[libc++] Introduce unversioned namespace macros (#133009)

We've started using `_LIBCPP_BEGIN_NAMESPACE_STD` and
`_LIBCPP_END_NAMESPACE_STD` for more than just the namespace for a while
now. For example, we're using it to add visibility annotations to types.
This works very well and avoids a bunch of annotations, but doesn't work
for the few places where we have an unversioned namespace. This adds
`_LIBCPP_BEGIN_UNVERSIONED_NAMESPACE_STD` and
`_LIBCPP_END_UNVERSIONED_NAMESPACE_STD` to make it simpler to add new
annotations consistently across the library as well as making it more
explicit that the unversioned namespace is indeed intended.

Author: philnik777

libcxx/include/__config

@@ -577,15 +577,27 @@ typedef __char32_t char32_t;
 #    define _LIBCPP_POP_EXTENSION_DIAGNOSTICS
 #  endif
 
-// Inline namespaces are available in Clang/GCC/MSVC regardless of C++ dialect.
 // clang-format off
-#  define _LIBCPP_BEGIN_NAMESPACE_STD _LIBCPP_PUSH_EXTENSION_DIAGNOSTICS                                               \
-                                      namespace _LIBCPP_TYPE_VISIBILITY_DEFAULT std {                                  \
-                               inline namespace _LIBCPP_ABI_NAMESPACE {
-#  define _LIBCPP_END_NAMESPACE_STD }} _LIBCPP_POP_EXTENSION_DIAGNOSTICS
 
-#define _LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL namespace std { namespace experimental {
-#define _LIBCPP_END_NAMESPACE_EXPERIMENTAL }}
+// The unversioned namespace is used when we want to be ABI compatible with other standard libraries in some way. There
+// are two main categories where that's the case:
+// - Historically, we have made exception types ABI compatible with libstdc++ to allow throwing them between libstdc++
+//   and libc++. This is not used anymore for new exception types, since there is no use-case for it anymore.
+// - Types and functions which are used by the compiler are in the unversioned namespace, since the compiler has to know
+//   their mangling without the appropriate declaration in some cases.
+// If it's not clear whether using the unversioned namespace is the correct thing to do, it's not. The versioned
+// namespace (_LIBCPP_BEGIN_NAMESPACE_STD) should almost always be used.
+#  define _LIBCPP_BEGIN_UNVERSIONED_NAMESPACE_STD                                                                      \
+    _LIBCPP_PUSH_EXTENSION_DIAGNOSTICS namespace _LIBCPP_TYPE_VISIBILITY_DEFAULT std {
+
+#  define _LIBCPP_END_UNVERSIONED_NAMESPACE_STD } _LIBCPP_POP_EXTENSION_DIAGNOSTICS
+
+#  define _LIBCPP_BEGIN_NAMESPACE_STD _LIBCPP_BEGIN_UNVERSIONED_NAMESPACE_STD inline namespace _LIBCPP_ABI_NAMESPACE {
+#  define _LIBCPP_END_NAMESPACE_STD } _LIBCPP_END_UNVERSIONED_NAMESPACE_STD
+
+// TODO: This should really be in the versioned namespace
+#define _LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL _LIBCPP_BEGIN_UNVERSIONED_NAMESPACE_STD namespace experimental {
+#define _LIBCPP_END_NAMESPACE_EXPERIMENTAL } _LIBCPP_END_UNVERSIONED_NAMESPACE_STD
 
 #define _LIBCPP_BEGIN_NAMESPACE_LFTS _LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL inline namespace fundamentals_v1 {
 #define _LIBCPP_END_NAMESPACE_LFTS } _LIBCPP_END_NAMESPACE_EXPERIMENTAL

libcxx/include/__cstddef/byte.h

@@ -19,7 +19,7 @@
 #endif
 
 #if _LIBCPP_STD_VER >= 17
-namespace std { // purposefully not versioned
+_LIBCPP_BEGIN_UNVERSIONED_NAMESPACE_STD
 
 enum class byte : unsigned char {};
 
@@ -79,7 +79,7 @@ template <class _Integer, __enable_if_t<is_integral<_Integer>::value, int> = 0>
   return static_cast<_Integer>(__b);
 }
 
-} // namespace std
+_LIBCPP_END_UNVERSIONED_NAMESPACE_STD
 #endif // _LIBCPP_STD_VER >= 17
 
 #endif // _LIBCPP___CSTDDEF_BYTE_H

libcxx/include/__exception/exception.h

@@ -21,7 +21,7 @@
 #  pragma GCC system_header
 #endif
 
-namespace std { // purposefully not using versioning namespace
+_LIBCPP_BEGIN_UNVERSIONED_NAMESPACE_STD
 
 #if defined(_LIBCPP_ABI_VCRUNTIME) && (!defined(_HAS_EXCEPTIONS) || _HAS_EXCEPTIONS != 0)
 // The std::exception class was already included above, but we're explicit about this condition here for clarity.
@@ -89,6 +89,6 @@ class _LIBCPP_EXPORTED_FROM_ABI bad_exception : public exception {
 };
 #endif // !_LIBCPP_ABI_VCRUNTIME
 
-} // namespace std
+_LIBCPP_END_UNVERSIONED_NAMESPACE_STD
 
 #endif // _LIBCPP___EXCEPTION_EXCEPTION_H

libcxx/include/__exception/exception_ptr.h

@@ -52,7 +52,7 @@ _LIBCPP_OVERRIDABLE_FUNC_VIS __cxa_exception* __cxa_init_primary_exception(
 
