[libc++] Deprecate _LIBCPP_ENABLE_ASSERTIONS & friends

[ldionne]

In LLVM 19, the old xxx_ENABLE_ASSERTIONS settings should be deprecated with the goal of removing them entirely in LLVM 20.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Louis Dionne (ldionne)

Changes

In LLVM 19, the old xxx_ENABLE_ASSERTIONS settings should be deprecated with the goal of removing them entirely in LLVM 20.

---

Full diff: https://github.com/llvm/llvm-project/pull/98364.diff

3 Files Affected:

• (modified) libcxx/CMakeLists.txt (+1)
• (modified) libcxx/docs/ReleaseNotes/19.rst (+1-1)
• (modified) libcxx/include/__config (+2-2)

diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
index e098bd574eec7..7d0b8d12674bd 100644
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -786,6 +786,7 @@ config_define_if_not(LIBCXX_ENABLE_VENDOR_AVAILABILITY_ANNOTATIONS _LIBCPP_HAS_N
 
 # TODO(LLVM 19): Produce a deprecation warning.
 if (LIBCXX_ENABLE_ASSERTIONS)
+  message(DEPRECATION "LIBCXX_ENABLE_ASSERTIONS is deprecated and will be removed in LLVM 20. Please use LIBCXX_HARDENING_MODE instead.")
   set(LIBCXX_HARDENING_MODE "extensive")
 endif()
 if (LIBCXX_HARDENING_MODE STREQUAL "none")
diff --git a/libcxx/docs/ReleaseNotes/19.rst b/libcxx/docs/ReleaseNotes/19.rst
index 893e418d32d1d..a72a583ffb0be 100644
--- a/libcxx/docs/ReleaseNotes/19.rst
+++ b/libcxx/docs/ReleaseNotes/19.rst
@@ -85,7 +85,7 @@ Deprecations and Removals
   This decision is based on LEWGs discussion on `P3122 <https://wg21.link/P3122>` and `P3162 <https://wg21.link/P3162>`
   to not use ``[[nodiscard]]`` in the standard.
 
-- TODO: The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable that was used to enable the safe mode has been deprecated and setting
+- The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable that was used to enable the safe mode has been deprecated and setting
   it triggers an error; use the ``LIBCXX_HARDENING_MODE`` CMake variable with the value ``extensive`` instead. Similarly,
   the ``_LIBCPP_ENABLE_ASSERTIONS`` macro has been deprecated (setting it to ``1`` still enables the extensive mode in
   the LLVM 19 release while also issuing a deprecation warning). See :ref:`the hardening documentation
diff --git a/libcxx/include/__config b/libcxx/include/__config
index dfb14fd6a380c..91bc6fc21df8e 100644
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -38,10 +38,10 @@
 
 // HARDENING {
 
-// TODO(hardening): deprecate this in LLVM 19.
 // This is for backward compatibility -- make enabling `_LIBCPP_ENABLE_ASSERTIONS` (which predates hardening modes)
-// equivalent to setting the extensive mode.
+// equivalent to setting the extensive mode. This is deprecated and will be removed in LLVM 20.
 #  ifdef _LIBCPP_ENABLE_ASSERTIONS
+#    warning "_LIBCPP_ENABLE_ASSERTIONS is deprecated, please use _LIBCPP_HARDENING_MODE instead"
 #    if _LIBCPP_ENABLE_ASSERTIONS != 0 && _LIBCPP_ENABLE_ASSERTIONS != 1
 #      error "_LIBCPP_ENABLE_ASSERTIONS must be set to 0 or 1"
 #    endif