[libcxx] [llvm] [libc++] Remove `get_temporary_buffer`/`return_temporary_buffer` (PR #100914)

[Nico Weber via llvm-commits]

================
@@ -0,0 +1,92 @@
+// -*- 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___MEMORY_UNIQUE_TEMPORARY_BUFFER_H
+#define _LIBCPP___MEMORY_UNIQUE_TEMPORARY_BUFFER_H
+
+#include <__assert>
+#include <__config>
+
+#include <__memory/allocator.h>
+#include <__memory/unique_ptr.h>
+#include <__type_traits/is_constant_evaluated.h>
+#include <cstddef>
+#include <new>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _Tp>
+struct __temporary_buffer_deleter {
+  ptrdiff_t __count_; // ignored in non-constant evaluation
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR __temporary_buffer_deleter() _NOEXCEPT : __count_(0) {}
+  _LIBCPP_HIDE_FROM_ABI
+  _LIBCPP_CONSTEXPR explicit __temporary_buffer_deleter(ptrdiff_t __count) _NOEXCEPT : __count_(__count) {}
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 void operator()(_Tp* __ptr) _NOEXCEPT {
+    if (__libcpp_is_constant_evaluated()) {
+      allocator<_Tp>().deallocate(__ptr, __count_);
+      return;
+    }
+
+    std::__libcpp_deallocate_unsized((void*)__ptr, _LIBCPP_ALIGNOF(_Tp));
+  }
+};
+
+template <class _Tp>
+using __unique_temporary_buffer = unique_ptr<_Tp, __temporary_buffer_deleter<_Tp> >;
+
+template <class _Tp>
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 __unique_temporary_buffer<_Tp>
+__allocate_unique_temporary_buffer(ptrdiff_t __count) {
+  using __deleter_type       = __temporary_buffer_deleter<_Tp>;
+  using __unique_buffer_type = __unique_temporary_buffer<_Tp>;
+
+  if (__libcpp_is_constant_evaluated()) {
+    return __unique_buffer_type(allocator<_Tp>().allocate(__count), __deleter_type(__count));
+  }
+
+  _Tp* __ptr = nullptr;
+  const ptrdiff_t __max_count =
+      (~ptrdiff_t(0) ^ ptrdiff_t(ptrdiff_t(1) << (sizeof(ptrdiff_t) * __CHAR_BIT__ - 1))) / sizeof(_Tp);
+  if (__count > __max_count)
+    __count = __max_count;
+  while (__count > 0) {
+#if !defined(_LIBCPP_HAS_NO_ALIGNED_ALLOCATION)
+    if (__is_overaligned_for_new(_LIBCPP_ALIGNOF(_Tp))) {
+      align_val_t __al = align_val_t(_LIBCPP_ALIGNOF(_Tp));
+      __ptr            = static_cast<_Tp*>(::operator new(__count * sizeof(_Tp), __al, nothrow));
+    } else {
+      __ptr = static_cast<_Tp*>(::operator new(__count * sizeof(_Tp), nothrow));
----------------
nico wrote:

Have a `void*` for the uninitialized tail and a `_Tp*` for the initialized head, I suppose? Not sure :)

https://github.com/llvm/llvm-project/pull/100914