Index: include/clang/Tooling/Tooling.h
===================================================================
--- include/clang/Tooling/Tooling.h
+++ include/clang/Tooling/Tooling.h
@@ -205,7 +205,7 @@
 ///
 /// \return The resulting AST or null if an error occurred.
 std::unique_ptr<ASTUnit>
-buildASTFromCode(const Twine &Code, const Twine &FileName = "input.cc",
+buildASTFromCode(StringRef Code, const Twine &FileName = "input.cc",
                  std::shared_ptr<PCHContainerOperations> PCHContainerOps =
                      std::make_shared<PCHContainerOperations>());
 
@@ -223,7 +223,7 @@
 ///
 /// \return The resulting AST or null if an error occurred.
 std::unique_ptr<ASTUnit> buildASTFromCodeWithArgs(
-    const Twine &Code, const std::vector<std::string> &Args,
+    StringRef Code, const std::vector<std::string> &Args,
     const Twine &FileName = "input.cc", const Twine &ToolName = "clang-tool",
     std::shared_ptr<PCHContainerOperations> PCHContainerOps =
       std::make_shared<PCHContainerOperations>(),
Index: lib/Tooling/Tooling.cpp
===================================================================
--- lib/Tooling/Tooling.cpp
+++ lib/Tooling/Tooling.cpp
@@ -574,15 +574,15 @@
 namespace tooling {
 
 std::unique_ptr<ASTUnit>
-buildASTFromCode(const Twine &Code, const Twine &FileName,
+buildASTFromCode(StringRef Code, const Twine &FileName,
                  std::shared_ptr<PCHContainerOperations> PCHContainerOps) {
   return buildASTFromCodeWithArgs(Code, std::vector<std::string>(), FileName,
                                   "clang-tool", std::move(PCHContainerOps));
 }
 
 std::unique_ptr<ASTUnit> buildASTFromCodeWithArgs(
-    const Twine &Code, const std::vector<std::string> &Args,
-    const Twine &FileName, const Twine &ToolName,
+    StringRef Code, const std::vector<std::string> &Args, const Twine &FileName,
+    const Twine &ToolName,
     std::shared_ptr<PCHContainerOperations> PCHContainerOps,
     ArgumentsAdjuster Adjuster) {
   SmallString<16> FileNameStorage;
@@ -602,10 +602,8 @@
       getSyntaxOnlyToolArgs(ToolName, Adjuster(Args, FileNameRef), FileNameRef),
       &Action, Files.get(), std::move(PCHContainerOps));
 
-  SmallString<1024> CodeStorage;
   InMemoryFileSystem->addFile(FileNameRef, 0,
-                              llvm::MemoryBuffer::getMemBuffer(
-                                  Code.toNullTerminatedStringRef(CodeStorage)));
+                              llvm::MemoryBuffer::getMemBufferCopy(Code));
   if (!Invocation.run())
     return nullptr;
 
Index: unittests/Analysis/ExprMutationAnalyzerTest.cpp
===================================================================
--- unittests/Analysis/ExprMutationAnalyzerTest.cpp
+++ unittests/Analysis/ExprMutationAnalyzerTest.cpp
@@ -11,6 +11,7 @@
 #include "clang/ASTMatchers/ASTMatchFinder.h"
 #include "clang/ASTMatchers/ASTMatchers.h"
 #include "clang/Tooling/Tooling.h"
+#include "llvm/ADT/SmallString.h"
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include <cctype>
@@ -32,7 +33,9 @@
 std::unique_ptr<ASTUnit>
 buildASTFromCodeWithArgs(const Twine &Code,
                          const std::vector<std::string> &Args) {
-  auto AST = tooling::buildASTFromCodeWithArgs(Code, Args);
+  SmallString<1024> CodeStorage;
+  auto AST =
+      tooling::buildASTFromCodeWithArgs(Code.toStringRef(CodeStorage), Args);
   EXPECT_FALSE(AST->getDiagnostics().hasErrorOccurred());
   return AST;
 }