[libc++][memory_resource] Applied [[nodiscard]] (#172134)

`[[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/mem.res

Towards #172124

---------

Co-authored-by: Hristo Hristov <zingam@outlook.com>
Co-authored-by: Nikolas Klauser <nikolasklauser@berlin.de>

libcxx/include/__memory_resource/memory_resource.h

@@ -42,7 +42,9 @@ public:
     do_deallocate(__p, __bytes, __align);
   }
 
-  _LIBCPP_HIDE_FROM_ABI bool is_equal(const memory_resource& __other) const noexcept { return do_is_equal(__other); }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI bool is_equal(const memory_resource& __other) const noexcept {
+    return do_is_equal(__other);
+  }
 
 private:
   virtual void* do_allocate(size_t, size_t)                       = 0;
@@ -68,7 +70,7 @@ operator!=(const memory_resource& __lhs, const memory_resource& __rhs) noexcept
 
 // [mem.res.global]
 
-[[__gnu__::__returns_nonnull__]] _LIBCPP_AVAILABILITY_PMR _LIBCPP_EXPORTED_FROM_ABI memory_resource*
+[[nodiscard, __gnu__::__returns_nonnull__]] _LIBCPP_AVAILABILITY_PMR _LIBCPP_EXPORTED_FROM_ABI memory_resource*
 get_default_resource() noexcept;
 
 [[__gnu__::__returns_nonnull__]] _LIBCPP_AVAILABILITY_PMR _LIBCPP_EXPORTED_FROM_ABI memory_resource*

libcxx/include/__memory_resource/monotonic_buffer_resource.h

@@ -93,7 +93,7 @@ public:
     }
   }
 
-  _LIBCPP_HIDE_FROM_ABI memory_resource* upstream_resource() const { return __res_; }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI memory_resource* upstream_resource() const { return __res_; }
 
 protected:
   void* do_allocate(size_t __bytes, size_t __alignment) override; // key function

libcxx/include/__memory_resource/polymorphic_allocator.h

@@ -173,11 +173,13 @@ public:
     __p->~_Tp();
   }
 
-  _LIBCPP_HIDE_FROM_ABI polymorphic_allocator select_on_container_copy_construction() const noexcept {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI polymorphic_allocator select_on_container_copy_construction() const noexcept {
     return polymorphic_allocator();
   }
 
-  [[__gnu__::__returns_nonnull__]] _LIBCPP_HIDE_FROM_ABI memory_resource* resource() const noexcept { return __res_; }
+  [[nodiscard, __gnu__::__returns_nonnull__]] _LIBCPP_HIDE_FROM_ABI memory_resource* resource() const noexcept {
+    return __res_;
+  }
 
   _LIBCPP_HIDE_FROM_ABI friend bool
   operator==(const polymorphic_allocator& __lhs, const polymorphic_allocator& __rhs) noexcept {

libcxx/include/__memory_resource/synchronized_pool_resource.h

@@ -56,9 +56,11 @@ public:
     __unsync_.release();
   }
 
-  _LIBCPP_HIDE_FROM_ABI memory_resource* upstream_resource() const { return __unsync_.upstream_resource(); }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI memory_resource* upstream_resource() const {
+    return __unsync_.upstream_resource();
+  }
 
-  _LIBCPP_HIDE_FROM_ABI pool_options options() const { return __unsync_.options(); }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI pool_options options() const { return __unsync_.options(); }
 
 protected:
   _LIBCPP_HIDE_FROM_ABI_VIRTUAL void* do_allocate(size_t __bytes, size_t __align) override {

libcxx/include/__memory_resource/unsynchronized_pool_resource.h

@@ -76,7 +76,7 @@ public:
 
   void release();
 
-  _LIBCPP_HIDE_FROM_ABI memory_resource* upstream_resource() const { return __res_; }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI memory_resource* upstream_resource() const { return __res_; }
 
   [[__gnu__::__pure__]] pool_options options() const;
 

libcxx/test/libcxx/diagnostics/memory_resource.nodiscard.verify.cpp

@@ -1,25 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// 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
-//
-//===----------------------------------------------------------------------===//
-
-// UNSUPPORTED: c++03, c++11, c++14
-
-// check that <memory_resource> functions are marked [[nodiscard]]
-
-// clang-format off
-
-#include <memory_resource>
-
-#include "test_macros.h"
-
-void test() {
-  std::pmr::memory_resource* resource = std::pmr::null_memory_resource();
-  resource->allocate(1); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
-
-  std::pmr::polymorphic_allocator<int> polymorphic_allocator;
-  polymorphic_allocator.allocate(1); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
-}

libcxx/test/libcxx/mem/mem.res/nodiscard.verify.cpp

@@ -6,20 +6,66 @@
 //
 //===----------------------------------------------------------------------===//
 
-// UNSUPPORTED: c++03, c++11, c++14
+// REQUIRES: std-at-least-c++17
 
-// check that functions are marked [[nodiscard]] as an extension in C++17
-
-// [[nodiscard]] std::pmr::memory_resource::allocate(size_t, size_t);
-// [[nodiscard]] std::pmr::polymorphic_allocator<T>::allocate(size_t, size_t);
+// check that <memory_resource> functions are marked [[nodiscard]]
 
 #include <memory_resource>
 
-void f() {
-  std::pmr::memory_resource* res = nullptr;
-  res->allocate(0);    // expected-warning {{ignoring return value of function}}
-  res->allocate(0, 1); // expected-warning {{ignoring return value of function}}
+#include "test_macros.h"
 
-  std::pmr::polymorphic_allocator<int> poly;
-  poly.allocate(0); // expected-warning {{ignoring return value of function}}
+void test() {
+  {
+    std::pmr::memory_resource* r = std::pmr::null_memory_resource();
+
+    // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    r->allocate(1);
+    // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    r->allocate(1, 1);
+    // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    r->is_equal(*r);
+
+    // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    std::pmr::get_default_resource();
+    // expected-warning@+1 {{ignoring return value of function declared with const attribute}}
+    std::pmr::new_delete_resource();
+    // expected-warning@+1 {{ignoring return value of function declared with const attribute}}
+    std::pmr::null_memory_resource();
+  }
+
+  {
+    std::pmr::monotonic_buffer_resource r;
+
+    // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    r.upstream_resource();
+  }
+
+  {
+    std::pmr::polymorphic_allocator<int> a;
+
+    // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    a.allocate(1);
+    // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    a.select_on_container_copy_construction();
+    // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    a.resource();
+  }
+
+  {
+    std::pmr::synchronized_pool_resource r;
+
+    // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    r.upstream_resource();
+    // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    r.options();
+  }
+
+  {
+    std::pmr::unsynchronized_pool_resource r;
+
+    // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    r.upstream_resource();
+    // expected-warning@+1 {{ignoring return value of function declared with pure attribute}}
+    r.options();
+  }
 }