[libc++] __config cleanup; _LIBCPP_ABI_UNSTABLE should set _LIBCPP_ABI_VERSION

[philnik]

Summary

Some __config cleanup and _LIBCPP_ABI_UNSTABLE should set _LIBCPP_ABI_VERSION, since the latest ABI version is the unstable ABI.

[• Quuxplusone]

Requesting changes just to make sure this gets attention.
Skimming the existing code, it looks to me as if a vendor who compiles with -D_LIBCPP_ABI_UNSTABLE -U_LIBCPP_ABI_VERSION will currently get all their symbols in namespace std::__1. This PR changes that, so that they'll get all their symbols in namespace std::__2 instead. I suspect this is not something we want to do, but ABI/vendor issues are above my pay grade, so I don't want to say I'm objecting to this. Just calling out a change that I think is likely to be significant.

[philnik]

In D118989#3297171, @Quuxplusone wrote:

Requesting changes just to make sure this gets attention.
Skimming the existing code, it looks to me as if a vendor who compiles with -D_LIBCPP_ABI_UNSTABLE -U_LIBCPP_ABI_VERSION will currently get all their symbols in namespace std::__1. This PR changes that, so that they'll get all their symbols in namespace std::__2 instead. I suspect this is not something we want to do, but ABI/vendor issues are above my pay grade, so I don't want to say I'm objecting to this. Just calling out a change that I think is likely to be significant.

I think if you compile with _LIBCPP_ABI_UNSTABLE you shouldn't have a problem with ABI changes. That's the entire point of using this macro. I might be wrong, but that's how I understand it. @ldionne should be able to clarify. We can of course also ask #libc_vendors.

[ldionne]

This looks correct to me. I've actually wanted to get rid of _LIBCPP_ABI_UNSTABLE for a while, and I guess this is a great occasion to do this. Indeed, since this is the unstable ABI, we don't care about the namespace changing. If someone comes to this patch and they are broken by it, then they just figured out that they shouldn't be on the unstable ABI.

I'm not a big fan of mixing changes like this in the same commit, I'd have rather made all the NFC cleanups in one patch and then the ABI unstable change separately. But the patch itself is small enough, this is acceptable.

I'll follow-up with additional cleanups for the ABI macros, I've had something in mind for a while.

libcxx/include/__config

 #ifndef _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER
 #  pragma GCC system_header
 #endif
 
 #ifdef __cplusplus
 
 #define _LIBCPP_VERSION 15000
 
+#ifdef _LIBCPP_ABI_UNSTABLE
+#  define _LIBCPP_ABI_VERSION 2
+#endif
+
 #ifndef _LIBCPP_ABI_VERSION
 #  define _LIBCPP_ABI_VERSION 1
 #endif
 
 #if __STDC_HOSTED__ == 0
 #  define _LIBCPP_FREESTANDING
 #endif
 
 #ifndef _LIBCPP_STD_VER
 #  if  __cplusplus <= 201103L
 #    define _LIBCPP_STD_VER 11
 #  elif __cplusplus <= 201402L
 #    define _LIBCPP_STD_VER 14
 #  elif __cplusplus <= 201703L
 #    define _LIBCPP_STD_VER 17
 #  elif __cplusplus <= 202002L
 #    define _LIBCPP_STD_VER 20
 #  else
-#    define _LIBCPP_STD_VER 21  // current year, or date of c++2b ratification
+#    define _LIBCPP_STD_VER 22  // current year, or date of c++2b ratification
 #  endif
 #endif // _LIBCPP_STD_VER
 
 #if defined(__ELF__)
 #  define _LIBCPP_OBJECT_FORMAT_ELF   1
 #elif defined(__MACH__)
 #  define _LIBCPP_OBJECT_FORMAT_MACHO 1
 #elif defined(_WIN32)
 #  define _LIBCPP_OBJECT_FORMAT_COFF  1
 #elif defined(__wasm__)
 #  define _LIBCPP_OBJECT_FORMAT_WASM  1
 #else
    // ... add new file formats here ...
 #endif
 
-#if defined(_LIBCPP_ABI_UNSTABLE) || _LIBCPP_ABI_VERSION >= 2
+#if _LIBCPP_ABI_VERSION >= 2
 // Change short string representation so that string data starts at offset 0,
 // improving its alignment in some cases.
 #  define _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 // Fix deque iterator type in order to support incomplete types.
 #  define _LIBCPP_ABI_INCOMPLETE_TYPES_IN_DEQUE
 // Fix undefined behavior in how std::list stores its linked nodes.
 #  define _LIBCPP_ABI_LIST_REMOVE_NODE_POINTER_UB
 // Fix undefined behavior in  how __tree stores its end and parent nodes.

