[libc++] Add a way to enable lightweight assertions independent of the debug mode

[ldionne]

Summary

This is a (IMO superior) alternative to https://reviews.llvm.org/D119969.
This patch is more involved than D119969, but it has the benefit of being
cleaner in the long run, especially since we know our current Debug mode
doesn't work -- so perhaps we shouldn't try to reuse too many of its
mechanisms.

In particular, this is better than reusing the existing debug mode because
we now have the ability to set the assertion handler at compile-time,
which means that assertions can be enabled even in code that targets
platforms on which the debug mode support had not been shipped yet.

As-is, this patch provides backwards compatibility with any user that
might have been setting _LIBCPP_DEBUG=0 in order to get basic assertions
in their code. Hence, this patch should be entirely backwards compatible.

However, given that our debug mode has been broken since day 1, I think
we'll need a discussion around either fixing it or removing it, since the
status quo is good for nobody. In particular, an immediate next step to
simplify the library would be to remove __libcpp_debug_info and friends.
We can keep the symbols in the dylib to avoid linker errors, but we should
not support two distinct ways of setting an assertion handler.

[ldionne]

libcxx/CMakeLists.txt

85–88

A point of discussion I expect is whether LIBCXX_ENABLE_ASSERTIONS at CMake configure-time should control *both* whether the dylib contains assertions and what the default is for user code. I've done it that way for now because I didn't really expect the need for additional customizeability, but I could be convinced otherwise.

TLDR, the one thing you can't do with the current design is build libc++.dylib with assertions enabled, but have assertions disabled inside user code by default (or the other way around, i.e. no assertions in the dylib, but user code builds with assertions by default).

libcxx/docs/UsingLibcxx.rst

167

I am not very happy about requiring users to include an internal header to customize the assertion handler. IMO that's kind of hacky. I thought about using <version> instead, but that would require <version> to include <__assert>, and unfortunately that's a no-go because <__assert> needs to include <iosfwd> right now (due to the legacy debug mode support).

libcxx/src/debug.cpp

27

In retrospect, I should have left this code alone in D119769, but I didn't know I would end up creating a new assertion handler.

[ldionne]

Sorry for pinging the vendors so often lately, but this change is something they'll want to be aware of, too.

[philnik]

libcxx/CMakeLists.txt

85–88

I would expect it to be off by default regardless of how the vendor configures it. What's the point of a default if it can be different on every platform anyways?

libcxx/docs/UsingLibcxx.rst

167

Why not put it in <cassert>? That would make the most sense IMO.

[ldionne]

libcxx/CMakeLists.txt

85–88

I understand this is a bit surprising, however from a vendor's perspective, it may be relevant to ship a "hardened" library where assertions are enabled by default *for everyone*, by default. I know there are platforms like that out there, and in fact it is one of the use cases motivating this work.

libcxx/docs/UsingLibcxx.rst

167

I agree it makes some sense, however we still have the problem that we need to include <iosfwd> from this header.

In other words, I don't think we can easily change this part of the user-facing API unless we agree to drop <iosfwd> from <__assert> (and in that case we can even include it from e.g. <__config>).

[• Quuxplusone]

I have no particular opinion on whether this is a good direction that will eventually lead somewhere useful, or just a random walk. No particular objection, either.

libcxx/CMakeLists.txt

681–685

I see you're no longer fiddling with NDEBUG. Why then are you keeping the fiddling with _DEBUG? What goes wrong if you remove these 5 lines too?

libcxx/docs/BuildingLibcxx.rst

219–222

Here and several times below, where you talk about "the library," I think it's important to find a phrase that clarifies we're talking about the .so or .dylib (or I guess the .a, which is not a "shared library") — we're not just talking about code that happens to be inside "libc++," like in the headers or in the std namespace.
Given that this phrase has to work for both .so and .a, though, I have no great suggestion. :( The best I've thought of so far is "the compiled library."

