[libc++] Rename __construct_one_at_end to __emplace_back_assume_capacity

Also, simplify its implementation to avoid using _ConstructTransaction,
which has a confusing interface and doesn't provide much value in this
case. Indeed, if an exception is thrown when we construct the single
element at the end, we simply don't need to call __annotate_increase.

Author: ldionne

libcxx/include/__vector/vector.h

@@ -461,6 +461,17 @@ class _LIBCPP_TEMPLATE_VIS vector {
   emplace_back(_Args&&... __args);
 #endif
 
+  template <class... _Args>
+  _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void __emplace_back_assume_capacity(_Args&&... __args) {
+    _LIBCPP_ASSERT_INTERNAL(
+        size() < capacity(), "We assume that we have enough space to insert an element at the end of the vector");
+    __alloc_traits::construct(this->__alloc_, std::__to_address(this->__end_), std::forward<_Args>(__args)...);
+    ++this->__end_;
+#if _LIBCPP_HAS_ASAN
+    __annotate_increase(1);
+#endif
+  }
+
 #if _LIBCPP_STD_VER >= 23
   template <_ContainerCompatibleRange<_Tp> _Range>
   _LIBCPP_HIDE_FROM_ABI constexpr void append_range(_Range&& __range) {
@@ -758,13 +769,6 @@ class _LIBCPP_TEMPLATE_VIS vector {
     _ConstructTransaction& operator=(_ConstructTransaction const&) = delete;
   };
 
-  template <class... _Args>
-  _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void __construct_one_at_end(_Args&&... __args) {
-    _ConstructTransaction __tx(*this, 1);
-    __alloc_traits::construct(this->__alloc_, std::__to_address(__tx.__pos_), std::forward<_Args>(__args)...);
-    ++__tx.__pos_;
-  }
-
   _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void __base_destruct_at_end(pointer __new_last) _NOEXCEPT {
     pointer __soon_to_be_end = this->__end_;
     while (__new_last != __soon_to_be_end)
@@ -1152,7 +1156,7 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 inline
     vector<_Tp, _Allocator>::emplace_back(_Args&&... __args) {
   pointer __end = this->__end_;
   if (__end < this->__cap_) {
-    __construct_one_at_end(std::forward<_Args>(__args)...);
+    __emplace_back_assume_capacity(std::forward<_Args>(__args)...);
     ++__end;
   } else {
     __end = __emplace_back_slow_path(std::forward<_Args>(__args)...);
@@ -1206,7 +1210,7 @@ vector<_Tp, _Allocator>::insert(const_iterator __position, const_reference __x)
   pointer __p = this->__begin_ + (__position - begin());
   if (this->__end_ < this->__cap_) {
     if (__p == this->__end_) {
-      __construct_one_at_end(__x);
+      __emplace_back_assume_capacity(__x);
     } else {
       __move_range(__p, this->__end_, __p + 1);
       const_pointer __xr = pointer_traits<const_pointer>::pointer_to(__x);
@@ -1228,7 +1232,7 @@ vector<_Tp, _Allocator>::insert(const_iterator __position, value_type&& __x) {
   pointer __p = this->__begin_ + (__position - begin());
   if (this->__end_ < this->__cap_) {
     if (__p == this->__end_) {
-      __construct_one_at_end(std::move(__x));
+      __emplace_back_assume_capacity(std::move(__x));
     } else {
       __move_range(__p, this->__end_, __p + 1);
       *__p = std::move(__x);
@@ -1248,7 +1252,7 @@ vector<_Tp, _Allocator>::emplace(const_iterator __position, _Args&&... __args) {
   pointer __p = this->__begin_ + (__position - begin());
   if (this->__end_ < this->__cap_) {
     if (__p == this->__end_) {
-      __construct_one_at_end(std::forward<_Args>(__args)...);
+      __emplace_back_assume_capacity(std::forward<_Args>(__args)...);
     } else {
       __temp_value<value_type, _Allocator> __tmp(this->__alloc_, std::forward<_Args>(__args)...);
       __move_range(__p, this->__end_, __p + 1);
@@ -1300,7 +1304,7 @@ vector<_Tp, _Allocator>::__insert_with_sentinel(const_iterator __position, _Inpu
   pointer __p           = this->__begin_ + __off;
   pointer __old_last    = this->__end_;
   for (; this->__end_ != this->__cap_ && __first != __last; ++__first)
-    __construct_one_at_end(*__first);
+    __emplace_back_assume_capacity(*__first);
 
   if (__first == __last)
     (void)std::rotate(__p, __old_last, this->__end_);