;
+  /// Receives each successful rewrites as an \c AtomicChange.
+  ChangeConsumer Consumer;
+};
+} // namespace tooling
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLING_REFACTOR_TRANSFORMER_H_
diff --git a/clang/lib/Tooling/Refactoring/CMakeLists.txt b/clang/lib/Tooling/Refactoring/CMakeLists.txt
--- a/clang/lib/Tooling/Refactoring/CMakeLists.txt
+++ b/clang/lib/Tooling/Refactoring/CMakeLists.txt
@@ -13,6 +13,7 @@
   Rename/USRFindingAction.cpp
   Rename/USRLocFinder.cpp
   NodeId.cpp
+  Transformer.cpp
 
   LINK_LIBS
   clangAST
diff --git a/clang/lib/Tooling/Refactoring/Transformer.cpp b/clang/lib/Tooling/Refactoring/Transformer.cpp
new file mode 100644
--- /dev/null
+++ b/clang/lib/Tooling/Refactoring/Transformer.cpp
@@ -0,0 +1,197 @@
+//===--- Transformer.cpp - Transformer library implementation ---*- C++ -*-===//
+//
+// 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/Refactoring/Transformer.h"
+#include "clang/AST/Expr.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
+#include "clang/Basic/Diagnostic.h"
+#include "clang/Basic/SourceLocation.h"
+#include "clang/Rewrite/Core/Rewriter.h"
+#include "clang/Tooling/FixIt.h"
+#include "clang/Tooling/Refactoring.h"
+#include "clang/Tooling/Refactoring/AtomicChange.h"
+#include "llvm/ADT/Optional.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Errc.h"
+#include "llvm/Support/Error.h"
+#include <deque>
+#include <string>
+#include <utility>
+#include <vector>
+
+using namespace clang;
+using namespace tooling;
+
+using ::clang::ast_matchers::MatchFinder;
+using ::clang::ast_type_traits::ASTNodeKind;
+using ::clang::ast_type_traits::DynTypedNode;
+using ::llvm::Error;
+using ::llvm::Expected;
+using ::llvm::Optional;
+using ::llvm::StringError;
+using ::llvm::StringRef;
+
+using MatchResult = MatchFinder::MatchResult;
+
+static bool isOriginMacroBody(const clang::SourceManager &SM,
+                              clang::SourceLocation Loc) {
+  while (Loc.isMacroID()) {
+    if (SM.isMacroBodyExpansion(Loc))
+      return true;
+    // Otherwise, it must be in an argument, so we continue searching up the
+    // invocation stack. getImmediateMacroCallerLoc() gives the location of the
+    // argument text, inside the call text.
+    Loc = SM.getImmediateMacroCallerLoc(Loc);
+  }
+  return false;
+}
+
+static llvm::Error invalidArgumentError(llvm::Twine Message) {
+  return llvm::make_error<StringError>(llvm::errc::invalid_argument, Message);
+}
+
+static llvm::Error unboundNodeError(StringRef Role, StringRef Id) {
+  return invalidArgumentError(Role + " (=" + Id + ") references unbound node");
+}
+
+static llvm::Error typeError(llvm::Twine Message, const ASTNodeKind &Kind) {
+  return invalidArgumentError(Message + " (node kind is " + Kind.asStringRef() +
+                              ")");
+}
+
+static llvm::Error missingPropertyError(llvm::Twine Description,
+                                        StringRef Property) {
+  return invalidArgumentError(Description + " requires property '" + Property +
+                              "'");
+}
+
+static Expected<CharSourceRange> getTarget(const DynTypedNode &Node,
+                                           ASTNodeKind Kind,
+                                           NodePart TargetPart,
+                                           ASTContext &Context) {
+  switch (TargetPart) {
+  case NodePart::Node: {
+    // For non-expression statements, associate any trailing semicolon with the
+    // statement text.  However, if the target was intended as an expression (as
+    // indicated by its kind) then we do not associate any trailing semicolon
+    // with it.  We only associate the exact expression text.