libcxx/docs/ReleaseNotes.rst

71–72

More flexibility than what? Personally I'd cut this clause and just start the sentence at "Vendors can select..."

libcxx/docs/UsingLibcxx.rst

132

I thought some of them do majorly change the complexity of algorithms! E.g. __do_compare_assert in the debug comparator will definitely change some algorithms on vectors-of-vectors-of-vectors from linearish (each step does 1^k comparisons) to exponentialish (each step does 2^k comparisons).
I might be wrong about this (e.g. if your new assert mode doesn't turn on debug comparators). But if this is an intended invariant, this needs to be called out vocally and explicitly, like maybe in a bulleted list of "things libc++ guarantees about our assertions," so that it's very clear to users and maintainers what'll count as a bug here.

158–161

// In HelloWorldConfig.h (this must always be included before any other libc++ header)

I think this has come up before, like in the past six months: We shouldn't encourage users to #define _LIBCPP_WHATEVER at the top of their file, because that will tend to get moved down, or not work when their file is a .h, or break when they move to modules, or whatever. We should encourage them to pass -D_LIBCPP_WHATEVER in their build system's C(XX)FLAGS instead.

(Yikes, you're even suggesting that they do put it in a .h, with a note that this header must be included first in any .cpp file that needs it. That's asking for trouble.)

167

Tangent FWIW: it seems like we could use a <__stringfwd> helper header.

libcxx/src/assert.cpp

12–13

<string> is no longer needed.

libcxx/src/debug.cpp

43–46

I think it's weird that __libcpp_set_debug_function is a dylib function, when all it actually does is write to the extern-linkage global variable __libcpp_debug_function which is already accessed from many places in the header code. Consider making this an inline function in the header, instead?
Or is there a benefit to having it in one place in the dylib where I guess someone could set a breakpoint on it or something?

[ldionne]

In D121123#3364671, @Quuxplusone wrote:

I have no particular opinion on whether this is a good direction that will eventually lead somewhere useful, or just a random walk. No particular objection, either.

It does lead somewhere useful today. Indeed, as you can see, it adds an assertion handler in the dylib under all configurations, and _LIBCPP_ENABLE_ASSERTIONS is such that it can be defined by the user regardless of how the library was built. This, alone, fixes most of the issues with the debug mode.

Another way to look at this patch is that it makes the bits of the debug mode that matter the most (i.e. non-complexity-modifying assertions) available to everyone, outside of the debug mode. This sets up the stage for either removing the remaining debug mode assertions (which are broken as everyone knows), or fixing them but keeping them separate from basic assertions. This is all cleanup / future work, but the basic assertions this allows us to enable is a real benefit that we get today.

libcxx/CMakeLists.txt

681–685

Because that is related to LIBCXX_DEBUG_BUILD, and that appears to still be used (in principle) on Windows (see for example https://github.com/llvm/llvm-project/blob/main/libcxx/utils/libcxx/test/config.py#L273).

Basically, I'm trying to clean some stuff up, but I'm also trying to be careful not to change anything unrelated to this PR specifically. I do definitely plan to look into it when I get to it.

libcxx/docs/BuildingLibcxx.rst

219–222

Agreed, will change to "the compiled library".

libcxx/docs/UsingLibcxx.rst

132

You are right, however D121129 fixes that and makes __do_compare_assert a debug-mode assertion instead of a regular assertion.

IMO the current description is sufficient: it says that we aim not to change the complexity of algorithm, and I think that should be the criterion that we start with.

158–161

I agree, this is error prone and not very ergonomic. It's also not simple to change this:

• The assertion handler needs to have been declared before any libc++ code is included. Otherwise, the compiler will complain that we are using ::HelloWorldAssertionHandler in the code but it hasn't been declared yet.

• The assertion handler needs to be aware of the type std::__libcpp_assertion_info, which means that the declaration of HelloWorldAssertionHandler must come after including <__assert>, and before including anything else. That's why we have this weird include sandwich.

