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

[ldionne]

Summary

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

[ldionne]

@thakis I was able to find a reference to _LIBCPP_DEBUG in a gn file, you may want to have a look in case changes are needed.

[ldionne]

@DavidSpickett @ARMDavidSpickett The ARM bots seem to be down right now. Can someone please look into it? It's stalling our CI.

[thakis]

In D137975#3925699, @ldionne wrote:

@thakis I was able to find a reference to _LIBCPP_DEBUG in a gn file, you may want to have a look in case changes are needed.

Done in 1b829c3d3a8c0488f8aa553eb15c7ee658f491c9

[DavidSpickett]

@DavidSpickett @ARMDavidSpickett The ARM bots seem to be down right now. Can someone please look into it? It's stalling our CI.

Was away at llvm dev, looks like whatever it was is now fixed.

libcxx/docs/ReleaseNotes.rst

 
 - The ``_LIBCPP_ENABLE_CXX03_FUNCTION`` macro that allowed re-enabling the now-deprecated C++03 implementation of
   ``std::function`` has been removed. Users who need to use ``std::function`` should switch to C++11 and above.
 
 - The contents of ``<experimental/memory_resource>`` are now deprecated since libc++ ships ``<memory_resource>`` now.
   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
 ----------------------------------
 
 API Changes
 -----------
 - The comparison operators on ``thread::id`` are now defined as free-standing
   functions instead of as hidden friends, in conformance with the C++ standard.
   Also see `issue 56187 <https://github.com/llvm/llvm-project/issues/56187>`_.

libcxx/include/__assert

 
 #include <__config>
 #include <__verbose_abort>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  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.
 #if defined(_LIBCPP_ENABLE_DEBUG_MODE)
 # ifndef _LIBCPP_ENABLE_ASSERTIONS
 #   define _LIBCPP_ENABLE_ASSERTIONS 1
 # endif
 #endif

libcxx/include/__debug

 #include <__config>
 #include <cstddef>
 #include <type_traits>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  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
 
 #ifdef _LIBCPP_ENABLE_DEBUG_MODE
 #   define _LIBCPP_DEBUG_ASSERT(x, m) _LIBCPP_ASSERT(::std::__libcpp_is_constant_evaluated() || (x), m)
 #else
 #   define _LIBCPP_DEBUG_ASSERT(x, m) ((void)0)

libcxx/test/libcxx/assertions/debug_mode_compatibility.pass.cpp

-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-// This test ensures that assertions are still enabled when _LIBCPP_DEBUG=0 is
-// defined, for backwards compatibility with code that might have been using
-// it to enable assertions previously.
-
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=0
-
-// We flag uses of the assertion handler in older dylibs at compile-time to avoid runtime
-// failures when back-deploying.
-// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11.0|12.0}}
-
-#include <cstdlib>
-
-void std::__libcpp_verbose_abort(char const*, ...) {
-  std::exit(EXIT_SUCCESS);
-}
-
-int main(int, char**) {
-  _LIBCPP_ASSERT(false, "message");
-  return EXIT_FAILURE;
-}

libcxx/test/libcxx/debug/debug.catch-legacy-macro.verify.cpp

-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-// This test ensures that we issue an error if we try to enable the debug mode with
-// a library that was not built with support for the debug mode.
-
-// REQUIRES: !libcpp-has-debug-mode
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=1
-
-// This test fails when modules are enabled because we fail to build module 'std' instead of
-// issuing the preprocessor error.
-// UNSUPPORTED: modules-build
-
-#include <__debug>
-
-// expected-error@*:* {{Enabling the debug mode now requires having configured the library with support for the debug mode}}