+    if (Node.get<Stmt>() != nullptr) {
+      auto ExprKind = ASTNodeKind::getFromNodeKind<clang::Expr>();
+      if (!ExprKind.isBaseOf(Kind))
+        return fixit::getExtendedRange(Node, tok::TokenKind::semi, Context);
+    }
+    return CharSourceRange::getTokenRange(Node.getSourceRange());
+  }
+  case NodePart::Member:
+    if (auto *M = Node.get<clang::MemberExpr>())
+      return CharSourceRange::getTokenRange(
+          M->getMemberNameInfo().getSourceRange());
+    return typeError("NodePart::Member applied to non-MemberExpr",
+                     Node.getNodeKind());
+  case NodePart::Name:
+    if (const auto *D = Node.get<clang::NamedDecl>()) {
+      if (!D->getDeclName().isIdentifier())
+        return missingPropertyError("NodePart::Name", "identifier");
+      SourceLocation L = D->getLocation();
+      auto R = CharSourceRange::getTokenRange(L, L);
+      // Verify that the range covers exactly the name.
+      // FIXME: extend this code to support cases like `operator +` or
+      // `foo<int>` for which this range will be too short.  Doing so will
+      // require subcasing `NamedDecl`, because it doesn't provide virtual
+      // access to the \c DeclarationNameInfo.
+      if (fixit::internal::getText(R, Context) != D->getName())
+        return CharSourceRange();
+      return R;
+    }
+    if (const auto *E = Node.get<clang::DeclRefExpr>()) {
+      if (!E->getNameInfo().getName().isIdentifier())
+        return missingPropertyError("NodePart::Name", "identifier");
+      SourceLocation L = E->getLocation();
+      return CharSourceRange::getTokenRange(L, L);
+    }
+    if (const auto *I = Node.get<clang::CXXCtorInitializer>()) {
+      if (!I->isMemberInitializer() && I->isWritten())
+        return missingPropertyError("NodePart::Name",
+                                    "explicit member initializer");
+      SourceLocation L = I->getMemberLocation();
+      return CharSourceRange::getTokenRange(L, L);
+    }
+    return typeError(
+        "NodePart::Name applied to neither DeclRefExpr, NamedDecl nor "
+        "CXXCtorInitializer",
+        Node.getNodeKind());
+  }
+  llvm_unreachable("Unexpected case in NodePart type.");
+}
+
+namespace clang {
+namespace tooling {
+Expected<Transformation>
+applyRewriteRule(const RewriteRule &Rule,
+                 const ast_matchers::MatchFinder::MatchResult &Match) {
+  if (Match.Context->getDiagnostics().hasErrorOccurred())
+    return Transformation();
+
+  auto &NodesMap = Match.Nodes.getMap();
+  auto It = NodesMap.find(Rule.target());
+  if (It == NodesMap.end())
+    return unboundNodeError("rule.target", Rule.target());
+
+  Expected<CharSourceRange> TargetOrErr = getTarget(
+      It->second, Rule.targetKind(), Rule.targetPart(), *Match.Context);
+  if (auto Err =
+          llvm::handleErrors(TargetOrErr.takeError(), [&Rule](StringError &E) {
+            return invalidArgumentError("Failure targeting node" +
+                                        Rule.target() + ": " + E.getMessage());
+          }))
+    return std::move(Err);
+  auto &Target = *TargetOrErr;
+  if (Target.isInvalid() ||
+      isOriginMacroBody(*Match.SourceManager, Target.getBegin()))
+    return Transformation();
+
+  return Transformation{Target, Rule.replacement(Match)};
+}
+} // namespace tooling
+} // namespace clang
+
+constexpr char RewriteRule::RootId[];
+
+RewriteRuleBuilder &&RewriteRuleBuilder::replaceWith(TextGenerator T) && {
+  Rule.setReplacement(std::move(T));
+  return std::move(*this);
+}
+
+RewriteRuleBuilder &&RewriteRuleBuilder::because(TextGenerator T) && {
+  Rule.setExplanation(std::move(T));
+  return std::move(*this);
+}
+
+void Transformer::registerMatchers(MatchFinder *MatchFinder) {
+  MatchFinder->addDynamicMatcher(Rule.matcher(), this);
+}
+
+void Transformer::run(const MatchResult &Result) {
+  auto ChangeOrErr = applyRewriteRule(Rule, Result);
+  if (auto Err = ChangeOrErr.takeError()) {
+    llvm::errs() << "Rewrite failed: " << llvm::toString(std::move(Err))
+                 << "\n";
+    return;
+  }
+  auto &Change = *ChangeOrErr;
+  auto &Range = Change.Range;
+  if (Range.isInvalid()) {
+    // No rewrite applied (but no error encountered either).