I think I can get rid of the second requirement by passing primitive types to the assertion handler, i.e. passing char const* file, int line, char const* predicate, char const* message) directly to the function. However, that prevents me from being backwards compatible with users that might have been setting __libcpp_debug_function (because I use the conversion operator in __libcpp_assertion_info to achieve that).

I'm not sure how to solve the first problem, though. In practice, I would expect that people use -include <path-to-header-that-declares-the-assertion-handler>.

To make progress, would you be OK with me changing the documentation to:

// In HelloWorldAssert.h
#define _LIBCPP_ASSERTION_HANDLER(info) ::HelloWorldAssertionHandler(info)
#include <__assert>

// This could also be defined out-of-line, but we're defining it inline
// for simplicity in this example.
inline void HelloWorldAssertionHandler(std::__libcpp_assertion_info info) {
  std::printf("Assertion %s failed at %s:%d, more info: %s",
              info.__predicate(), info.file(), info.__line(), info.__message());
  std::abort();
}

// In HelloWorld.cpp
// Make sure to pass `-include path/to/HelloWorldAssert.h` to the compiler
#include <vector>

int main() {
  std::vector<int> v;
  int& x = v[0]; // Your assertion handler will be called here if _LIBCPP_ENABLE_ASSERTIONS=1
}

If we decide to break backwards compatibility with the debug mode, we can simplify this to (I'm happy to do it, but I would prefer doing it separately since it's a breaking change):

// In HelloWorldAssert.h
#define _LIBCPP_ASSERTION_HANDLER(info) ::HelloWorldAssertionHandler(info)
// This could also be defined out-of-line, but we're defining it inline
// for simplicity in this example.
inline void HelloWorldAssertionHandler(char const* file, int line, char const* expression, char const* message) {
  std::printf("Assertion %s failed at %s:%d, more info: %s", expression, file, line, message);
  std::abort();
}

// Same story from HelloWorld.cpp, still need `-include path/to/HelloWorldAssert.h`

167

I agree, and it's useful for e.g. D116950 anyways.

libcxx/src/debug.cpp

43–46

Well, we can't really remove it from the dylib because some folks may depend on that symbol being defined. So it has to stay in the dylib, at the very least.

And as I've hinted at, I believe we should remove those from our headers in the future: __libcpp_set_debug_function, __libcpp_debug_function and __libcpp_abort_debug_function. Indeed, those are effectively supplanted by this new suggested assertion handler.

[Mordante]

I haven't looked at the code and design in depth, but have some remarks already.

libcxx/docs/UsingLibcxx.rst

166

Do users need to do this in every translation unit? (Looking at the macro I assume yes.)

167

I think when we need an internal header it would be a good idea to add a new header specifically for that purpose.
__libcpp_assertion_hander for example. That header can then include __assert, but that way we can change our implementation without breaking user code.

171

How about adding a [[noreturn]] attribute on the function?

libcxx/include/__assert

21

I think this looks better in code that is visible to users.

libcxx/include/__availability

87

What happens when a user enables this on Apple? Will it result in a compilation or a linkage error?

[• Quuxplusone]

libcxx/docs/UsingLibcxx.rst

158–161

Ah, I see that I did forget something: you can't just -D_LIBCPP_ASSERTION_HANDLER=foo and then include libc++ headers, because you also need to provide a forward declaration of foo (otherwise it's an undeclared identifier). Gross.
(Backstory: My previous experiences with defining handlers for things via macros is all from https://github.com/troydhanson/uthash , where the handlers are used only within macro expansions themselves, and thus forward declarations are never needed; you just have to declare foo sometime before the first call to HASH_ADD-or-whatever.)

In absolute terms I'm not a huge fan of -include either... but yeah, I think putting -include HelloWorld.h into your build flags is a big improvement over putting forward declarations and stuff above the libc++ headers in every translation unit. :)

167

+1 Mordante's idea of <__libcpp_assertion_handler>, except that I worry that "one header per entry point" is too fine a level of granularity. Like maybe it should be <__libcpp_assert> and pull in all the stuff relevant to assertion handlers (__libcpp_assertion_handler, __libcpp_assertion_handler_type, __libcpp_set_assertion_handler...). And then that's just <__assert> (i.e. what Louis has done) but with a libcpp on the front. Which might still be a nice touch.

