[libc++[[ranges] Applied [[nodiscard]] to as_rvalue_view

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

- https://libcxx.llvm.org/CodingGuidelines.html
- https://wg21.link/ranges
- https://wg21.link/range.as.rvalue

Towards #172124

Author: H-G-Hristov

libcxx/include/__ranges/as_rvalue_view.h

@@ -48,27 +48,27 @@ class as_rvalue_view : public view_interface<as_rvalue_view<_View>> {
 
   _LIBCPP_HIDE_FROM_ABI constexpr explicit as_rvalue_view(_View __base) : __base_(std::move(__base)) {}
 
-  _LIBCPP_HIDE_FROM_ABI constexpr _View base() const&
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _View base() const&
     requires copy_constructible<_View>
   {
     return __base_;
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr _View base() && { return std::move(__base_); }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _View base() && { return std::move(__base_); }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr auto begin()
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto begin()
     requires(!__simple_view<_View>)
   {
     return move_iterator(ranges::begin(__base_));
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr auto begin() const
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto begin() const
     requires range<const _View>
   {
     return move_iterator(ranges::begin(__base_));
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr auto end()
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto end()
     requires(!__simple_view<_View>)
   {
     if constexpr (common_range<_View>) {
@@ -78,7 +78,7 @@ class as_rvalue_view : public view_interface<as_rvalue_view<_View>> {
     }
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr auto end() const
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto end() const
     requires range<const _View>
   {
     if constexpr (common_range<const _View>) {
@@ -88,13 +88,13 @@ class as_rvalue_view : public view_interface<as_rvalue_view<_View>> {
     }
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr auto size()
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto size()
     requires sized_range<_View>
   {
     return ranges::size(__base_);
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr auto size() const
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto size() const
     requires sized_range<const _View>
   {
     return ranges::size(__base_);

libcxx/test/libcxx/ranges/range.adaptors/range.as_rvalue_view/nodiscard.verify.cpp

@@ -0,0 +1,63 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// Check that functions are marked [[nodiscard]]
+
+#include <ranges>
+#include <utility>
+
+#include "test_range.h"
+
+struct NonSimpleView : std::ranges::view_base {
+  int* begin() const;
+  int* end() const;
+
+  const int* begin();
+  const int* end();
+
+  constexpr std::size_t size() { return 0; };
+};
+static_assert(!simple_view<NonSimpleView>);
+
+void test() {
+  NonSimpleView range;
+
+  auto v = std::views::as_rvalue(range);
+
+  // [range.as.rvalue.view]
+
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  v.base();
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::move(v).base();
+
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  v.begin();
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::as_const(v).begin();
+
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  v.end();
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::as_const(v).end();
+
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  v.size();
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::as_const(v).size();
+
+  // [range.as.rvalue.overview]
+
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::views::as_rvalue(range);
+
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::views::as_rvalue(v);
+}