+    return;
+  }
+  AtomicChange AC(*Result.SourceManager, Range.getBegin());
+  if (auto Err = AC.replace(*Result.SourceManager, Range, Change.Replacement))
+    AC.setError(llvm::toString(std::move(Err)));
+  Consumer(AC);
+}
diff --git a/clang/unittests/Tooling/CMakeLists.txt b/clang/unittests/Tooling/CMakeLists.txt
--- a/clang/unittests/Tooling/CMakeLists.txt
+++ b/clang/unittests/Tooling/CMakeLists.txt
@@ -50,6 +50,7 @@
   ReplacementsYamlTest.cpp
   RewriterTest.cpp
   ToolingTest.cpp
+  TransformerTest.cpp
   )
 
 target_link_libraries(ToolingTests
diff --git a/clang/unittests/Tooling/TransformerTest.cpp b/clang/unittests/Tooling/TransformerTest.cpp
new file mode 100644
--- /dev/null
+++ b/clang/unittests/Tooling/TransformerTest.cpp
@@ -0,0 +1,418 @@
+//===- unittest/Tooling/TransformerTest.cpp -------------------------------===//
+//
+// 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/Refactoring/Transformer.h"
+
+#include "clang/ASTMatchers/ASTMatchers.h"
+#include "clang/Tooling/Tooling.h"
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+namespace clang {
+namespace tooling {
+namespace {
+using ::clang::ast_matchers::anyOf;
+using ::clang::ast_matchers::argumentCountIs;
+using ::clang::ast_matchers::callee;
+using ::clang::ast_matchers::callExpr;
+using ::clang::ast_matchers::cxxMemberCallExpr;
+using ::clang::ast_matchers::cxxMethodDecl;
+using ::clang::ast_matchers::cxxRecordDecl;
+using ::clang::ast_matchers::declRefExpr;
+using ::clang::ast_matchers::expr;
+using ::clang::ast_matchers::functionDecl;
+using ::clang::ast_matchers::hasAnyName;
+using ::clang::ast_matchers::hasArgument;
+using ::clang::ast_matchers::hasDeclaration;
+using ::clang::ast_matchers::hasElse;
+using ::clang::ast_matchers::hasName;
+using ::clang::ast_matchers::hasType;
+using ::clang::ast_matchers::ifStmt;
+using ::clang::ast_matchers::member;
+using ::clang::ast_matchers::memberExpr;
+using ::clang::ast_matchers::namedDecl;
+using ::clang::ast_matchers::on;
+using ::clang::ast_matchers::pointsTo;
+using ::clang::ast_matchers::to;
+using ::clang::ast_matchers::unless;
+
+constexpr char KHeaderContents[] = R"cc(
+  struct string {
+    string(const char*);
+    char* c_str();
+    int size();
+  };
+  int strlen(const char*);
+
+  namespace proto {
+  struct PCFProto {
+    int foo();
+  };
+  struct ProtoCommandLineFlag : PCFProto {
+    PCFProto& GetProto();
+  };
+  }  // namespace proto
+)cc";
+} // namespace
+
+static clang::ast_matchers::internal::Matcher<clang::QualType>
+isOrPointsTo(const clang::ast_matchers::DeclarationMatcher &TypeMatcher) {
+  return anyOf(hasDeclaration(TypeMatcher), pointsTo(TypeMatcher));
+}
+
+static std::string format(llvm::StringRef Code) {
+  const std::vector<Range> Ranges(1, Range(0, Code.size()));
+  auto Style = format::getLLVMStyle();
+  const auto Replacements = format::reformat(Style, Code, Ranges);
+  auto Formatted = applyAllReplacements(Code, Replacements);
+  if (!Formatted) {
+    ADD_FAILURE() << "Could not format code: "
+                  << llvm::toString(Formatted.takeError());
+    return std::string();
+  }
+  return *Formatted;
+}
+
+void compareSnippets(llvm::StringRef Expected,
+                     const llvm::Optional<std::string> &MaybeActual) {
+  ASSERT_TRUE(MaybeActual) << "Rewrite failed. Expecting: " << Expected;
+  auto Actual = *MaybeActual;
+  std::string HL = "#include \"header.h\"\n";
+  auto I = Actual.find(HL);
+  if (I != std::string::npos) {
+    Actual.erase(I, HL.size());
+  }
+  EXPECT_EQ(format(Expected), format(Actual));
+}
+
+// FIXME: consider separating this class into its own file(s).
