[libc++] Improve error messages for disabled modes

[ldionne]

Summary

We should not surface CMake-level options like LIBCXX_ENABLE_FILESYSTEM
to our users, since they don't know what it means. Instead, use a slightly
more general wording.

Also, add an error in <ios> to improve the quality of errors for people
trying to use <iostream> when localization is disabled.

[Mordante]

Nice user improvement!

libcxx/include/filesystem

264

In general I like the approach to make errors easier to understand. I just wonder would it be useful to keep the CMake flag? Something along the lines of the suggested edit.

libcxx/include/future

2435

<shared_mutex> and <thread> seem to have a similar issue.

[kubamracek]

Very nice!

[ldionne]

Fix CI issues.

libcxx/include/filesystem

264

Frankly, I'm not sure. This is going to be seen by end-users exclusively, not by maintainers. They don't even know that we configure and build libc++ with CMake.

One could even argue that this can be confusing if a user is building *their* project with CMake -- it could be understood as saying that they should be passing LIBCXX_ENABLE_FILESYSTEM=ON to their own CMake configuration, which would be rather misleading.

libcxx/include/future

2435

Thanks for the catch!

[Mordante]

libcxx/include/filesystem

264

Fair point. I don't feel very strongly about it. I expect (and hope) disabled headers are uncommon in practice.

libcxx/include/barrier

 # include <memory>
 #endif
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 #ifdef _LIBCPP_HAS_NO_THREADS
-# error <barrier> is not supported on this single threaded system
+# error "<barrier> is not supported since libc++ has been configured without support for threads."
 #endif
 
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
 #if _LIBCPP_STD_VER >= 14
 
 _LIBCPP_BEGIN_NAMESPACE_STD

libcxx/include/filesystem

 #include <__filesystem/perms.h>
 #include <__filesystem/recursive_directory_iterator.h>
 #include <__filesystem/space_info.h>
 #include <__filesystem/u8path.h>
 #include <compare>
 #include <version>
 
 #if defined(_LIBCPP_HAS_NO_FILESYSTEM_LIBRARY)
-# error "The Filesystem library is not supported since libc++ has been configured with LIBCXX_ENABLE_FILESYSTEM disabled"
+# error "The <filesystem> library is not supported since libc++ has been configured without support for a filesystem."
 #endif
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 #endif // _LIBCPP_FILESYSTEM

libcxx/include/future

 #include <thread>
 #include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 #ifdef _LIBCPP_HAS_NO_THREADS
-#error <future> is not supported on this single threaded system
-#else // !_LIBCPP_HAS_NO_THREADS
+# error "<future> is not supported since libc++ has been configured without support for threads."
+#endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 //enum class future_errc
 _LIBCPP_DECLARE_STRONG_ENUM(future_errc)
 {
     future_already_retrieved = 1,
     promise_already_satisfied,

 shared_future<void>
 future<void>::share() _NOEXCEPT
 {
     return shared_future<void>(_VSTD::move(*this));
 }
 
 _LIBCPP_END_NAMESPACE_STD
 
-#endif // !_LIBCPP_HAS_NO_THREADS
-
 #endif // _LIBCPP_FUTURE

libcxx/include/ios

 error_code make_error_code(io_errc e) noexcept;
 error_condition make_error_condition(io_errc e) noexcept;
 storage-class-specifier const error_category& iostream_category() noexcept;
 
 }  // std
 
 */
 
-#include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
+
+#if defined(_LIBCPP_HAS_NO_LOCALIZATION)
+#   error "The iostreams library is not supported since libc++ has been configured without support for localization."
+#endif
+
+#include <__assert> // all public C++ headers provide the assertion handler
 #include <__ios/fpos.h>
 #include <__locale>
 #include <__utility/swap.h>
 #include <iosfwd>
 #include <system_error>
 #include <version>
 
 #if !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)

libcxx/include/latch

 #include <atomic>
 #include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 #ifdef _LIBCPP_HAS_NO_THREADS
-# error <latch> is not supported on this single threaded system
+# error "<latch> is not supported since libc++ has been configured without support for threads."
 #endif
 
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
 #if _LIBCPP_STD_VER >= 14
 
 _LIBCPP_BEGIN_NAMESPACE_STD

libcxx/include/locale.h

Functions:
    setlocale
    localeconv
 
 */
 
 #include <__config>
 
 #if defined(_LIBCPP_HAS_NO_LOCALIZATION)
-#   error "The Localization library is not supported since libc++ has been configured with LIBCXX_ENABLE_LOCALIZATION disabled"
+#   error "<locale.h> is not supported since libc++ has been configured without support for localization."
 #endif
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 #include_next <locale.h>
 
 #endif // _LIBCPP_LOCALE_H

libcxx/include/semaphore

 #include <atomic>
 #include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 #ifdef _LIBCPP_HAS_NO_THREADS
-# error <semaphore> is not supported on this single threaded system
+# error "<semaphore> is not supported since libc++ has been configured without support for threads."
 #endif
 
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
 #if _LIBCPP_STD_VER >= 14
 
 _LIBCPP_BEGIN_NAMESPACE_STD

libcxx/include/shared_mutex

 
 #include <__mutex_base>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 #ifdef _LIBCPP_HAS_NO_THREADS
-#error <shared_mutex> is not supported on this single threaded system
-#else // !_LIBCPP_HAS_NO_THREADS
+# error "<shared_mutex> is not supported since libc++ has been configured without support for threads."
+#endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 struct _LIBCPP_TYPE_VIS _LIBCPP_AVAILABILITY_SHARED_MUTEX _LIBCPP_THREAD_SAFETY_ANNOTATION(capability("shared_mutex"))
 __shared_mutex_base
 {
     mutex               __mut_;
     condition_variable  __gate1_;

 template <class _Mutex>
 inline _LIBCPP_INLINE_VISIBILITY
 void
 swap(shared_lock<_Mutex>& __x, shared_lock<_Mutex>& __y) _NOEXCEPT
     {__x.swap(__y);}
 
 _LIBCPP_END_NAMESPACE_STD
 
-#endif // !_LIBCPP_HAS_NO_THREADS
-
 #endif // _LIBCPP_STD_VER > 11
 
 _LIBCPP_POP_MACROS
 
 #endif // _LIBCPP_SHARED_MUTEX

libcxx/include/thread

 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
 #ifdef _LIBCPP_HAS_NO_THREADS
-#error <thread> is not supported on this single threaded system
-#else // !_LIBCPP_HAS_NO_THREADS
+# error "<thread> is not supported since libc++ has been configured without support for threads."
+#endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Tp> class __thread_specific_ptr;
 class _LIBCPP_TYPE_VIS __thread_struct;
 class _LIBCPP_HIDDEN __thread_struct_imp;
 class __assoc_sub_state;
 

 
 inline _LIBCPP_INLINE_VISIBILITY
 void yield() _NOEXCEPT {__libcpp_thread_yield();}
 
 } // namespace this_thread
 
 _LIBCPP_END_NAMESPACE_STD
 
-#endif // !_LIBCPP_HAS_NO_THREADS
-
 _LIBCPP_POP_MACROS
 
 #endif // _LIBCPP_THREAD