[libc++][format] Adds availability macros for std::format.

[Mordante]

Summary

This prevents std::format to be available until there's an ABI stable
version. (This only impacts the Apple platform.)

Depends on D102703

[Mordante]

Poke CI.

[ldionne]

libcxx/include/__availability

145

Are there any feature test macros associated to this feature? If so, we'll need to disable the feature test macros when we disable the format feature (otherwise, the feature test macro will pretend that std::format is available, but one will get an availability error due to the attribute when trying to use it).

[Mordante]

libcxx/include/__availability

145

Yes it's depends on __cpp_lib_format, which is currently marked as not implemented. I'll update the patch.

[Mordante]

Properly disable std::format when the availablity macro requires it.

[ldionne]

Thanks!

libcxx/include/__availability

#   define _LIBCPP_AVAILABILITY_TO_CHARS
     // which requires shared library support for various operations
     // (see libcxx/src/atomic.cpp).
 #   define _LIBCPP_AVAILABILITY_SYNC
 // #   define _LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait
 // #   define _LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_barrier
 // #   define _LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_latch
 // #   define _LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_semaphore
 
+    // This controls the availability of the C++20 format library.
+    // The library is in development and not ABI stable yet. Currently
+    // P2216 is aiming to be retroactively accepted in C++20. This paper
+    // contains ABI breaking changes.
+#   define _LIBCPP_AVAILABILITY_FORMAT
+
 #elif defined(__APPLE__)
 
 #   define _LIBCPP_AVAILABILITY_SHARED_MUTEX                                    \
         __attribute__((availability(macosx,strict,introduced=10.12)))           \
         __attribute__((availability(ios,strict,introduced=10.0)))               \
         __attribute__((availability(tvos,strict,introduced=10.0)))              \
         __attribute__((availability(watchos,strict,introduced=3.0)))
 #   if (defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ < 101200) ||    \
#   define _LIBCPP_AVAILABILITY_TO_CHARS                                        \
     // Note: Those are not ABI-stable yet, so we can't ship them.
 #   define _LIBCPP_AVAILABILITY_SYNC                                            \
         __attribute__((unavailable))
 #   define _LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait
 #   define _LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_barrier
 #   define _LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_latch
 #   define _LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_semaphore
 
+    // This controls the availability of the C++20 format library.
+    // The library is in development and not ABI stable yet. Currently
+    // P2216 is aiming to be retroactively accepted in C++20. This paper
+    // contains ABI breaking changes.
+#   define _LIBCPP_AVAILABILITY_FORMAT                                          \
+        __attribute__((unavailable))
 #else
 
 // ...New vendors can add availability markup here...
 
 #   error "It looks like you're trying to enable vendor availability markup, but you haven't defined the corresponding macros yet!"
 
 #endif
 

libcxx/include/__format/format_parse_context.h

 // TODO FMT Remove this once we require compilers with proper C++20 support.
 // If the compiler has no concepts support, the format header will be disabled.
 // Without concepts support enable_if needs to be used and that too much effort
 // to support compilers with partial C++20 support.
 #if !defined(_LIBCPP_HAS_NO_CONCEPTS) &&                                       \
     !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
 
 template <class _CharT>
-class _LIBCPP_TEMPLATE_VIS basic_format_parse_context {
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT basic_format_parse_context {
 public:
   using char_type = _CharT;
   using const_iterator = typename basic_string_view<_CharT>::const_iterator;
   using iterator = const_iterator;
 
   _LIBCPP_INLINE_VISIBILITY
   constexpr explicit basic_format_parse_context(basic_string_view<_CharT> __fmt,
                                                 size_t __num_args = 0) noexcept