+class ClangRefactoringTestBase : public testing::Test {
+protected:
+  void appendToHeader(llvm::StringRef S) { FileContents[0].second += S; }
+
+  void addFile(llvm::StringRef Filename, llvm::StringRef Content) {
+    FileContents.emplace_back(Filename, Content);
+  }
+
+  llvm::Optional<std::string> rewrite(llvm::StringRef Input) {
+    std::string Code = ("#include \"header.h\"\n" + Input).str();
+    auto Factory = newFrontendActionFactory(&MatchFinder);
+    if (!runToolOnCodeWithArgs(
+            Factory->create(), Code, std::vector<std::string>(), "input.cc",
+            "clang-tool", std::make_shared<PCHContainerOperations>(),
+            FileContents)) {
+      return None;
+    }
+    auto ChangedCodeOrErr =
+        applyAtomicChanges("input.cc", Code, Changes, ApplyChangesSpec());
+    if (auto Err = ChangedCodeOrErr.takeError()) {
+      llvm::errs() << "Change failed: " << llvm::toString(std::move(Err))
+                   << "\n";
+      return None;
+    }
+    return *ChangedCodeOrErr;
+  }
+
+  clang::ast_matchers::MatchFinder MatchFinder;
+  AtomicChanges Changes;
+
+private:
+  FileContentMappings FileContents = {{"header.h", ""}};
+};
+
+class TransformerTest : public ClangRefactoringTestBase {
+protected:
+  TransformerTest() { appendToHeader(KHeaderContents); }
+
+  Transformer::ChangeConsumer changeRecorder() {
+    return [this](const AtomicChange &C) { Changes.push_back(C); };
+  }
+};
+
+// Given string s, change strlen($s.c_str()) to $s.size().
+RewriteRule ruleStrlenSize() {
+  ExprId StringExpr;
+  auto StringType = namedDecl(hasAnyName("::basic_string", "::string"));
+  return buildRule(
+             callExpr(
+                 callee(functionDecl(hasName("strlen"))),
+                 hasArgument(0, cxxMemberCallExpr(
+                                    on(expr(StringExpr.bind(),
+                                            hasType(isOrPointsTo(StringType)))),
+                                    callee(cxxMethodDecl(hasName("c_str")))))))
+      // Specify the intended type explicitly, because the matcher "type" of
+      // `callExpr()` is `Stmt`, not `Expr`.
+      .as<clang::Expr>()
+      .replaceWith("REPLACED")
+      .because("Use size() method directly on string.");
+}
+
+TEST_F(TransformerTest, StrlenSize) {
+  std::string Input = "int f(string s) { return strlen(s.c_str()); }";
+  std::string Expected = "int f(string s) { return REPLACED; }";
+
+  Transformer T(ruleStrlenSize(), changeRecorder());
+  T.registerMatchers(&MatchFinder);
+  compareSnippets(Expected, rewrite(Input));
+}
+
+// Tests that no change is applied when a match is not expected.
