[RFC][libc++]] Adds _LIBCPP_HIDE_FROM_ABI_TYPE.

[Mordante]

Summary

As discussed in D136775.

[Mordante]

libcxx/include/__format/parser_std_format_spec.h

126

I think it would be good to document what should and what should not be hidden from the ABI.
For example should this implementation field use an ABI tag or not.
Then we probably should update https://libcxx.llvm.org/DesignDocs/VisibilityMacros.html too and if possible have some more clang-tidy validations.

[ldionne]

I think I like this approach, but we should discuss it in the context of D153658. Also, I think we want to punt this change to LLVM 18, I don't believe we're blocking ourselves from anything if we wait.

libcxx/include/__format/parser_std_format_spec.h

126

I think that whatever we put an ABI tag on would also need to have hidden visibility. Otherwise, it is possible for users to start exporting symbols whose name depends on the libc++ version on their dylib boundaries, which is not good.

[ldionne]

libcxx/include/__config

630

I think it would be useful to add this attribute to a few other internal-only types that are not meant to appear at ABI boundaries, just to make it clear how this attribute is intended to be used. A good candidate would be libcxx/include/__utility/exception_guard.h.

638

This documentation needs to touch on the fact that we don't have a way to enforce that these types are not used at ABI boundaries, but that if it happens in e.g. a function signature the ABI break will be quite loud because the mangling of the type changes with the ODR signature (which includes e.g. the version of the library).

We should also say that it's fine to use these types as return types and parameter types of functions, but only if said functions are marked as HIDE_FROM_ABI.

We should also say that such types must never be used as a member of another type, except if that other type is *also* marked as HIDE_FROM_ABI_TYPE.

639

I think I would do this now:

#define _LIBCPP_HIDE_FROM_ABI_TYPE \
    _LIBCPP_HIDDEN \
    __attribute__((__abi_tag__(_LIBCPP_TOSTRING(_LIBCPP_ODR_SIGNATURE))))

libcxx/include/__config

 // TODO: We provide a escape hatch with _LIBCPP_NO_ABI_TAG for folks who want to avoid increasing
 //       the length of symbols with an ABI tag. In practice, we should remove the escape hatch and
 //       use compression mangling instead, see https://github.com/itanium-cxx-abi/cxx-abi/issues/70.
 #  ifndef _LIBCPP_NO_ABI_TAG
 #    define _LIBCPP_HIDE_FROM_ABI                                                                                      \
       _LIBCPP_HIDDEN _LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION                                                       \
           __attribute__((__abi_tag__(_LIBCPP_TOSTRING(_LIBCPP_VERSIONED_IDENTIFIER))))
 #  else
 #    define _LIBCPP_HIDE_FROM_ABI _LIBCPP_HIDDEN _LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION
 #  endif
 #  define _LIBCPP_HIDE_FROM_ABI_VIRTUAL _LIBCPP_HIDDEN _LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION
 
 // This macro provides a HIDE_FROM_ABI equivalent that can be applied to extern
 // "C" function, as those lack mangling.
 #  define _LIBCPP_HIDE_FROM_ABI_C _LIBCPP_HIDDEN _LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION
 
+// This macro provides a HIDE_FROM_ABI equivalent for types.
+#  define _LIBCPP_HIDE_FROM_ABI_TYPE                                                                                   \
+    _LIBCPP_HIDDEN __attribute__((__abi_tag__(_LIBCPP_TOSTRING(_LIBCPP_VERSIONED_IDENTIFIER))))
+
 #  ifdef _LIBCPP_BUILDING_LIBRARY
 #    if _LIBCPP_ABI_VERSION > 1
 #      define _LIBCPP_HIDE_FROM_ABI_AFTER_V1 _LIBCPP_HIDE_FROM_ABI
 #    else
 #      define _LIBCPP_HIDE_FROM_ABI_AFTER_V1
 #    endif
 #  else
 #    define _LIBCPP_HIDE_FROM_ABI_AFTER_V1 _LIBCPP_HIDE_FROM_ABI

libcxx/include/__format/parser_std_format_spec.h

return _VSTD::__visit_format_arg(
       },
       __format_arg);
 }
 
 /// These fields are a filter for which elements to parse.
 ///
 /// They default to false so when a new field is added it needs to be opted in
 /// explicitly.
-// TODO FMT Use an ABI tag for this struct.
-struct __fields {
+struct _LIBCPP_HIDE_FROM_ABI_TYPE __fields {
   uint8_t __sign_ : 1 {false};
   uint8_t __alternate_form_ : 1 {false};
   uint8_t __zero_padding_ : 1 {false};
   uint8_t __precision_ : 1 {false};
   uint8_t __locale_specific_form_ : 1 {false};
   uint8_t __type_ : 1 {false};
   // Determines the valid values for fill.
   //
   // Originally the fill could be any character except { and }. Range-based
   // formatters use the colon to mark the beginning of the
   // underlying-format-spec. To avoid parsing ambiguities these formatter
   // specializations prohibit the use of the colon as a fill character.
   uint8_t __allow_colon_in_fill_ : 1 {false};
 };
 
 // By not placing this constant in the formatter class it's not duplicated for
 // char and wchar_t.
 inline constexpr __fields __fields_integral{
     .__sign_                 = true,
     .__alternate_form_       = true,
     .__zero_padding_         = true,
     .__locale_specific_form_ = true,
     .__type_                 = true};
 inline constexpr __fields __fields_floating_point{
     .__sign_                 = true,
     .__alternate_form_       = true,