diff --git a/clang/include/clang/StaticAnalyzer/Checkers/SValExplainer.h b/clang/include/clang/StaticAnalyzer/Checkers/SValExplainer.h
--- a/clang/include/clang/StaticAnalyzer/Checkers/SValExplainer.h
+++ b/clang/include/clang/StaticAnalyzer/Checkers/SValExplainer.h
@@ -32,7 +32,8 @@
     std::string Str;
     llvm::raw_string_ostream OS(Str);
     S->printPretty(OS, nullptr, PrintingPolicy(ACtx.getLangOpts()));
-    return OS.str();
+    OS.flush();
+    return Str;
   }
 
   bool isThisObject(const SymbolicRegion *R) {
@@ -69,7 +70,8 @@
     std::string Str;
     llvm::raw_string_ostream OS(Str);
     OS << "concrete memory address '" << I << "'";
-    return OS.str();
+    OS.flush();
+    return Str;
   }
 
   std::string VisitNonLocSymbolVal(nonloc::SymbolVal V) {
@@ -82,7 +84,8 @@
     llvm::raw_string_ostream OS(Str);
     OS << (I.isSigned() ? "signed " : "unsigned ") << I.getBitWidth()
        << "-bit integer '" << I << "'";
-    return OS.str();
+    OS.flush();
+    return Str;
   }
 
   std::string VisitNonLocLazyCompoundVal(nonloc::LazyCompoundVal V) {
@@ -123,7 +126,8 @@
     OS << "(" << Visit(S->getLHS()) << ") "
        << std::string(BinaryOperator::getOpcodeStr(S->getOpcode())) << " "
        << S->getRHS();
-    return OS.str();
+    OS.flush();
+    return Str;
   }
 
   // TODO: IntSymExpr doesn't appear in practice.
@@ -177,7 +181,8 @@
     else
       OS << "'" << Visit(R->getIndex()) << "'";
     OS << " of " + Visit(R->getSuperRegion());
-    return OS.str();
+    OS.flush();
+    return Str;
   }
 
   std::string VisitNonParamVarRegion(const NonParamVarRegion *R) {
diff --git a/clang/include/clang/Testing/TestClangConfig.h b/clang/include/clang/Testing/TestClangConfig.h
--- a/clang/include/clang/Testing/TestClangConfig.h
+++ b/clang/include/clang/Testing/TestClangConfig.h
@@ -73,7 +73,8 @@
     std::string Result;
     llvm::raw_string_ostream OS(Result);
     OS << "{ Language=" << Language << ", Target=" << Target << " }";
-    return OS.str();
+    OS.flush();
+    return Result;
   }
 
   friend std::ostream &operator<<(std::ostream &OS,
diff --git a/clang/lib/AST/ASTDiagnostic.cpp b/clang/lib/AST/ASTDiagnostic.cpp
--- a/clang/lib/AST/ASTDiagnostic.cpp
+++ b/clang/lib/AST/ASTDiagnostic.cpp
@@ -312,7 +312,8 @@
       OS << "'" << S << "' (vector of " << VTy->getNumElements() << " '"
          << VTy->getElementType().getAsString(Context.getPrintingPolicy())
          << "' " << Values << ")";
-      return OS.str();
+      OS.flush();
+      return DecoratedString;
     }
   }
 
diff --git a/clang/lib/AST/AttrImpl.cpp b/clang/lib/AST/AttrImpl.cpp
--- a/clang/lib/AST/AttrImpl.cpp
+++ b/clang/lib/AST/AttrImpl.cpp
@@ -60,7 +60,8 @@
   else
     OS << "disable";
   OS << ")";
-  return OS.str();
+  OS.flush();
+  return ValueName;
 }
 
 // Return a string suitable for identifying this attribute in diagnostics.
diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -1589,7 +1589,8 @@
   std::string QualName;
   llvm::raw_string_ostream OS(QualName);
   printQualifiedName(OS, getASTContext().getPrintingPolicy());
-  return OS.str();
+  OS.flush();
+  return QualName;
 }
 
 void NamedDecl::printQualifiedName(raw_ostream &OS) const {
diff --git a/clang/lib/AST/DeclarationName.cpp b/clang/lib/AST/DeclarationName.cpp
--- a/clang/lib/AST/DeclarationName.cpp
+++ b/clang/lib/AST/DeclarationName.cpp
@@ -236,7 +236,8 @@
   std::string Result;
   llvm::raw_string_ostream OS(Result);
   OS << *this;
-  return OS.str();
+  OS.flush();
+  return Result;
 }
 
 void *DeclarationName::getFETokenInfoSlow() const {
@@ -460,7 +461,8 @@
   std::string Result;
   llvm::raw_string_ostream OS(Result);
   OS << *this;
-  return OS.str();
+  OS.flush();
+  return Result;
 }
 
 raw_ostream &clang::operator<<(raw_ostream &OS, DeclarationNameInfo DNInfo) {
diff --git a/clang/lib/AST/OpenMPClause.cpp b/clang/lib/AST/OpenMPClause.cpp
--- a/clang/lib/AST/OpenMPClause.cpp
+++ b/clang/lib/AST/OpenMPClause.cpp
@@ -2454,7 +2454,8 @@
                                                 Property.RawString);
     }
   }
-  return OS.str();
+  OS.flush();
+  return MangledName;
 }
 
