Introduce _LIBCPP_ABI_UNSTABLE to libc++

[mclow.lists]

Summary

First step towards letting vendors manage ABI changes in libc++.

Introduce the _LIBCPP_ABI_UNSTABLE macro, which turns on all the ABI changes.

Introduce the _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT macro, which controls the layout of std::string objects. This is a synonym for the _LIBCPP_ALTERNATE_STRING_LAYOUT macro.

[danalbert]

Will probably want to expose _LIBCPP_ABI_UNSTABLE as a cmake option.

[mclow.lists]

Added support for lit and CMAKE - thanks to ErikWF for these bits

[chandlerc]

FYI, as I commented on the original discussion, I'd personally like to see this have some versioning scheme before it goes in rather than a binary "unstable". Or some plan for getting there.

Otherwise this looks good as a first step.

[EricWF]

This is really old and should be closed.

include/__config

 #define __has_builtin(__x) 0
 #endif
 
+#ifdef _LIBCPP_ALTERNATE_STRING_LAYOUT
+#define _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
+#endif
+#ifdef _LIBCPP_ABI_UNSTABLE
+#define _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
+#endif
+
 #if __has_feature(cxx_explicit_conversions) || defined(__IBMCPP__)
 #   define _LIBCPP_EXPLICIT explicit
 #else

include/string

 #pragma warning( pop )
 #endif // _LIBCPP_MSVC
 
-#ifdef _LIBCPP_ALTERNATE_STRING_LAYOUT
+#ifdef _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
 template <class _CharT, size_t = sizeof(_CharT)>
 struct __padding

 {
 };
 
-#endif  // _LIBCPP_ALTERNATE_STRING_LAYOUT
+#endif  // _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
 template<class _CharT, class _Traits, class _Allocator>
 class _LIBCPP_TYPE_VIS_ONLY basic_string

 
 private:
 
-#ifdef _LIBCPP_ALTERNATE_STRING_LAYOUT
+#ifdef _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
     struct __long
     {

         value_type __data_[__min_cap];
     };
 
-#endif  // _LIBCPP_ALTERNATE_STRING_LAYOUT
+#endif  // _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
     union __ulx{__long __lx; __short __lxx;};
 

     const allocator_type& __alloc() const _NOEXCEPT
         {return __r_.second();}
 
-#ifdef _LIBCPP_ALTERNATE_STRING_LAYOUT
+#ifdef _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
     _LIBCPP_INLINE_VISIBILITY
     void __set_short_size(size_type __s) _NOEXCEPT

         {return __r_.first().__s.__size_;}
 #   endif
 
-#else  // _LIBCPP_ALTERNATE_STRING_LAYOUT
+#else  // _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
     _LIBCPP_INLINE_VISIBILITY
     void __set_short_size(size_type __s) _NOEXCEPT

         {return __r_.first().__s.__size_ >> 1;}
 #   endif
 
-#endif  // _LIBCPP_ALTERNATE_STRING_LAYOUT
+#endif  // _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
     _LIBCPP_INLINE_VISIBILITY
     void __set_long_size(size_type __s) _NOEXCEPT