[libc++] Improves _LIBCPP_HAS_NO_THREADS guards. (#76624)

Previously the header included several headers, possibly granularized
threading headers. This could lead to build errors when these headers
were incompatible with threading disabled.

Now test the guard before inclusion. This matches the pattern used for
no localization and no wide characters.

Fixes: https://github.com/llvm/llvm-project/issues/76620

libcxx/include/barrier

@@ -45,11 +45,16 @@ namespace std
 
 */
 
+#include <__config>
+
+#ifdef _LIBCPP_HAS_NO_THREADS
+#  error "<barrier> is not supported since libc++ has been configured without support for threads."
+#endif
+
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__atomic/atomic_base.h>
 #include <__atomic/memory_order.h>
 #include <__availability>
-#include <__config>
 #include <__memory/unique_ptr.h>
 #include <__thread/poll_with_backoff.h>
 #include <__thread/timed_backoff_policy.h>
@@ -63,10 +68,6 @@ namespace std
 #  pragma GCC system_header
 #endif
 
-#ifdef _LIBCPP_HAS_NO_THREADS
-#  error "<barrier> is not supported since libc++ has been configured without support for threads."
-#endif
-
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 

libcxx/include/future

@@ -362,11 +362,16 @@ template <class R, class Alloc> struct uses_allocator<packaged_task<R>, Alloc>;
 
 */
 
+#include <__config>
+
+#ifdef _LIBCPP_HAS_NO_THREADS
+#  error "<future> is not supported since libc++ has been configured without support for threads."
+#endif
+
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__availability>
 #include <__chrono/duration.h>
 #include <__chrono/time_point.h>
-#include <__config>
 #include <__exception/exception_ptr.h>
 #include <__memory/addressof.h>
 #include <__memory/allocator.h>
@@ -396,10 +401,6 @@ template <class R, class Alloc> struct uses_allocator<packaged_task<R>, Alloc>;
 #  pragma GCC system_header
 #endif
 
-#ifdef _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

libcxx/include/latch

@@ -40,12 +40,17 @@ namespace std
 
 */
 
+#include <__config>
+
+#ifdef _LIBCPP_HAS_NO_THREADS
+#  error "<latch> is not supported since libc++ has been configured without support for threads."
+#endif
+
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__atomic/atomic_base.h>
 #include <__atomic/atomic_sync.h>
 #include <__atomic/memory_order.h>
 #include <__availability>
-#include <__config>
 #include <cstddef>
 #include <limits>
 #include <version>
@@ -54,10 +59,6 @@ namespace std
 #  pragma GCC system_header
 #endif
 
-#ifdef _LIBCPP_HAS_NO_THREADS
-#  error "<latch> is not supported since libc++ has been configured without support for threads."
-#endif
-
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 

libcxx/include/semaphore

@@ -45,13 +45,18 @@ using binary_semaphore = counting_semaphore<1>;
 
 */
 
+#include <__config>
+
+#ifdef _LIBCPP_HAS_NO_THREADS
+#  error "<semaphore> is not supported since libc++ has been configured without support for threads."
+#endif
+
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__atomic/atomic_base.h>
 #include <__atomic/atomic_sync.h>
 #include <__atomic/memory_order.h>
 #include <__availability>
 #include <__chrono/time_point.h>
-#include <__config>
 #include <__thread/poll_with_backoff.h>
 #include <__thread/timed_backoff_policy.h>
 #include <__threading_support>
@@ -63,10 +68,6 @@ using binary_semaphore = counting_semaphore<1>;
 #  pragma GCC system_header
 #endif
 
-#ifdef _LIBCPP_HAS_NO_THREADS
-#  error "<semaphore> is not supported since libc++ has been configured without support for threads."
-#endif
-
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 

libcxx/include/shared_mutex

@@ -122,13 +122,18 @@ template <class Mutex>
 
 */
 
+#include <__config>
+
+#  ifdef _LIBCPP_HAS_NO_THREADS
+#    error "<shared_mutex> is not supported since libc++ has been configured without support for threads."
+#  endif
+
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__availability>
 #include <__chrono/duration.h>
 #include <__chrono/steady_clock.h>
 #include <__chrono/time_point.h>
 #include <__condition_variable/condition_variable.h>
-#include <__config>
 #include <__memory/addressof.h>
 #include <__mutex/mutex.h>
 #include <__mutex/tag_types.h>
@@ -147,10 +152,6 @@ _LIBCPP_PUSH_MACROS
 #    pragma GCC system_header
 #  endif
 
-#  ifdef _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_EXPORTED_FROM_ABI __shared_mutex_base {

libcxx/include/stop_token

@@ -31,8 +31,13 @@ namespace std {
 
 */
 
-#include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
+
+#ifdef _LIBCPP_HAS_NO_THREADS
+#  error "<stop_token> is not supported since libc++ has been configured without support for threads."
+#endif
+
+#include <__assert> // all public C++ headers provide the assertion handler
 #include <__stop_token/stop_callback.h>
 #include <__stop_token/stop_source.h>
 #include <__stop_token/stop_token.h>
@@ -42,10 +47,6 @@ namespace std {
 #  pragma GCC system_header
 #endif
 
-#ifdef _LIBCPP_HAS_NO_THREADS
-#  error "<stop_token> is not supported since libc++ has been configured without support for threads."
-#endif
-
 #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
 #  include <iosfwd>
 #endif

libcxx/include/thread

@@ -86,9 +86,14 @@ void sleep_for(const chrono::duration<Rep, Period>& rel_time);
 
 */
 
+#include <__config>
+
+#ifdef _LIBCPP_HAS_NO_THREADS
+#  error "<thread> is not supported since libc++ has been configured without support for threads."
+#endif
+
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__availability>
-#include <__config>
 #include <__thread/formatter.h>
 #include <__thread/jthread.h>
 #include <__thread/this_thread.h>
@@ -105,10 +110,6 @@ void sleep_for(const chrono::duration<Rep, Period>& rel_time);
 #  pragma GCC system_header
 #endif
 
-#ifdef _LIBCPP_HAS_NO_THREADS
-#  error "<thread> is not supported since libc++ has been configured without support for threads."
-#endif
-
 #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES)
 #  include <cstddef>
 #  include <ctime>