[libc++] Apply [[nodiscard]] to in/out_ptr (#167097)

...according to Coding Guidelines: `[[nodiscard]]` should be applied to
functions where discarding the return value is most likely a correctness
issue.

Changes to:
  - [x] `inout_ptr()` 
  - [x] `out_ptr()` 


At the time of impelentation the `[[nodiscard]]` policy has not been
established yet.

---------

Co-authored-by: Hristo Hristov <[email protected]>

Author: H-G-Hristov

libcxx/include/__memory/inout_ptr.h

@@ -96,7 +96,7 @@ class inout_ptr_t {
 };
 
 template <class _Pointer = void, class _Smart, class... _Args>
-_LIBCPP_HIDE_FROM_ABI auto inout_ptr(_Smart& __s, _Args&&... __args) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI auto inout_ptr(_Smart& __s, _Args&&... __args) {
   using _Ptr = conditional_t<is_void_v<_Pointer>, __pointer_of_t<_Smart>, _Pointer>;
   return std::inout_ptr_t<_Smart, _Ptr, _Args&&...>(__s, std::forward<_Args>(__args)...);
 }

libcxx/include/__memory/out_ptr.h

@@ -88,7 +88,7 @@ class out_ptr_t {
 };
 
 template <class _Pointer = void, class _Smart, class... _Args>
-_LIBCPP_HIDE_FROM_ABI auto out_ptr(_Smart& __s, _Args&&... __args) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI auto out_ptr(_Smart& __s, _Args&&... __args) {
   using _Ptr = conditional_t<is_void_v<_Pointer>, __pointer_of_t<_Smart>, _Pointer>;
   return std::out_ptr_t<_Smart, _Ptr, _Args&&...>(__s, std::forward<_Args>(__args)...);
 }

libcxx/test/libcxx/utilities/smartptr/nodiscard.verify.cpp

@@ -0,0 +1,29 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: std-at-least-c++23
+
+// <memory>
+
+// Check that functions are marked [[nodiscard]]
+
+#include <memory>
+
+#include "test_macros.h"
+
+void test() {
+#if TEST_STD_VER >= 23
+  {
+    std::unique_ptr<int> uPtr;
+    // [inout.ptr]
+    std::inout_ptr(uPtr); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+    // [out.ptr]
+    std::out_ptr(uPtr); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  }
+#endif
+}