[libc++][format][6/6] Improve vformat.

[Mordante]

Summary

Directly use the string's insert operation instead of using a
back_insert_iterator wrapper. With the other changes in this series the
back_insert_iterator will merely be a wrapper to select the strings
insert operation.

Depends on D110499

[Mordante]

Needs to be updated due to the removal of __put in D110495.

[Mordante]

Rebased and adjusted to changes in __put.

[Mordante]

Rebased to test with wchar_t changes.

[vitaut]

LGTM

[vitaut]

libcxx/include/format

564

Why pass a pointer rather than a reference (and take a pointer in the ctor)?

[Mordante]

This patch is outdated and no longer really needed.

libcxx/include/__format/buffer.h

 // -*- 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
 //
 //===----------------------------------------------------------------------===//
 

 class _LIBCPP_TEMPLATE_VIS __container_buffer {
 public:
   using _CharT = typename _Container::value_type;
 
   _LIBCPP_HIDE_FROM_ABI explicit __container_buffer(
       back_insert_iterator<_Container> __it)
       : __container_(__it.__get_container()) {}
 
+  _LIBCPP_HIDE_FROM_ABI explicit __container_buffer(_Container* __container)
+      : __container_(__container) {}
+
   _LIBCPP_HIDE_FROM_ABI void put(_CharT __c) {
     *__buffer_it_++ = __c;
     // Profiling showed flushing after adding is more efficient than flushing
     // when entering the function.
     if (__buffer_it_ == __buffer_.end())
       __flush();
   }
 

libcxx/include/format

format_to(_OutIt __out_it, wstring_view __fmt, const _Args&... __args) {
   return _VSTD::vformat_to(_VSTD::move(__out_it), __fmt,
                            _VSTD::make_wformat_args(__args...));
 }
 #endif
 
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT string
 vformat(string_view __fmt, format_args __args) {
   string __res;
-  _VSTD::vformat_to(_VSTD::back_inserter(__res), __fmt, __args);
+  using _Buffer = typename __format::__container_buffer<std::string>;
+  _Buffer __buffer{&__res};
+  _VSTD::__vformat_to(
+      __format::__output_iterator<char>{_VSTD::addressof(__buffer)}, __fmt,
+      __args);
+  __buffer.out();
   return __res;
 }
 
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT wstring
 vformat(wstring_view __fmt, wformat_args __args) {
+  using _Buffer = typename __format::__container_buffer<std::wstring>;
   wstring __res;
-  _VSTD::vformat_to(_VSTD::back_inserter(__res), __fmt, __args);
+  _Buffer __buffer{&__res};
+  _VSTD::__vformat_to(
+      __format::__output_iterator<wchar_t>{_VSTD::addressof(__buffer)}, __fmt,
+      __args);
+  __buffer.out();
   return __res;
 }
 #endif
 
 template <class... _Args>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT string
 format(string_view __fmt, const _Args&... __args) {
   return _VSTD::vformat(__fmt, _VSTD::make_format_args(__args...));
_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT _OutIt format_to(
     _OutIt __out_it, locale __loc, wstring_view __fmt, const _Args&... __args) {
   return _VSTD::vformat_to(_VSTD::move(__out_it), _VSTD::move(__loc), __fmt,
                            _VSTD::make_wformat_args(__args...));
 }
 #endif
 
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT string
 vformat(locale __loc, string_view __fmt, format_args __args) {
+  using _Buffer = typename __format::__container_buffer<std::string>;
   string __res;
-  _VSTD::vformat_to(_VSTD::back_inserter(__res), _VSTD::move(__loc), __fmt,
-                    __args);
+  _Buffer __buffer{&__res};
+  _VSTD::__vformat_to(
+      __format::__output_iterator<char>{_VSTD::addressof(__buffer)},
+      _VSTD::move(__loc), __fmt, __args);
+  __buffer.out();
   return __res;
 }
 
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT wstring
 vformat(locale __loc, wstring_view __fmt, wformat_args __args) {
+  using _Buffer = typename __format::__container_buffer<std::wstring>;
   wstring __res;
-  _VSTD::vformat_to(_VSTD::back_inserter(__res), _VSTD::move(__loc), __fmt,
-                    __args);
+  _Buffer __buffer{&__res};
+  _VSTD::__vformat_to(
+      __format::__output_iterator<wchar_t>{_VSTD::addressof(__buffer)},
+      _VSTD::move(__loc), __fmt, __args);
+  __buffer.out();
   return __res;
 }
 #endif
 
 template <class... _Args>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT string
 format(locale __loc, string_view __fmt, const _Args&... __args) {
   return _VSTD::vformat(_VSTD::move(__loc), __fmt,