Index: llvm/include/llvm/Support/raw_ostream.h
===================================================================
--- llvm/include/llvm/Support/raw_ostream.h
+++ llvm/include/llvm/Support/raw_ostream.h
@@ -721,7 +721,11 @@
 
 public:
   buffer_unique_ostream(std::unique_ptr<raw_ostream> OS)
-      : raw_svector_ostream(Buffer), OS(std::move(OS)) {}
+      : raw_svector_ostream(Buffer), OS(std::move(OS)) {
+    // Turn off buffering on OS, which we now own, to avoid allocating a buffer
+    // when the destructor writes only to be immediately flushed again.
+    this->OS->SetUnbuffered();
+  }
   ~buffer_unique_ostream() override { *OS << str(); }
 };
 
Index: llvm/unittests/Support/CMakeLists.txt
===================================================================
--- llvm/unittests/Support/CMakeLists.txt
+++ llvm/unittests/Support/CMakeLists.txt
@@ -92,6 +92,7 @@
   WithColorTest.cpp
   YAMLIOTest.cpp
   YAMLParserTest.cpp
+  buffer_ostream_test.cpp
   formatted_raw_ostream_test.cpp
   raw_fd_stream_test.cpp
   raw_ostream_test.cpp
Index: llvm/unittests/Support/buffer_ostream_test.cpp
===================================================================
--- /dev/null
+++ llvm/unittests/Support/buffer_ostream_test.cpp
@@ -0,0 +1,77 @@
+//===- buffer_ostream_test.cpp - buffer_ostream tests ---------------------===//
+//
+// 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 "llvm/ADT/SmallString.h"
+#include "llvm/Support/raw_ostream.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+
+namespace {
+
+/// Naive version of raw_svector_ostream that is buffered (by default) and
+/// doesn't support pwrite.
+class NaiveSmallVectorStream : public raw_ostream {
+public:
+  uint64_t current_pos() const override { return Vector.size(); }
+  void write_impl(const char *Ptr, size_t Size) override {
+    Vector.append(Ptr, Ptr + Size);
+  }
+
+  explicit NaiveSmallVectorStream(SmallVectorImpl<char> &Vector)
+      : Vector(Vector) {}
+  ~NaiveSmallVectorStream() override { flush(); }
+
+  SmallVectorImpl<char> &Vector;
+};
+
+TEST(buffer_ostreamTest, Reference) {
+  SmallString<128> Dest;
+  {
+    NaiveSmallVectorStream DestOS(Dest);
+    buffer_ostream BufferOS(DestOS);
+
+    // Writing and flushing should have no effect on Dest.
+    BufferOS << "abcd";
+    static_cast<raw_ostream &>(BufferOS).flush();
+    EXPECT_EQ("", Dest);
+    DestOS.flush();
+    EXPECT_EQ("", Dest);
+  }
+
+  // Write should land when constructor is called.
+  EXPECT_EQ("abcd", Dest);
+}
+
+TEST(buffer_ostreamTest, Owned) {
+  SmallString<128> Dest;
+  {
+    auto DestOS = std::make_unique<NaiveSmallVectorStream>(Dest);
+
+    // Confirm that NaiveSmallVectorStream is buffered by default.
+    EXPECT_NE(0u, DestOS->GetBufferSize());
+
+    // Confirm that passing ownership to buffer_unique_ostream sets it to
+    // unbuffered. Also steal a reference to DestOS.
+    NaiveSmallVectorStream &DestOSRef = *DestOS;
+    buffer_unique_ostream BufferOS(std::move(DestOS));
+    EXPECT_EQ(0u, DestOSRef.GetBufferSize());
+
+    // Writing and flushing should have no effect on Dest.
+    BufferOS << "abcd";
+    static_cast<raw_ostream &>(BufferOS).flush();
+    EXPECT_EQ("", Dest);
+    DestOSRef.flush();
+    EXPECT_EQ("", Dest);
+  }
+
+  // Write should land when constructor is called.
+  EXPECT_EQ("abcd", Dest);
+}
+
+} // namespace