diff --git a/libcxx/docs/DesignDocs/DebugMode.rst b/libcxx/docs/DesignDocs/DebugMode.rst
--- a/libcxx/docs/DesignDocs/DebugMode.rst
+++ b/libcxx/docs/DesignDocs/DebugMode.rst
@@ -80,3 +80,66 @@
     str.insert(bad_it, '!'); // causes debug assertion
     // control flow doesn't return
   }
+
+The above example overrides the bug handler at run-time, but it is also possible
+to override it at compile time using the ``_LIBCPP_DEBUG_FUNCTION_INVOCATION``
+macro. This macro takes 2 arguments: the expression ``x`` to trigger the
+assertion, and an error message ``m``. The default invocation uses these
+arguments to construct the ``std::__libcpp_debug_info`` argument, but you can do
+whatever you like with them (including nothing):
+
+.. code-block:: cpp
+
+  #define _LIBCPP_DEBUG_FUNCTION_INVOCATION(x, m) abort()
+  #include <string>
+  int main(int, char**) {
+    std::string::iterator bad_it;
+    std::string str("hello world");
+    str.insert(bad_it, '!'); // causes debug assertion
+    // control flow doesn't return
+  }
+
+Debug Mode Checks
+=================
+
+Libc++'s debug mode offers two levels of checking. The first enables various
+precondition checks throughout libc++. The second additionally enables
+"iterator debugging" which checks the validity of iterators used by the program.
+
+Basic Checks
+============
+
+These checks are enabled when ``_LIBCPP_DEBUG`` is defined to either 0 or 1.
+
+The following checks are enabled by ``_LIBCPP_DEBUG``:
+
+  * Many algorithms, such as ``binary_search``, ``merge``, ``next_permutation``, and ``sort``,
+    wrap the user-provided comparator to assert that `!comp(y, x)` whenever
+    `comp(x, y)`. This can cause the user-provided comparator to be evaluated
+    up to twice as many times as it would be without ``_LIBCPP_DEBUG``, and
+    causes the library to violate some of the Standard's complexity clauses.
+
+  * Bounds checks in containers (e.g ``operator[]``)
+
+  * Validity of dereferencing (e.g. ``operator*``, ``operator->``)
+
+  * Invariants and preconditions (e.g. ``has_value`` for ``std::optional``)
+
+Iterator Debugging Checks
+=========================
+
+These checks are enabled when ``_LIBCPP_DEBUG`` is defined to 1.
+
+The following containers and STL classes support iterator debugging:
+
+  * ``std::string``
+  * ``std::vector<T>`` (``T != bool``)
+  * ``std::list``
+  * ``std::locale``
+  * ``std::unordered_map``
+  * ``std::unordered_multimap``
+  * ``std::unordered_set``
+  * ``std::unordered_multiset``
+
+The remaining includes do not currently support iterator debugging.
+Patches welcome.
diff --git a/libcxx/include/__debug b/libcxx/include/__debug
--- a/libcxx/include/__debug
+++ b/libcxx/include/__debug
@@ -27,15 +27,19 @@
 #   include <cstddef>
 #endif
 
+#if !defined(_LIBCPP_DEBUG_FUNCTION_INVOCATION)
+#   define _LIBCPP_DEBUG_FUNCTION_INVOCATION(x, m) _VSTD::__libcpp_debug_function(_VSTD::__libcpp_debug_info(__FILE__, __LINE__, x, m))
+#endif
+
 #if _LIBCPP_DEBUG_LEVEL == 0
 #   define _LIBCPP_DEBUG_ASSERT(x, m) ((void)0)
 #   define _LIBCPP_ASSERT_IMPL(x, m) ((void)0)
 #elif _LIBCPP_DEBUG_LEVEL == 1
 #   define _LIBCPP_DEBUG_ASSERT(x, m) ((void)0)
-#   define _LIBCPP_ASSERT_IMPL(x, m) ((x) ? (void)0 : _VSTD::__libcpp_debug_function(_VSTD::__libcpp_debug_info(__FILE__, __LINE__, #x, m)))
+#   define _LIBCPP_ASSERT_IMPL(x, m) ((x) ? (void)0 : _LIBCPP_DEBUG_FUNCTION_INVOCATION(#x, m))
 #elif _LIBCPP_DEBUG_LEVEL == 2
 #   define _LIBCPP_DEBUG_ASSERT(x, m) _LIBCPP_ASSERT(x, m)
-#   define _LIBCPP_ASSERT_IMPL(x, m) ((x) ? (void)0 : _VSTD::__libcpp_debug_function(_VSTD::__libcpp_debug_info(__FILE__, __LINE__, #x, m)))
+#   define _LIBCPP_ASSERT_IMPL(x, m) ((x) ? (void)0 : _LIBCPP_DEBUG_FUNCTION_INVOCATION(#x, m))
 #else
 #   error _LIBCPP_DEBUG_LEVEL must be one of 0, 1, 2
 #endif
diff --git a/libcxx/test/libcxx/debug/debug_function_invocation_macro.pass.cpp b/libcxx/test/libcxx/debug/debug_function_invocation_macro.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/libcxx/debug/debug_function_invocation_macro.pass.cpp
@@ -0,0 +1,25 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DEBUG=1
+// UNSUPPORTED: libcxx-no-debug-mode
+
+// Test that the _LIBCPP_DEBUG_FUNCTION_INVOCATION macro works.
+
+// Define macro so that it doesn't abort.
+#define _LIBCPP_DEBUG_FUNCTION_INVOCATION(x, m) std::_Exit(EXIT_SUCCESS)
+
+#include <string>
+
+int main(int, char**) {
+  std::string::iterator bad_it;
+  std::string str("hello world");
+  str.insert(bad_it, '!');
+  return EXIT_FAILURE;
+}