[libc++] Make it an error to define _LIBCPP_DEBUG

We have been transitioning off of that macro since LLVM 15.

Differential Revision: https://reviews.llvm.org/D137975

Author: ldionne

libcxx/docs/ReleaseNotes.rst

@@ -97,6 +97,9 @@ Deprecations and Removals
   Please migrate to ``<memory_resource>`` instead. Per libc++'s TS deprecation policy,
   ``<experimental/memory_resource>`` will be removed in LLVM 18.
 
+- The ``_LIBCPP_DEBUG`` macro is not honored anymore, and it is an error to try to use it. Please migrate to
+  ``_LIBCPP_ENABLE_DEBUG_MODE`` instead.
+
 Upcoming Deprecations and Removals
 ----------------------------------
 

libcxx/include/__assert

@@ -17,13 +17,9 @@
 #  pragma GCC system_header
 #endif
 
-// This is for backwards compatibility with code that might have been enabling
-// assertions through the Debug mode previously.
-// TODO: In LLVM 16, make it an error to define _LIBCPP_DEBUG
+// TODO: Remove in LLVM 17.
 #if defined(_LIBCPP_DEBUG)
-# ifndef _LIBCPP_ENABLE_ASSERTIONS
-#   define _LIBCPP_ENABLE_ASSERTIONS 1
-# endif
+#   error "Defining _LIBCPP_DEBUG is not supported anymore. Please use _LIBCPP_ENABLE_DEBUG_MODE instead."
 #endif
 
 // Automatically enable assertions when the debug mode is enabled.

libcxx/include/__debug

@@ -19,11 +19,6 @@
 #  pragma GCC system_header
 #endif
 
-// Catch invalid uses of the legacy _LIBCPP_DEBUG toggle.
-#if defined(_LIBCPP_DEBUG) && _LIBCPP_DEBUG != 0 && !defined(_LIBCPP_ENABLE_DEBUG_MODE)
-#   error "Enabling the debug mode now requires having configured the library with support for the debug mode"
-#endif
-
 #if defined(_LIBCPP_ENABLE_DEBUG_MODE) && !defined(_LIBCPP_CXX03_LANG) && !defined(_LIBCPP_DEBUG_RANDOMIZE_UNSPECIFIED_STABILITY)
 # define _LIBCPP_DEBUG_RANDOMIZE_UNSPECIFIED_STABILITY
 #endif