 OMPTraitInfo::OMPTraitInfo(StringRef MangledName) {
diff --git a/clang/lib/ASTMatchers/Dynamic/Diagnostics.cpp b/clang/lib/ASTMatchers/Dynamic/Diagnostics.cpp
--- a/clang/lib/ASTMatchers/Dynamic/Diagnostics.cpp
+++ b/clang/lib/ASTMatchers/Dynamic/Diagnostics.cpp
@@ -204,7 +204,8 @@
   std::string S;
   llvm::raw_string_ostream OS(S);
   printToStream(OS);
-  return OS.str();
+  OS.flush();
+  return S;
 }
 
 void Diagnostics::printToStreamFull(llvm::raw_ostream &OS) const {
@@ -223,7 +224,8 @@
   std::string S;
   llvm::raw_string_ostream OS(S);
   printToStreamFull(OS);
-  return OS.str();
+  OS.flush();
+  return S;
 }
 
 }  // namespace dynamic
diff --git a/clang/lib/Analysis/AnalysisDeclContext.cpp b/clang/lib/Analysis/AnalysisDeclContext.cpp
--- a/clang/lib/Analysis/AnalysisDeclContext.cpp
+++ b/clang/lib/Analysis/AnalysisDeclContext.cpp
@@ -387,7 +387,8 @@
     OS << ' ' << OMD->getSelector().getAsString() << ']';
   }
 
-  return OS.str();
+  OS.flush();
+  return Str;
 }
 
 LocationContextManager &AnalysisDeclContext::getLocationContextManager() {
diff --git a/clang/lib/Basic/SourceLocation.cpp b/clang/lib/Basic/SourceLocation.cpp
--- a/clang/lib/Basic/SourceLocation.cpp
+++ b/clang/lib/Basic/SourceLocation.cpp
@@ -90,7 +90,8 @@
   std::string S;
   llvm::raw_string_ostream OS(S);
   print(OS, SM);
-  return OS.str();
+  OS.flush();
+  return S;
 }
 
 LLVM_DUMP_METHOD void SourceLocation::dump(const SourceManager &SM) const {
@@ -149,7 +150,8 @@
   std::string S;
   llvm::raw_string_ostream OS(S);
   print(OS, SM);
-  return OS.str();
+  OS.flush();
+  return S;
 }
 
 //===----------------------------------------------------------------------===//
diff --git a/clang/lib/Basic/Version.cpp b/clang/lib/Basic/Version.cpp
--- a/clang/lib/Basic/Version.cpp
+++ b/clang/lib/Basic/Version.cpp
@@ -82,7 +82,8 @@
       OS << LLVMRepo << ' ';
     OS << LLVMRev << ')';
   }
-  return OS.str();
+  OS.flush();
+  return buf;
 }
 
 std::string getClangFullVersion() {
@@ -102,7 +103,8 @@
     OS << " " << repo;
   }
 
-  return OS.str();
+  OS.flush();
+  return buf;
 }
 
 std::string getClangFullCPPVersion() {
@@ -120,7 +122,8 @@
     OS << " " << repo;
   }
 
-  return OS.str();
+  OS.flush();
+  return buf;
 }
 
 } // end namespace clang
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -1285,7 +1285,8 @@
   std::string Buffer;
   llvm::raw_string_ostream OS(Buffer);
   llvm::interleave(BundleParts, OS, [&OS](StringRef Part) { OS << Part; }, ",");
-  return OS.str();
+  OS.flush();
+  return Buffer;
 }
 
