[libc++] Move unused basic_string function definition to the dylib sources (llvm#126219)

`__init(const value_type*, size_type, size_type)` is part of our ABI,
but we don't actually use the function anymore in the dylib. THis moves
the definition to the `src/` directory to make it clear that the code is
unused. This also allows us to remove it entirely in the unstable ABI.

Author: philnik777

libcxx/include/__string/extern_template_lists.h

@@ -32,7 +32,6 @@
 #define _LIBCPP_STRING_V1_EXTERN_TEMPLATE_LIST(_Func, _CharType) \
   _Func(_LIBCPP_EXPORTED_FROM_ABI basic_string<_CharType>& basic_string<_CharType>::replace(size_type, size_type, value_type const*, size_type)) \
   _Func(_LIBCPP_EXPORTED_FROM_ABI basic_string<_CharType>::size_type basic_string<_CharType>::rfind(value_type const*, size_type, size_type) const) \
-  _Func(_LIBCPP_EXPORTED_FROM_ABI void basic_string<_CharType>::__init(value_type const*, size_type, size_type)) \
   _Func(_LIBCPP_EXPORTED_FROM_ABI basic_string<_CharType>::basic_string(basic_string const&)) \
   _Func(_LIBCPP_EXPORTED_FROM_ABI basic_string<_CharType>& basic_string<_CharType>::replace(size_type, size_type, value_type const*)) \
   _Func(_LIBCPP_EXPORTED_FROM_ABI basic_string<_CharType>::basic_string(basic_string const&, allocator<_CharType> const&)) \
@@ -82,7 +81,6 @@
 #define _LIBCPP_STRING_UNSTABLE_EXTERN_TEMPLATE_LIST(_Func, _CharType) \
   _Func(_LIBCPP_EXPORTED_FROM_ABI basic_string<_CharType>& basic_string<_CharType>::replace(size_type, size_type, value_type const*, size_type)) \
   _Func(_LIBCPP_EXPORTED_FROM_ABI basic_string<_CharType>::size_type basic_string<_CharType>::rfind(value_type const*, size_type, size_type) const) \
-  _Func(_LIBCPP_EXPORTED_FROM_ABI void basic_string<_CharType>::__init(value_type const*, size_type, size_type)) \
   _Func(_LIBCPP_EXPORTED_FROM_ABI basic_string<_CharType>& basic_string<_CharType>::replace(size_type, size_type, value_type const*)) \
   _Func(_LIBCPP_EXPORTED_FROM_ABI basic_string<_CharType>::size_type basic_string<_CharType>::find_last_not_of(value_type const*, size_type, size_type) const) \
   _Func(_LIBCPP_EXPORTED_FROM_ABI basic_string<_CharType>::~basic_string()) \

libcxx/include/string

@@ -2254,7 +2254,6 @@ private:
     return __guess;
   }
 
-  inline _LIBCPP_CONSTEXPR_SINCE_CXX20 void __init(const value_type* __s, size_type __sz, size_type __reserve);
   inline _LIBCPP_CONSTEXPR_SINCE_CXX20 void __init(const value_type* __s, size_type __sz);
   inline _LIBCPP_CONSTEXPR_SINCE_CXX20 void __init(size_type __n, value_type __c);
 
@@ -2439,6 +2438,12 @@ private:
   template <class _CharT2, class _Traits2, class _Allocator2>
   friend inline _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI bool
   operator==(const basic_string<_CharT2, _Traits2, _Allocator2>&, const _CharT2*) _NOEXCEPT;
+
+  // These functions aren't used anymore but are part of out ABI, so we need to provide them in the dylib for backwards
+  // compatibility
+#  ifdef _LIBCPP_BUILDING_LIBRARY
+  void __init(const value_type* __s, size_type __sz, size_type __reserve);
+#  endif
 };
 
 // These declarations must appear before any functions are implicitly used
@@ -2490,30 +2495,6 @@ basic_string(from_range_t, _Range&&, _Allocator = _Allocator())
     -> basic_string<ranges::range_value_t<_Range>, char_traits<ranges::range_value_t<_Range>>, _Allocator>;
 #  endif
 
-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())
-    __rep_ = __rep();
-  if (__reserve > max_size())
-    __throw_length_error();
-  pointer __p;
-  if (__fits_in_sso(__reserve)) {
-    __set_short_size(__sz);
-    __p = __get_short_pointer();
-  } else {
-    auto __allocation = std::__allocate_at_least(__alloc_, __recommend(__reserve) + 1);
-    __p               = __allocation.ptr;
-    __begin_lifetime(__p, __allocation.count);
-    __set_long_pointer(__p);
-    __set_long_cap(__allocation.count);
-    __set_long_size(__sz);
-  }
-  traits_type::copy(std::__to_address(__p), __s, __sz);
-  traits_type::assign(__p[__sz], value_type());
-  __annotate_new(__sz);
-}
-
 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) {

libcxx/src/string.cpp

@@ -37,6 +37,44 @@ void __basic_string_common<true>::__throw_out_of_range() const { std::__throw_ou
 
 #endif // _LIBCPP_ABI_DO_NOT_EXPORT_BASIC_STRING_COMMON
 
+// Define legacy ABI functions
+// ---------------------------
+
+#ifndef _LIBCPP_ABI_STRING_OPTIMIZED_EXTERNAL_INSTANTIATION
+
+template <class _CharT, class _Traits, class _Allocator>
+void basic_string<_CharT, _Traits, _Allocator>::__init(const value_type* __s, size_type __sz, size_type __reserve) {
+  if (__libcpp_is_constant_evaluated())
+    __rep_ = __rep();
+  if (__reserve > max_size())
+    __throw_length_error();
+  pointer __p;
+  if (__fits_in_sso(__reserve)) {
+    __set_short_size(__sz);
+    __p = __get_short_pointer();
+  } else {
+    auto __allocation = std::__allocate_at_least(__alloc_, __recommend(__reserve) + 1);
+    __p               = __allocation.ptr;
+    __begin_lifetime(__p, __allocation.count);
+    __set_long_pointer(__p);
+    __set_long_cap(__allocation.count);
+    __set_long_size(__sz);
+  }
+  traits_type::copy(std::__to_address(__p), __s, __sz);
+  traits_type::assign(__p[__sz], value_type());
+  __annotate_new(__sz);
+}
+
+#  define STRING_LEGACY_API(CharT)                                                                                     \
+    template _LIBCPP_EXPORTED_FROM_ABI void basic_string<CharT>::__init(const value_type*, size_type, size_type)
+
+STRING_LEGACY_API(char);
+#  if _LIBCPP_HAS_WIDE_CHARACTERS
+STRING_LEGACY_API(wchar_t);
+#  endif
+
+#endif // _LIBCPP_ABI_STRING_OPTIMIZED_EXTERNAL_INSTANTIATION
+
 #define _LIBCPP_EXTERN_TEMPLATE_DEFINE(...) template __VA_ARGS__;
 #ifdef _LIBCPP_ABI_STRING_OPTIMIZED_EXTERNAL_INSTANTIATION
 _LIBCPP_STRING_UNSTABLE_EXTERN_TEMPLATE_LIST(_LIBCPP_EXTERN_TEMPLATE_DEFINE, char)