+TEST_F(TransformerTest, NoMatch) {
+  std::string Input = "int f(string s) { return s.size(); }";
+
+  Transformer T(ruleStrlenSize(), changeRecorder());
+  T.registerMatchers(&MatchFinder);
+  // Input should not be changed.
+  compareSnippets(Input, rewrite(Input));
+}
+
+// Tests that expressions in macro arguments are rewritten (when applicable).
+TEST_F(TransformerTest, StrlenSizeMacro) {
+  std::string Input = R"cc(
+#define ID(e) e
+    int f(string s) { return ID(strlen(s.c_str())); })cc";
+  std::string Expected = R"cc(
+#define ID(e) e
+    int f(string s) { return ID(REPLACED); })cc";
+
+  Transformer T(ruleStrlenSize(), changeRecorder());
+  T.registerMatchers(&MatchFinder);
+  compareSnippets(Expected, rewrite(Input));
+}
+
+// Tests replacing an expression.
+TEST_F(TransformerTest, Flag) {
+  ExprId Flag;
+  RewriteRule Rule =
+      buildRule(cxxMemberCallExpr(
+                    on(expr(Flag.bind(), hasType(cxxRecordDecl(hasName(
+                                             "proto::ProtoCommandLineFlag"))))),
+                    unless(callee(cxxMethodDecl(hasName("GetProto"))))))
+          .change(Flag)
+          .replaceWith("EXPR")
+          .because("Use GetProto() to access proto fields.");
+
+  std::string Input = R"cc(
+    proto::ProtoCommandLineFlag flag;
+    int x = flag.foo();
+    int y = flag.GetProto().foo();
+  )cc";
+  std::string Expected = R"cc(
+    proto::ProtoCommandLineFlag flag;
+    int x = EXPR.foo();
+    int y = flag.GetProto().foo();
+  )cc";
+
+  Transformer T(Rule, changeRecorder());
+  T.registerMatchers(&MatchFinder);
+  compareSnippets(Expected, rewrite(Input));
+}
+
+TEST_F(TransformerTest, NodePartNameNamedDecl) {
+  DeclId Fun;
+  RewriteRule Rule = buildRule(functionDecl(hasName("bad"), Fun.bind()))
+                         .change(Fun, NodePart::Name)
+                         .replaceWith("good");
+
+  std::string Input = R"cc(
+    int bad(int x);
+    int bad(int x) { return x * x; }
+  )cc";
+  std::string Expected = R"cc(
+    int good(int x);
+    int good(int x) { return x * x; }
+  )cc";
+
+  Transformer T(Rule, changeRecorder());
+  T.registerMatchers(&MatchFinder);
+  compareSnippets(Expected, rewrite(Input));
+}
+
+TEST_F(TransformerTest, NodePartNameDeclRef) {
+  std::string Input = R"cc(
+    template <typename T>
+    T bad(T x) {
+      return x;
+    }
+    int neutral(int x) { return bad<int>(x) * x; }
+  )cc";
+  std::string Expected = R"cc(
+    template <typename T>
+    T bad(T x) {
+      return x;
+    }
+    int neutral(int x) { return good<int>(x) * x; }
+  )cc";
+
+  ExprId Ref;
+  Transformer T(
+      buildRule(declRefExpr(to(functionDecl(hasName("bad"))), Ref.bind()))
+          .change(Ref, NodePart::Name)
+          .replaceWith("good"),
+      changeRecorder());
+  T.registerMatchers(&MatchFinder);
+  compareSnippets(Expected, rewrite(Input));
+}
+
+TEST_F(TransformerTest, NodePartNameDeclRefFailure) {
+  std::string Input = R"cc(
+    struct Y {
+      int operator*();
+    };
+    int neutral(int x) {
+      Y y;
+      int (Y::*ptr)() = &Y::operator*;
+      return *y + x;
+    }
+  )cc";
+
+  ExprId Ref;
+  Transformer T(buildRule(declRefExpr(to(functionDecl()), Ref.bind()))
+                    .change(Ref, NodePart::Name)
+                    .replaceWith("good"),
+                changeRecorder());
+  T.registerMatchers(&MatchFinder);
+  compareSnippets(Input, rewrite(Input));
+}
+
