[libc++][PMR] Add attributes

[philnik]

Summary

This allows the compiler to do more optimizations.

[philnik]

Try to fix CI

[philnik]

Fix diff

[philnik]

Try to fix CI

[ldionne]

We went over these attributes just now and I believe they are correctly applied. We also discussed how to test this, but since it's not observable in most cases (except those that produce a diagnostic), I think this is acceptable as-is.

[philnik]

Rebased

[philnik]

Rebased

[philnik]

Rebased; Fix CI

[philnik]

Next try

[philnik]

Next try

libcxx/include/__memory_resource/memory_resource.h

 _LIBCPP_DIAGNOSTIC_PUSH
 _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wweak-vtables") // TODO: move destructor into the dylib
 class _LIBCPP_TYPE_VIS memory_resource {
   static const size_t __max_align = alignof(max_align_t);
 
 public:
   virtual ~memory_resource() = default;
 
-  _LIBCPP_HIDE_FROM_ABI void* allocate(size_t __bytes, size_t __align = __max_align) {
+  [[using __gnu__: __returns_nonnull__, __alloc_size__(2), __alloc_align__(3)]] _LIBCPP_HIDE_FROM_ABI void*
+  allocate(size_t __bytes, size_t __align = __max_align) {
     return do_allocate(__bytes, __align);
   }
 
-  _LIBCPP_HIDE_FROM_ABI void deallocate(void* __p, size_t __bytes, size_t __align = __max_align) {
+  [[__gnu__::__nonnull__]] _LIBCPP_HIDE_FROM_ABI void
+  deallocate(void* __p, size_t __bytes, size_t __align = __max_align) {
     do_deallocate(__p, __bytes, __align);
   }
 
   _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;
   virtual void do_deallocate(void*, size_t, size_t)               = 0;
   virtual bool do_is_equal(memory_resource const&) const noexcept = 0;
 };
 _LIBCPP_DIAGNOSTIC_POP
 
 // [mem.res.eq]
 
 inline _LIBCPP_HIDE_FROM_ABI bool operator==(const memory_resource& __lhs, const memory_resource& __rhs) noexcept {
   return &__lhs == &__rhs || __lhs.is_equal(__rhs);
 }
 
 inline _LIBCPP_HIDE_FROM_ABI bool operator!=(const memory_resource& __lhs, const memory_resource& __rhs) noexcept {
   return !(__lhs == __rhs);
 }
 
 // [mem.res.global]
 
-_LIBCPP_AVAILABILITY_MEMORY_RESOURCE _LIBCPP_FUNC_VIS memory_resource* get_default_resource() noexcept;
-_LIBCPP_AVAILABILITY_MEMORY_RESOURCE _LIBCPP_FUNC_VIS memory_resource* set_default_resource(memory_resource*) noexcept;
-_LIBCPP_AVAILABILITY_MEMORY_RESOURCE _LIBCPP_FUNC_VIS memory_resource* new_delete_resource() noexcept;
-_LIBCPP_AVAILABILITY_MEMORY_RESOURCE _LIBCPP_FUNC_VIS memory_resource* null_memory_resource() noexcept;
+[[using __gnu__: __returns_nonnull__, __pure__]] _LIBCPP_AVAILABILITY_MEMORY_RESOURCE _LIBCPP_FUNC_VIS memory_resource*
+get_default_resource() noexcept;
+[[using __gnu__: __returns_nonnull__, __pure__]] _LIBCPP_AVAILABILITY_MEMORY_RESOURCE _LIBCPP_FUNC_VIS memory_resource*
+set_default_resource(memory_resource*) noexcept;
+[[using __gnu__: __returns_nonnull__, __const__]] _LIBCPP_AVAILABILITY_MEMORY_RESOURCE _LIBCPP_FUNC_VIS memory_resource*
+new_delete_resource() noexcept;
+[[using __gnu__: __returns_nonnull__, __const__]] _LIBCPP_AVAILABILITY_MEMORY_RESOURCE _LIBCPP_FUNC_VIS memory_resource*
+null_memory_resource() noexcept;
 
 } // namespace pmr
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_STD_VER > 14
 
 #endif // _LIBCPP___MEMORY_RESOURCE_MEMORY_RESOURCE_H

libcxx/include/__memory_resource/unsynchronized_pool_resource.h

_LIBCPP_HIDE_FROM_ABI explicit unsynchronized_pool_resource(const pool_options& __opts)
       : unsynchronized_pool_resource(__opts, get_default_resource()) {}
 
   unsynchronized_pool_resource(const unsynchronized_pool_resource&) = delete;
 
   _LIBCPP_HIDE_FROM_ABI ~unsynchronized_pool_resource() override { release(); }
 
   unsynchronized_pool_resource& operator=(const unsynchronized_pool_resource&) = delete;
 
-  _LIBCPP_AVAILABILITY_MEMORY_RESOURCE void release();
+  [[__gnu__::__pure__]] _LIBCPP_AVAILABILITY_MEMORY_RESOURCE void release();
 
   _LIBCPP_HIDE_FROM_ABI memory_resource* upstream_resource() const { return __res_; }
 
-  _LIBCPP_AVAILABILITY_MEMORY_RESOURCE pool_options options() const;
+  [[__gnu__::__pure__]] _LIBCPP_AVAILABILITY_MEMORY_RESOURCE pool_options options() const;
 
 protected:
   _LIBCPP_AVAILABILITY_MEMORY_RESOURCE void* do_allocate(size_t __bytes, size_t __align) override; // key function
 
   _LIBCPP_AVAILABILITY_MEMORY_RESOURCE void do_deallocate(void* __p, size_t __bytes, size_t __align) override;
 
   _LIBCPP_HIDE_FROM_ABI bool do_is_equal(const memory_resource& __other) const _NOEXCEPT override {
     return &__other == this;