[libcxx] Remove empty ~__no_destroy

Primary motivation is that after #84651 msan will
complain if fields accessed after ~__no_destroy.

Previously msan assumed that __obj_ will have own
destructor, were we will poison the field, but it never
happened before.

After #84651 msan will complain on any field access
after `~__no_destroy`.

As is Msan does validate fields destruction order for
classes with trivial destructor.

Additionally empty destructor will register __cxa_atexit with -O0.
https://gcc.godbolt.org/z/hce587b65

We can not remove the destructor with union where
_Tp can have non-trivial destructor.

But we can remove destructor if we use in-place
new instead of union.
https://gcc.godbolt.org/z/Yqxx57eEd - empty even with -O0.

Reviewers: #reviewers-libcxx

Pull Request: #89882

Author: vitalybuka

libcxx/include/__utility/no_destroy.h

@@ -12,6 +12,7 @@
 #include <__config>
 #include <__type_traits/is_constant_evaluated.h>
 #include <__utility/forward.h>
+#include <new>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -29,33 +30,23 @@ struct __uninitialized_tag {};
 // initialization using __emplace.
 template <class _Tp>
 struct __no_destroy {
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI explicit __no_destroy(__uninitialized_tag) : __dummy_() {
-    if (__libcpp_is_constant_evaluated()) {
-      __dummy_ = char();
-    }
-  }
-  _LIBCPP_HIDE_FROM_ABI ~__no_destroy() {
-    // nothing
-  }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR explicit __no_destroy(__uninitialized_tag) {}
 
   template <class... _Args>
-  _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI explicit __no_destroy(_Args&&... __args)
-      : __obj_(std::forward<_Args>(__args)...) {}
+  _LIBCPP_HIDE_FROM_ABI explicit __no_destroy(_Args&&... __args) {
+    ::new ((void*)__obj_) _Tp(std::forward<_Args>(__args)...);
+  }
 
   template <class... _Args>
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI _Tp& __emplace(_Args&&... __args) {
-    new (&__obj_) _Tp(std::forward<_Args>(__args)...);
-    return __obj_;
+  _LIBCPP_HIDE_FROM_ABI _Tp& __emplace(_Args&&... __args) {
+    return *(::new ((void*)__obj_) _Tp(std::forward<_Args>(__args)...));
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI _Tp& __get() { return __obj_; }
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI _Tp const& __get() const { return __obj_; }
+  _LIBCPP_HIDE_FROM_ABI _Tp& __get() { return *reinterpret_cast<_Tp*>(__obj_); }
+  _LIBCPP_HIDE_FROM_ABI _Tp const& __get() const { return *reinterpret_cast<const _Tp*>(__obj_); }
 
 private:
-  union {
-    _Tp __obj_;
-    char __dummy_; // so we can initialize a member even with __uninitialized_tag for constexpr-friendliness
-  };
+  _ALIGNAS_TYPE(_Tp) unsigned char __obj_[sizeof(_Tp)] = {};
 };
 
 _LIBCPP_END_NAMESPACE_STD

libcxx/test/libcxx/utilities/no_destroy.pass.cpp

@@ -0,0 +1,28 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 <__utility/no_destroy.h>
+#include <cassert>
+
+#include "test_macros.h"
+
+struct DestroyLast {
+  ~DestroyLast() { assert(*ptr == 5); }
+
+  int* ptr;
+} last;
+
+static std::__no_destroy<int> nd_int(5);
+
+void test() { last.ptr = &nd_int.__get(); }
+
+int main(int, char**) {
+  test();
+
+  return 0;
+}