diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -128,6 +128,7 @@
   __config
   __debug
   __errc
+  __format/buffer.h
   __format/format_arg.h
   __format/format_args.h
   __format/format_context.h
diff --git a/libcxx/include/__format/buffer.h b/libcxx/include/__format/buffer.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__format/buffer.h
@@ -0,0 +1,94 @@
+// -*- 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___FORMAT_BUFFER_H
+#define _LIBCPP___FORMAT_BUFFER_H
+
+#include <__config>
+#include <__format/formatter.h> // for __char_type TODO FMT Move the concept?
+#include <__iterator/concepts.h>
+#include <__utility/move.h>
+#include <cstddef>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 17
+
+// TODO FMT Remove this once we require compilers with proper C++20 support.
+// If the compiler has no concepts support, the format header will be disabled.
+// Without concepts support enable_if needs to be used and that too much effort
+// to support compilers with partial C++20 support.
+#if !defined(_LIBCPP_HAS_NO_CONCEPTS)
+
+namespace __format {
+
+/// The output iterator used for the @ref format_context.
+///
+/// In order to specialize the iterator only for the character type used it
+/// uses a type-erased function pointer to call the underlying buffer storage
+/// function.
+template <__formatter::__char_type _CharT>
+class _LIBCPP_TEMPLATE_VIS __output_iterator {
+public:
+  // No Legacy tag since the code isn't available prior to C++20.
+  using value_type = void;
+  using difference_type = ptrdiff_t;
+  using pointer = void;
+  using reference = void;
+
+  template <class _Tp>
+  _LIBCPP_HIDE_FROM_ABI explicit __output_iterator(_Tp* __obj)
+      : __fun_([](_CharT __c, void* __o) { static_cast<_Tp*>(__o)->put(__c); }),
+        __obj_(__obj) {}
+
+  _LIBCPP_HIDE_FROM_ABI __output_iterator& operator=(_CharT __c) {
+    __fun_(__c, __obj_);
+    return *this;
+  }
+  _LIBCPP_HIDE_FROM_ABI __output_iterator& operator*() { return *this; }
+  _LIBCPP_HIDE_FROM_ABI __output_iterator& operator++() { return *this; }
+  _LIBCPP_HIDE_FROM_ABI __output_iterator& operator++(int) { return *this; }
+
+private:
+  void (*__fun_)(_CharT, void*);
+  void* __obj_;
+};
+
+/// A "buffer" that directly writes to the supplied iterator.
+///
+/// This helper is specialized for all different output iterators used in the
+/// format function. Together with the @ref __output_iterator this offers a
+/// type erasure, reducing the number of formatter specializations.
+template <class _OutIt, __formatter::__char_type _CharT>
+requires(output_iterator<_OutIt, const _CharT&>) class _LIBCPP_TEMPLATE_VIS
+    __iterator_wrapper_buffer {
+public:
+  _LIBCPP_HIDE_FROM_ABI explicit __iterator_wrapper_buffer(_OutIt __it)
+      : __it_(_VSTD::move(__it)) {}
+
+  _LIBCPP_HIDE_FROM_ABI void put(_CharT __c) { *__it_++ = __c; }
+
+  _LIBCPP_HIDE_FROM_ABI _OutIt out() { return __it_; }
+
+private:
+  _OutIt __it_;
+};
+
+} // namespace __format
+
+#endif // !defined(_LIBCPP_HAS_NO_CONCEPTS)
+#endif //_LIBCPP_STD_VER > 17
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___FORMAT_BUFFER_H
diff --git a/libcxx/include/__format/format_context.h b/libcxx/include/__format/format_context.h
--- a/libcxx/include/__format/format_context.h
+++ b/libcxx/include/__format/format_context.h
@@ -12,12 +12,11 @@
 
 #include <__availability>
 #include <__config>
+#include <__format/buffer.h>
 #include <__format/format_args.h>
 #include <__format/format_fwd.h>
 #include <__iterator/concepts.h>
 #include <concepts>
-#include <iterator>
-#include <string>
 
 #ifndef _LIBCPP_HAS_NO_LOCALIZATION
 #include <locale>
@@ -145,9 +144,10 @@
 // storage by wrapping those in temporary objects with a uniform interface
 // (such as a span<charT>) and polymorphic reallocation. - end note]
 
-using format_context = basic_format_context<back_insert_iterator<string>, char>;
+using format_context =
+    basic_format_context<__format::__output_iterator<char>, char>;
 using wformat_context =
-    basic_format_context<back_insert_iterator<wstring>, wchar_t>;
+    basic_format_context<__format::__output_iterator<wchar_t>, wchar_t>;
 
 #endif // !defined(_LIBCPP_HAS_NO_CONCEPTS)
 
diff --git a/libcxx/include/format b/libcxx/include/format
--- a/libcxx/include/format
+++ b/libcxx/include/format
@@ -266,6 +266,7 @@
 
 #include <__config>
 #include <__debug>
+#include <__format/buffer.h>
 #include <__format/format_arg.h>
 #include <__format/format_args.h>
 #include <__format/format_context.h>
@@ -495,11 +496,15 @@
         _OutIt __out_it, basic_string_view<_CharT> __fmt,
         basic_format_args<basic_format_context<_FormatOutIt, _CharT>> __args) {
 