 // Set the profile kind using fprofile-instrument-use-path.
diff --git a/clang/lib/Frontend/TestModuleFileExtension.cpp b/clang/lib/Frontend/TestModuleFileExtension.cpp
--- a/clang/lib/Frontend/TestModuleFileExtension.cpp
+++ b/clang/lib/Frontend/TestModuleFileExtension.cpp
@@ -133,5 +133,6 @@
   llvm::raw_string_ostream OS(Buffer);
   OS << BlockName << ":" << MajorVersion << ":" << MinorVersion << ":" << Hashed
      << ":" << UserInfo;
-  return OS.str();
+  OS.flush();
+  return Buffer;
 }
diff --git a/clang/lib/Rewrite/HTMLRewrite.cpp b/clang/lib/Rewrite/HTMLRewrite.cpp
--- a/clang/lib/Rewrite/HTMLRewrite.cpp
+++ b/clang/lib/Rewrite/HTMLRewrite.cpp
@@ -202,8 +202,8 @@
     case '&': os << "&amp;"; break;
     }
   }
-
-  return os.str();
+  os.flush();
+  return Str;
 }
 
 static void AddLineNumber(RewriteBuffer &RB, unsigned LineNo,
diff --git a/clang/lib/Sema/CodeCompleteConsumer.cpp b/clang/lib/Sema/CodeCompleteConsumer.cpp
--- a/clang/lib/Sema/CodeCompleteConsumer.cpp
+++ b/clang/lib/Sema/CodeCompleteConsumer.cpp
@@ -335,7 +335,8 @@
       break;
     }
   }
-  return OS.str();
+  OS.flush();
+  return Result;
 }
 
 const char *CodeCompletionString::getTypedText() const {
@@ -640,7 +641,8 @@
       break;
     }
   }
-  return OS.str();
+  OS.flush();
+  return Result;
 }
 
 void PrintingCodeCompleteConsumer::ProcessOverloadCandidates(
diff --git a/clang/lib/Sema/SemaAttr.cpp b/clang/lib/Sema/SemaAttr.cpp
--- a/clang/lib/Sema/SemaAttr.cpp
+++ b/clang/lib/Sema/SemaAttr.cpp
@@ -792,7 +792,8 @@
       OS << (I.index() == Rules.size() - 1 ? ", and " : ", ");
     OS << "'" << attr::getSubjectMatchRuleSpelling(I.value()) << "'";
   }
-  return OS.str();
+  OS.flush();
+  return Result;
 }
 
 } // end anonymous namespace
diff --git a/clang/lib/StaticAnalyzer/Checkers/FuchsiaHandleChecker.cpp b/clang/lib/StaticAnalyzer/Checkers/FuchsiaHandleChecker.cpp
--- a/clang/lib/StaticAnalyzer/Checkers/FuchsiaHandleChecker.cpp
+++ b/clang/lib/StaticAnalyzer/Checkers/FuchsiaHandleChecker.cpp
@@ -389,7 +389,8 @@
         llvm::raw_string_ostream OS(SBuf);
         OS << "Function '" << FuncDecl->getDeclName()
            << "' returns an open handle";
-        return OS.str();
+        OS.flush();
+        return SBuf;
       } else
         return "";
     });
@@ -405,7 +406,8 @@
         llvm::raw_string_ostream OS(SBuf);
         OS << "Function '" << FuncDecl->getDeclName()
            << "' returns an unowned handle";
-        return OS.str();
+        OS.flush();
+        return SBuf;
       } else
         return "";
     });
@@ -439,7 +441,8 @@
               llvm::raw_string_ostream OS(SBuf);
               OS << "Handle released through " << ParamDiagIdx
                  << llvm::getOrdinalSuffix(ParamDiagIdx) << " parameter";
-              return OS.str();
+              OS.flush();
+              return SBuf;
             } else
               return "";
           });
@@ -453,7 +456,8 @@
             llvm::raw_string_ostream OS(SBuf);
             OS << "Handle allocated through " << ParamDiagIdx
                << llvm::getOrdinalSuffix(ParamDiagIdx) << " parameter";
-            return OS.str();
+            OS.flush();
+            return SBuf;
           } else
             return "";
         });
@@ -467,7 +471,8 @@
             llvm::raw_string_ostream OS(SBuf);
             OS << "Unowned handle allocated through " << ParamDiagIdx
                << llvm::getOrdinalSuffix(ParamDiagIdx) << " parameter";
-            return OS.str();
+            OS.flush();
+            return SBuf;
           } else
             return "";
         });
diff --git a/clang/lib/StaticAnalyzer/Core/HTMLDiagnostics.cpp b/clang/lib/StaticAnalyzer/Core/HTMLDiagnostics.cpp
--- a/clang/lib/StaticAnalyzer/Core/HTMLDiagnostics.cpp
+++ b/clang/lib/StaticAnalyzer/Core/HTMLDiagnostics.cpp
@@ -437,7 +437,8 @@
   for (auto BI : *Buf)
     os << BI;
 
-  return os.str();
+  os.flush();
+  return file;
 }
 
 void HTMLDiagnostics::dumpCoverageData(
@@ -534,7 +535,8 @@
 </form>
 )<<<";
 
