[libc++][NFC] Copy the whole union instead of a member; also remove __zero()

[philnik]

Summary

This doesn't affect code-gen

[ldionne]

@philnik tells me he looked at the codegen for these functions as explicitly instantiated in the dylib and it didn't make a difference.

libcxx/include/string

#endif // _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
     // don't initialize the characters. The contents of the string, including the null terminator, must be
     // initialized separately.
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
     explicit basic_string(__uninitialized_size_tag, size_type __size, const allocator_type& __a)
             : __r_(__default_init_tag(), __a) {
         if (__size > max_size())
             __throw_length_error();
         if (__fits_in_sso(__size)) {
-            __zero();
+            __r_.first() = __rep();
             __set_short_size(__size);
         } else {
             auto __capacity = __recommend(__size) + 1;
             auto __allocation = __alloc_traits::allocate(__alloc(), __capacity);
             __begin_lifetime(__allocation, __capacity);
             __set_long_cap(__capacity);
             __set_long_pointer(__allocation);
             __set_long_size(__size);
#if _LIBCPP_STD_VER > 17
         }
 #else
         (void)__begin;
         (void)__n;
 #endif // _LIBCPP_STD_VER > 17
     }
 
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __default_init() {
-        __zero();
+        __r_.first() = __rep();
         if (__libcpp_is_constant_evaluated()) {
             size_type __sz = __recommend(0) + 1;
             pointer __ptr = __alloc_traits::allocate(__alloc(), __sz);
             __begin_lifetime(__ptr, __sz);
             __set_long_pointer(__ptr);
             __set_long_cap(__sz);
             __set_long_size(0);
         }
const_pointer __get_short_pointer() const _NOEXCEPT
         {return pointer_traits<const_pointer>::pointer_to(__r_.first().__s.__data_[0]);}
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
     pointer __get_pointer() _NOEXCEPT
         {return __is_long() ? __get_long_pointer() : __get_short_pointer();}
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
     const_pointer __get_pointer() const _NOEXCEPT
         {return __is_long() ? __get_long_pointer() : __get_short_pointer();}
 
-    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
-    void __zero() _NOEXCEPT {
-        __r_.first() = __rep();
-    }
-
     template <size_type __a> static
         _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
         size_type __align_it(size_type __s) _NOEXCEPT
             {return (__s + (__a-1)) & ~(__a-1);}
     enum {__alignment = 16};
     static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
     size_type __recommend(size_type __s) _NOEXCEPT
     {

 
 template <class _CharT, class _Traits, class _Allocator>
 _LIBCPP_CONSTEXPR_SINCE_CXX20
 void basic_string<_CharT, _Traits, _Allocator>::__init(const value_type* __s,
                                                        size_type __sz,
                                                        size_type __reserve)
 {
     if (__libcpp_is_constant_evaluated())
-        __zero();
+        __r_.first() = __rep();
     if (__reserve > max_size())
         __throw_length_error();
     pointer __p;
     if (__fits_in_sso(__reserve))
     {
         __set_short_size(__sz);
         __p = __get_short_pointer();
     }

 }
 
 template <class _CharT, class _Traits, class _Allocator>
 _LIBCPP_CONSTEXPR_SINCE_CXX20
 void
 basic_string<_CharT, _Traits, _Allocator>::__init(const value_type* __s, size_type __sz)
 {
     if (__libcpp_is_constant_evaluated())
-        __zero();
+        __r_.first() = __rep();
     if (__sz > max_size())
         __throw_length_error();
     pointer __p;
     if (__fits_in_sso(__sz))
     {
         __set_short_size(__sz);
         __p = __get_short_pointer();
     }