 #endif
 
-namespace std { // purposefully not using versioning namespace
+_LIBCPP_BEGIN_UNVERSIONED_NAMESPACE_STD
 
 #ifndef _LIBCPP_ABI_MICROSOFT
 
@@ -171,6 +171,6 @@ _LIBCPP_HIDE_FROM_ABI exception_ptr make_exception_ptr(_Ep __e) _NOEXCEPT {
 }
 
 #endif // _LIBCPP_ABI_MICROSOFT
-} // namespace std
+_LIBCPP_END_UNVERSIONED_NAMESPACE_STD
 
 #endif // _LIBCPP___EXCEPTION_EXCEPTION_PTR_H

libcxx/include/__exception/nested_exception.h

@@ -27,7 +27,7 @@
 #  pragma GCC system_header
 #endif
 
-namespace std { // purposefully not using versioning namespace
+_LIBCPP_BEGIN_UNVERSIONED_NAMESPACE_STD
 
 class _LIBCPP_EXPORTED_FROM_ABI nested_exception {
   exception_ptr __ptr_;
@@ -95,6 +95,6 @@ inline _LIBCPP_HIDE_FROM_ABI void rethrow_if_nested(const _Ep& __e) {
 template <class _Ep, __enable_if_t<!__can_dynamic_cast<_Ep, nested_exception>::value, int> = 0>
 inline _LIBCPP_HIDE_FROM_ABI void rethrow_if_nested(const _Ep&) {}
 
-} // namespace std
+_LIBCPP_END_UNVERSIONED_NAMESPACE_STD
 
 #endif // _LIBCPP___EXCEPTION_NESTED_EXCEPTION_H

libcxx/include/__exception/operations.h

@@ -15,7 +15,7 @@
 #  pragma GCC system_header
 #endif
 
-namespace std { // purposefully not using versioning namespace
+_LIBCPP_BEGIN_UNVERSIONED_NAMESPACE_STD
 #if _LIBCPP_STD_VER <= 14 || defined(_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS) ||                             \
     defined(_LIBCPP_BUILDING_LIBRARY)
 using unexpected_handler = void (*)();
@@ -37,6 +37,6 @@ class _LIBCPP_EXPORTED_FROM_ABI exception_ptr;
 
 _LIBCPP_EXPORTED_FROM_ABI exception_ptr current_exception() _NOEXCEPT;
 [[__noreturn__]] _LIBCPP_EXPORTED_FROM_ABI void rethrow_exception(exception_ptr);
-} // namespace std
+_LIBCPP_END_UNVERSIONED_NAMESPACE_STD
 
 #endif // _LIBCPP___EXCEPTION_OPERATIONS_H

libcxx/include/__exception/terminate.h

@@ -15,8 +15,8 @@
 #  pragma GCC system_header
 #endif
 
-namespace std { // purposefully not using versioning namespace
+_LIBCPP_BEGIN_UNVERSIONED_NAMESPACE_STD
 [[__noreturn__]] _LIBCPP_EXPORTED_FROM_ABI void terminate() _NOEXCEPT;
-} // namespace std
+_LIBCPP_END_UNVERSIONED_NAMESPACE_STD
 
 #endif // _LIBCPP___EXCEPTION_TERMINATE_H

libcxx/include/__fwd/byte.h

@@ -16,11 +16,11 @@
 #endif
 
 #if _LIBCPP_STD_VER >= 17
-namespace std { // purposefully not versioned
+_LIBCPP_BEGIN_UNVERSIONED_NAMESPACE_STD
 
 enum class byte : unsigned char;
 
-} // namespace std
+_LIBCPP_END_UNVERSIONED_NAMESPACE_STD
 #endif // _LIBCPP_STD_VER >= 17
 
 #endif // _LIBCPP___FWD_BYTE_H

libcxx/include/__new/align_val_t.h

@@ -16,15 +16,14 @@
 #  pragma GCC system_header
 #endif
 
-// purposefully not using versioning namespace
-namespace std {
+_LIBCPP_BEGIN_UNVERSIONED_NAMESPACE_STD
 #if _LIBCPP_HAS_LIBRARY_ALIGNED_ALLOCATION && !defined(_LIBCPP_ABI_VCRUNTIME)
 #  ifndef _LIBCPP_CXX03_LANG
 enum class align_val_t : size_t {};
 #  else
 enum align_val_t { __zero = 0, __max = (size_t)-1 };
 #  endif
 #endif
-} // namespace std
+_LIBCPP_END_UNVERSIONED_NAMESPACE_STD
 
 #endif // _LIBCPP___NEW_ALIGN_VAL_T_H

libcxx/include/__new/destroying_delete_t.h

@@ -16,15 +16,14 @@
 #endif
 
 #if _LIBCPP_STD_VER >= 20
-// purposefully not using versioning namespace
-namespace std {
+_LIBCPP_BEGIN_UNVERSIONED_NAMESPACE_STD
 // Enable the declaration even if the compiler doesn't support the language
 // feature.
 struct destroying_delete_t {
   explicit destroying_delete_t() = default;
 };
 inline constexpr destroying_delete_t destroying_delete{};
-} // namespace std
+_LIBCPP_END_UNVERSIONED_NAMESPACE_STD
 #endif
 
 #endif // _LIBCPP___NEW_DESTROYING_DELETE_T_H