-  basic_string<_CharT> __str;
+  using _Buffer = __format::__iterator_wrapper_buffer<_OutIt, _CharT>;
+  _Buffer __buffer{_VSTD::move(__out_it)};
   _VSTD::__format::__vformat_to(
       basic_format_parse_context{__fmt, __args.__size()},
-      _VSTD::__format_context_create(_VSTD::back_inserter(__str), __args));
-  return _VSTD::copy_n(__str.begin(), __str.size(), _VSTD::move(__out_it));
+      _VSTD::__format_context_create(
+          _VSTD::__format::__output_iterator<_CharT>{
+              _VSTD::addressof(__buffer)},
+          __args));
+  return __buffer.out();
 }
 
 template <class _OutIt, class _CharT, class _FormatOutIt>
@@ -621,12 +626,15 @@
         _OutIt __out_it, locale __loc, basic_string_view<_CharT> __fmt,
         basic_format_args<basic_format_context<_FormatOutIt, _CharT>> __args) {
 
-  basic_string<_CharT> __str;
+  using _Buffer = __format::__iterator_wrapper_buffer<_OutIt, _CharT>;
+  _Buffer __buffer{_VSTD::move(__out_it)};
   _VSTD::__format::__vformat_to(
       basic_format_parse_context{__fmt, __args.__size()},
-      _VSTD::__format_context_create(_VSTD::back_inserter(__str), __args,
-                                     _VSTD::move(__loc)));
-  return _VSTD::copy_n(__str.begin(), __str.size(), _VSTD::move(__out_it));
+      _VSTD::__format_context_create(
+          _VSTD::__format::__output_iterator<_CharT>{
+              _VSTD::addressof(__buffer)},
+          __args, _VSTD::move(__loc)));
+  return __buffer.out();
 }
 
 template <class _OutIt, class _CharT, class _FormatOutIt>
diff --git a/libcxx/include/module.modulemap b/libcxx/include/module.modulemap
--- a/libcxx/include/module.modulemap
+++ b/libcxx/include/module.modulemap
@@ -438,6 +438,7 @@
         export optional
         export locale
       }
+      module buffer                 { private header "__format/buffer.h"                 }
       module format_error           { private header "__format/format_error.h"           }
       module format_fwd             { private header "__format/format_fwd.h"             }
       module format_parse_context   { private header "__format/format_parse_context.h"   }
diff --git a/libcxx/test/libcxx/diagnostics/detail.headers/format/buffer.module.verify.cpp b/libcxx/test/libcxx/diagnostics/detail.headers/format/buffer.module.verify.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/libcxx/diagnostics/detail.headers/format/buffer.module.verify.cpp
@@ -0,0 +1,16 @@
+// -*- 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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: modules-build
+
+// WARNING: This test was generated by 'generate_private_header_tests.py'
+// and should not be edited manually.
+
+// expected-error@*:* {{use of private header from outside its module: '__format/buffer.h'}}
+#include <__format/buffer.h>
diff --git a/libcxx/test/std/utilities/format/format.formatter/format.context/types.compile.pass.cpp b/libcxx/test/libcxx/utilities/format/format.formatter/format.context/types.compile.pass.cpp
rename from libcxx/test/std/utilities/format/format.formatter/format.context/types.compile.pass.cpp
rename to libcxx/test/libcxx/utilities/format/format.formatter/format.context/types.compile.pass.cpp
--- a/libcxx/test/std/utilities/format/format.formatter/format.context/types.compile.pass.cpp
+++ b/libcxx/test/libcxx/utilities/format/format.formatter/format.context/types.compile.pass.cpp
@@ -25,7 +25,6 @@
 // using wformat_context = basic_format_context<unspecified, wchar_t>;
 
 #include <format>
-#include <string>
 #include <string_view>
 #include <type_traits>
 
@@ -98,22 +97,15 @@
 }
 
 constexpr void test() {
-  test<std::back_insert_iterator<std::basic_string<char>>, char>();
-  test<std::back_insert_iterator<std::basic_string<wchar_t>>, wchar_t>();
-  test<std::back_insert_iterator<std::basic_string<char8_t>>, char8_t>();
-  test<std::back_insert_iterator<std::basic_string<char16_t>>, char16_t>();
-  test<std::back_insert_iterator<std::basic_string<char32_t>>, char32_t>();
+  test<std::__format::__output_iterator<char>, char>();
+  test<std::__format::__output_iterator<wchar_t>, wchar_t>();
 }
 
-static_assert(std::is_same_v<
-              std::format_context,
-              std::basic_format_context<
-                  std::back_insert_iterator<std::basic_string<char>>, char>>);
 static_assert(
-    std::is_same_v<
-        std::wformat_context,
-        std::basic_format_context<
-            std::back_insert_iterator<std::basic_string<wchar_t>>, wchar_t>>);
-
-// Required for MSVC internal test runner compatibility.
-int main(int, char**) { return 0; }
+    std::is_same_v<std::format_context,
+                   std::basic_format_context<
+                       std::__format::__output_iterator<char>, char>>);
+static_assert(
+    std::is_same_v<std::wformat_context,
+                   std::basic_format_context<
+                       std::__format::__output_iterator<wchar_t>, wchar_t>>);