 }
 
 template <class _CharT, class _Traits, class _Allocator>
 _LIBCPP_CONSTEXPR_SINCE_CXX20
 basic_string<_CharT, _Traits, _Allocator>::basic_string(const basic_string& __str)
     : __r_(__default_init_tag(), __alloc_traits::select_on_container_copy_construction(__str.__alloc()))
 {
     if (!__str.__is_long())
-        __r_.first().__r = __str.__r_.first().__r;
+        __r_.first() = __str.__r_.first();
     else
         __init_copy_ctor_external(std::__to_address(__str.__get_long_pointer()),
                                   __str.__get_long_size());
     std::__debug_db_insert_c(this);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 _LIBCPP_CONSTEXPR_SINCE_CXX20
 basic_string<_CharT, _Traits, _Allocator>::basic_string(
     const basic_string& __str, const allocator_type& __a)
     : __r_(__default_init_tag(), __a)
 {
     if (!__str.__is_long())
-        __r_.first().__r = __str.__r_.first().__r;
+        __r_.first() = __str.__r_.first();
     else
         __init_copy_ctor_external(std::__to_address(__str.__get_long_pointer()),
                                   __str.__get_long_size());
     std::__debug_db_insert_c(this);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 _LIBCPP_CONSTEXPR_SINCE_CXX20
 void basic_string<_CharT, _Traits, _Allocator>::__init_copy_ctor_external(
     const value_type* __s, size_type __sz) {
   if (__libcpp_is_constant_evaluated())
-    __zero();
+    __r_.first() = __rep();
+
   pointer __p;
   if (__fits_in_sso(__sz)) {
     __p = __get_short_pointer();
     __set_short_size(__sz);
   } else {
     if (__sz > max_size())
       __throw_length_error();
     auto __allocation = std::__allocate_at_least(__alloc(), __recommend(__sz) + 1);

 inline _LIBCPP_CONSTEXPR_SINCE_CXX20
 basic_string<_CharT, _Traits, _Allocator>::basic_string(basic_string&& __str, const allocator_type& __a)
     : __r_(__default_init_tag(), __a)
 {
     if (__str.__is_long() && __a != __str.__alloc()) // copy, not move
         __init(std::__to_address(__str.__get_long_pointer()), __str.__get_long_size());
     else
     {
-        if (__libcpp_is_constant_evaluated()) {
-            __zero();
-            __r_.first().__l = __str.__r_.first().__l;
-        } else {
-            __r_.first().__r = __str.__r_.first().__r;
-        }
+        if (__libcpp_is_constant_evaluated())
+            __r_.first() = __rep();
+        __r_.first() = __str.__r_.first();
         __str.__default_init();
     }
     std::__debug_db_insert_c(this);
     if (__is_long())
         std::__debug_db_swap(this, &__str);
 }
 
 #endif // _LIBCPP_CXX03_LANG
 
 template <class _CharT, class _Traits, class _Allocator>
 _LIBCPP_CONSTEXPR_SINCE_CXX20
 void
 basic_string<_CharT, _Traits, _Allocator>::__init(size_type __n, value_type __c)
 {
     if (__libcpp_is_constant_evaluated())
-        __zero();
+        __r_.first() = __rep();
+
     if (__n > max_size())
         __throw_length_error();
     pointer __p;
     if (__fits_in_sso(__n))
     {
         __set_short_size(__n);
         __p = __get_short_pointer();
     }

 _LIBCPP_CONSTEXPR_SINCE_CXX20
 __enable_if_t
 <
     __is_cpp17_forward_iterator<_ForwardIterator>::value
 >
 basic_string<_CharT, _Traits, _Allocator>::__init(_ForwardIterator __first, _ForwardIterator __last)
 {
     if (__libcpp_is_constant_evaluated())
-        __zero();
+        __r_.first() = __rep();
     size_type __sz = static_cast<size_type>(std::distance(__first, __last));
     if (__sz > max_size())
         __throw_length_error();
     pointer __p;
     if (__fits_in_sso(__sz))
     {
         __set_short_size(__sz);
         __p = __get_short_pointer();

 _LIBCPP_CONSTEXPR_SINCE_CXX20
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::operator=(const basic_string& __str)
 {
   if (this != &__str) {
     __copy_assign_alloc(__str);
     if (!__is_long()) {
       if (!__str.__is_long()) {
-        __r_.first().__r = __str.__r_.first().__r;
+        __r_.first() = __str.__r_.first();
       } else {
         return __assign_no_alias<true>(__str.data(), __str.size());
       }
     } else {
       return __assign_no_alias<false>(__str.data(), __str.size());
     }
   }
   return *this;