[libcxx-commits] [libcxx] [libcxx] adds a size-based representation for `vector`'s unstable ABI (PR #155330)

[Christopher Di Bella via libcxx-commits]

================
@@ -82,25 +83,334 @@ _LIBCPP_PUSH_MACROS
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+#ifdef _LIBCPP_ABI_SIZE_BASED_VECTOR
+template <class _Vector, class _Tp, class _Allocator>
+class __vector_layout {
+public:
+  using value_type                      = _Tp;
+  using allocator_type                  = _Allocator;
+  using __alloc_traits _LIBCPP_NODEBUG  = allocator_traits<allocator_type>;
+  using reference                       = value_type&;
+  using const_reference                 = const value_type&;
+  using size_type                       = typename __alloc_traits::size_type;
+  using difference_type                 = typename __alloc_traits::difference_type;
+  using pointer                         = typename __alloc_traits::pointer;
+  using const_pointer                   = typename __alloc_traits::const_pointer;
+  using __split_buffer _LIBCPP_NODEBUG  = std::__split_buffer<_Tp, _Allocator&, __split_buffer_size_layout>;
+  using __sentinel_type _LIBCPP_NODEBUG = size_type;
+
+  _LIBCPP_HIDE_FROM_ABI __vector_layout() = default;
+
+  _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI explicit __vector_layout(allocator_type const& __a)
+      _NOEXCEPT_(is_nothrow_copy_constructible<allocator_type>::value)
+      : __alloc_(__a) {}
+
+  _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI explicit __vector_layout(allocator_type&& __a)
+      _NOEXCEPT_(is_nothrow_move_constructible<allocator_type>::value)
+      : __alloc_(std::move(__a)) {}
+
+  _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI __vector_layout(__vector_layout&& __other)
+      _NOEXCEPT_(is_nothrow_move_constructible<allocator_type>::value)
+      : __begin_(std::move(__other.__begin_)),
+        __size_(std::move(__other.__size_)),
+        __cap_(std::move(__other.__cap_)),
+        __alloc_(std::move(__other.__alloc_)) {
+    __other.__begin_ = nullptr;
+    __other.__size_  = 0;
+    __other.__cap_   = 0;
+  }
+
+  // Capacity
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI size_type size() const _NOEXCEPT {
+    return __size_;
+  }
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI size_type capacity() const _NOEXCEPT {
+    return __cap_;
+  }
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI bool empty() const _NOEXCEPT {
+    return __size_ == 0;
+  }
+
+  // Access
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI reference back() _NOEXCEPT {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!empty(), "back() called on an empty vector");
+    return __begin_[__size_ - 1];
+  }
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI const_reference back() const _NOEXCEPT {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!empty(), "back() called on an empty vector");
+    return __begin_[__size_ - 1];
+  }
+
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI allocator_type& __allocator_ref() _NOEXCEPT {
+    return __alloc_;
+  }
+
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI allocator_type const&
+  __allocator_ref() const _NOEXCEPT {
+    return __alloc_;
+  }
+
+  _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void swap(__vector_layout& __x)
----------------
cjdb wrote:

`__raw_sentinel` and friends return values, not references, so I don't think we can.

https://github.com/llvm/llvm-project/pull/155330