 //
 // IMPORTANT: IF YOU ARE READING THIS AND YOU TURN THIS MACRO ON, PLEASE LEAVE
 //            A COMMENT ON https://reviews.llvm.org/D109459 OR YOU WILL BE BROKEN
 //            IN THE FUTURE WHEN WE REMOVE THE ABILITY TO USE THE C++03 EMULATION.
 #ifndef _LIBCPP_ABI_USE_CXX03_NULLPTR_EMULATION
 # define _LIBCPP_ABI_ALWAYS_USE_CXX11_NULLPTR
 #endif
 
-#if defined(_LIBCPP_BUILDING_LIBRARY) || defined(_LIBCPP_ABI_UNSTABLE) || _LIBCPP_ABI_VERSION >= 2
+#if defined(_LIBCPP_BUILDING_LIBRARY) || _LIBCPP_ABI_VERSION >= 2
 // Enable additional explicit instantiations of iostreams components. This
 // reduces the number of weak definitions generated in programs that use
 // iostreams by providing a single strong definition in the shared library.
 # define _LIBCPP_ABI_ENABLE_ADDITIONAL_IOSTREAM_EXPLICIT_INSTANTIATIONS_1
 
 // Define a key function for `bad_function_call` in the library, to centralize
 // its vtable and typeinfo to libc++ rather than having all other libraries
 // using that class define their own copies.

 #else
 #define _LIBCPP_BEGIN_NAMESPACE_FILESYSTEM \
   _LIBCPP_BEGIN_NAMESPACE_STD namespace __fs { namespace filesystem {
 #endif
 
 #define _LIBCPP_END_NAMESPACE_FILESYSTEM \
   _LIBCPP_END_NAMESPACE_STD } }
 
-#define _VSTD_FS _VSTD::__fs::filesystem
+#define _VSTD_FS std::__fs::filesystem
 
 #if __has_attribute(__enable_if__)
 #   define _LIBCPP_PREFERRED_OVERLOAD __attribute__ ((__enable_if__(true, "")))
 #endif
 
 #ifndef _LIBCPP_HAS_NO_NOEXCEPT
 #  define _NOEXCEPT noexcept
 #  define _NOEXCEPT_(x) noexcept(x)

 #endif
 
 #if !defined(_LIBCPP_CXX03_LANG)
 #  define _LIBCPP_DEPRECATED_IN_CXX11 _LIBCPP_DEPRECATED
 #else
 #  define _LIBCPP_DEPRECATED_IN_CXX11
 #endif
 
-#if _LIBCPP_STD_VER >= 14
+#if _LIBCPP_STD_VER > 11
 #  define _LIBCPP_DEPRECATED_IN_CXX14 _LIBCPP_DEPRECATED
 #else
 #  define _LIBCPP_DEPRECATED_IN_CXX14
 #endif
 
-#if _LIBCPP_STD_VER >= 17
+#if _LIBCPP_STD_VER > 14
 #  define _LIBCPP_DEPRECATED_IN_CXX17 _LIBCPP_DEPRECATED
 #else
 #  define _LIBCPP_DEPRECATED_IN_CXX17
 #endif
 
 #if _LIBCPP_STD_VER > 17
 #  define _LIBCPP_DEPRECATED_IN_CXX20 _LIBCPP_DEPRECATED
 #else

 #  define _LIBCPP_DIAGNOSE_ERROR(...) \
      __attribute__((diagnose_if(__VA_ARGS__, "error")))
 #else
 #  define _LIBCPP_DIAGNOSE_WARNING(...)
 #  define _LIBCPP_DIAGNOSE_ERROR(...)
 #endif
 
 // Use a function like macro to imply that it must be followed by a semicolon
-#if __cplusplus > 201402L && __has_cpp_attribute(fallthrough)
+#if _LIBCPP_STD_VER > 14 && __has_cpp_attribute(fallthrough)
 #  define _LIBCPP_FALLTHROUGH() [[fallthrough]]
 #elif __has_cpp_attribute(clang::fallthrough)
 #  define _LIBCPP_FALLTHROUGH() [[clang::fallthrough]]
 #elif __has_attribute(__fallthrough__)
 #  define _LIBCPP_FALLTHROUGH() __attribute__((__fallthrough__))
 #else
 #  define _LIBCPP_FALLTHROUGH() ((void)0)
 #endif