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
@@ -581,7 +581,7 @@
 }
 
 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, const Twine &ToolName,
     std::shared_ptr<PCHContainerOperations> PCHContainerOps,
     ArgumentsAdjuster Adjuster) {
@@ -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;