-  return os.str();
+  os.flush();
+  return s;
 }
 
 void HTMLDiagnostics::FinalizeHTML(const PathDiagnostic& D, Rewriter &R,
@@ -1202,7 +1204,8 @@
   std::string Result;
   llvm::raw_string_ostream OS(Result);
   OS << "<span id=\"" << ClassName << Index << "\">";
-  return OS.str();
+  OS.flush();
+  return Result;
 }
 
 std::string getSpanBeginForControlStart(unsigned Index) {
diff --git a/clang/lib/StaticAnalyzer/Core/MemRegion.cpp b/clang/lib/StaticAnalyzer/Core/MemRegion.cpp
--- a/clang/lib/StaticAnalyzer/Core/MemRegion.cpp
+++ b/clang/lib/StaticAnalyzer/Core/MemRegion.cpp
@@ -444,7 +444,8 @@
   std::string s;
   llvm::raw_string_ostream os(s);
   dumpToStream(os);
-  return os.str();
+  os.flush();
+  return s;
 }
 
 void MemRegion::dumpToStream(raw_ostream &os) const {
diff --git a/clang/lib/Tooling/Syntax/Tokens.cpp b/clang/lib/Tooling/Syntax/Tokens.cpp
--- a/clang/lib/Tooling/Syntax/Tokens.cpp
+++ b/clang/lib/Tooling/Syntax/Tokens.cpp
@@ -927,5 +927,6 @@
           M.EndExpanded);
     }
   }
-  return OS.str();
+  OS.flush();
+  return Dump;
 }
diff --git a/clang/lib/Tooling/Syntax/Tree.cpp b/clang/lib/Tooling/Syntax/Tree.cpp
--- a/clang/lib/Tooling/Syntax/Tree.cpp
+++ b/clang/lib/Tooling/Syntax/Tree.cpp
@@ -263,7 +263,8 @@
       OS << " ";
     }
   });
-  return OS.str();
+  OS.flush();
+  return Storage;
 }
 
 void syntax::Node::assertInvariants() const {
diff --git a/clang/tools/clang-fuzzer/handle-llvm/handle_llvm.cpp b/clang/tools/clang-fuzzer/handle-llvm/handle_llvm.cpp
--- a/clang/tools/clang-fuzzer/handle-llvm/handle_llvm.cpp
+++ b/clang/tools/clang-fuzzer/handle-llvm/handle_llvm.cpp
@@ -139,7 +139,8 @@
 
   Passes.run(*M);
 
-  return OS.str();
+  OS.flush();
+  return outString;
 }
 
 // Takes a function and runs it on a set of inputs
diff --git a/clang/unittests/AST/ASTTraverserTest.cpp b/clang/unittests/AST/ASTTraverserTest.cpp
--- a/clang/unittests/AST/ASTTraverserTest.cpp
+++ b/clang/unittests/AST/ASTTraverserTest.cpp
@@ -111,7 +111,8 @@
 
   Dumper.Visit(std::forward<NodeType &&>(N)...);
 
-  return OS.str();
+  OS.flush();
+  return Buffer;
 }
 
 template <typename... NodeType>
@@ -126,7 +127,8 @@
 
   Dumper.Visit(std::forward<NodeType &&>(N)...);
 
-  return OS.str();
+  OS.flush();
+  return Buffer;
 }
 
 const FunctionDecl *getFunctionNode(clang::ASTUnit *AST,
diff --git a/clang/unittests/Analysis/MacroExpansionContextTest.cpp b/clang/unittests/Analysis/MacroExpansionContextTest.cpp
--- a/clang/unittests/Analysis/MacroExpansionContextTest.cpp
+++ b/clang/unittests/Analysis/MacroExpansionContextTest.cpp
@@ -95,14 +95,16 @@
     std::string Buf;
     llvm::raw_string_ostream OS{Buf};
     Ctx.dumpExpandedTextsToStream(OS);
-    return OS.str();
+    OS.flush();
+    return Buf;
   }
 
   static std::string dumpExpansionRanges(const MacroExpansionContext &Ctx) {
     std::string Buf;
     llvm::raw_string_ostream OS{Buf};
     Ctx.dumpExpansionRangesToStream(OS);
-    return OS.str();
+    OS.flush();
+    return Buf;
   }
 };
 
diff --git a/clang/unittests/Tooling/Syntax/TreeTest.cpp b/clang/unittests/Tooling/Syntax/TreeTest.cpp
--- a/clang/unittests/Tooling/Syntax/TreeTest.cpp
+++ b/clang/unittests/Tooling/Syntax/TreeTest.cpp
@@ -203,7 +203,8 @@
 
     OS << "]";
 
-    return OS.str();
+    OS.flush();
+    return Storage;
   }
 
   std::string dumpNodes(ArrayRef<Node *> Nodes) {
@@ -218,7 +219,8 @@
 
     OS << "]";
 
-    return OS.str();
+    OS.flush();
+    return Storage;
   }
 };