+TEST_F(TransformerTest, NodePartMember) {
+  ExprId E;
+  RewriteRule Rule = buildRule(memberExpr(member(hasName("bad")), E.bind()))
+                         .change(E, NodePart::Member)
+                         .replaceWith("good");
+
+  std::string Input = R"cc(
+    struct S {
+      int bad;
+    };
+    int g() {
+      S s;
+      return s.bad;
+    }
+  )cc";
+  std::string Expected = R"cc(
+    struct S {
+      int bad;
+    };
+    int g() {
+      S s;
+      return s.good;
+    }
+  )cc";
+
+  Transformer T(Rule, changeRecorder());
+  T.registerMatchers(&MatchFinder);
+  compareSnippets(Expected, rewrite(Input));
+}
+
+TEST_F(TransformerTest, NodePartMemberQualified) {
+  ExprId E;
+  RewriteRule Rule = buildRule(memberExpr(E.bind()))
+                         .change(E, NodePart::Member)
+                         .replaceWith("good");
+
+  std::string Input = R"cc(
+    struct S {
+      int bad;
+      int good;
+    };
+    struct T : public S {
+      int bad;
+    };
+    int g() {
+      T t;
+      return t.S::bad;
+    }
+  )cc";
+  std::string Expected = R"cc(
+    struct S {
+      int bad;
+      int good;
+    };
+    struct T : public S {
+      int bad;
+    };
+    int g() {
+      T t;
+      return t.S::good;
+    }
+  )cc";
+
+  Transformer T(Rule, changeRecorder());
+  T.registerMatchers(&MatchFinder);
+  compareSnippets(Expected, rewrite(Input));
+}
+
+TEST_F(TransformerTest, NodePartMemberMultiToken) {
+  std::string Input = R"cc(
+    struct Y {
+      int operator*();
+      int good();
+      template <typename T> void foo(T t);
+    };
+    int neutral(int x) {
+      Y y;
+      y.template foo<int>(3);
+      return y.operator *();
+    }
+  )cc";
+  std::string Expected = R"cc(
+    struct Y {
+      int operator*();
+      int good();
+      template <typename T> void foo(T t);
+    };
+    int neutral(int x) {
+      Y y;
+      y.template good<int>(3);
+      return y.good();
+    }
+  )cc";
+
+  ExprId MemExpr;
+  Transformer T(buildRule(memberExpr(MemExpr.bind()))
+                    .change(MemExpr, NodePart::Member)
+                    .replaceWith("good"),
+                changeRecorder());
+  T.registerMatchers(&MatchFinder);
+  compareSnippets(Expected, rewrite(Input));
+}
+
+//
+// Negative tests (where we expect no transformation to occur).
+//
+
+TEST_F(TransformerTest, NoTransformationInMacro) {
+  std::string Input = R"cc(
+#define MACRO(str) strlen((str).c_str())
+    int f(string s) { return MACRO(s); })cc";
+
+  Transformer T(ruleStrlenSize(), changeRecorder());
+  T.registerMatchers(&MatchFinder);
+  // The macro should be ignored.
+  compareSnippets(Input, rewrite(Input));
+}
+
+// This test handles the corner case where a macro called within another macro
+// expands to matching code, but the matched code is an argument to the nested
+// macro.  A simple check of isMacroArgExpansion() vs. isMacroBodyExpansion()
+// will get this wrong, and transform the code. This test verifies that no such
+// transformation occurs.
+TEST_F(TransformerTest, NoTransformationInNestedMacro) {
+  std::string Input = R"cc(
+#define NESTED(e) e
+#define MACRO(str) NESTED(strlen((str).c_str()))
+    int f(string s) { return MACRO(s); })cc";
+
+  Transformer T(ruleStrlenSize(), changeRecorder());
+  T.registerMatchers(&MatchFinder);
+  // The macro should be ignored.
+  compareSnippets(Input, rewrite(Input));
+}
+} // namespace tooling
+} // namespace clang