If we were to rename __assert to __libcpp_assert, would that hurt any users? (No, because __assert hasn't shipped yet?)
If we were to rename it, what would be connoted by the symmetry between __libcpp_assert and __libcpp_version? Would that be a good thing or a bad thing? (Is __libcpp_version even shipped? I have never bothered to understand its job.)

Is this a job for a subdirectory, like <libcpp/assert.h> or <ext/libcpp_assertions.h> or something?

[ldionne]

libcxx/docs/UsingLibcxx.rst

166

Yes, they do, but as we've mentioned, this should be done from the compiler command-line.

167

I'm not sure I understand the request here. We already have a header that includes "everything related to assertions", and the name of that header is <__assert>. We can discuss changing the name of that header, no problem, but I'm not sure I understand your request if it's not just to rename the header. What would < __libcpp_assertion_hander> contain?

libcxx/include/__availability

87

If a user tries to enable assertions on Apple and is deploying to a platform that doesn't support the default handler (which is all platforms for the time being), they will get a compilation error saying they are using the default handler, which hasn't been introduced yet.

If they define their own custom handler, they won't get any error.

Eventually, once we actually ship a dylib with support for the assertion handler, this will move from __attribute__((unavailable)) on line 160 below to __attribute__((availability(macosx,strict,introduced=SOME VERSION))).

[ldionne]

Address review comments except the ask for <__libcpp_assertion_hander>, which I don't understand yet.

[• Quuxplusone]

libcxx/docs/UsingLibcxx.rst

167

FWIW, I interpreted @Mordante's comment as a reply to @philnik's suggestion; i.e. my interpretation of this thread has been [all words are my own and quite possibly not what people were actually intending to convey]:

L: "I'm not thrilled about making people include <__assert>, because it isn't very discoverable, and it might encourage them to include other detail headers like <__string>, which we do not want them to."
P: "How about <cassert> then? it's more discoverable, and obviously user-facing."
M: "How about <__libcpp_assertion_handler>? it's named exactly the same as the facility they must already know about because they're trying to use it, and maybe the libcpp prefix indicates that it's more implementation-specific and also more special than <__string> and so on."
Q: "Yeah, not <cassert>, and I like the libcpp prefix idea. Or at least some sort of prefix that separates this thing from <__string> and <__hash_table>. How about put it in a subdirectory?"

@ldionne wrote:

I'm not sure I understand your request if it's not just to rename the header.

My interpretation was that it did boil down to just a request to rename the header, but I could be wrong.

[Mordante]

I've given this review some more thoughts and I'm not sure whether this is the best approach. I've send you a message to look at a slightly different approach.

libcxx/benchmarks/algorithms.bench.cpp

154

Is this intended or for testing. When intended please commit it separately.

libcxx/docs/UsingLibcxx.rst

167

Correct. Except I wasn't aware __debug was recent. Hence my proposal to have a separate header to include __debug. When __debug is still new renaming seems better than adding an additional header.

[ldionne]

libcxx/benchmarks/algorithms.bench.cpp

154

Agreed -- see D121244.

libcxx/docs/UsingLibcxx.rst

167

I'm OK with having a header named __libcpp_assertion_handler or ideally __libcpp_assert for users to include, instead of <__assert>. However, I do not want to merge __debug and __assert (however it ends up being named), specifically because I want to keep a clear distinction between the notion of assertions and the notion of a debugging mode. This lack of distinction is what has been muddying the waters for so long, and specifically what I'm trying to fix.

Most of our code wants the ability to add assertions for preconditions, UB and stuff like that. Only a tiny fraction of our code cares about the full debug mode -- basically only containers and a couple of algorithms.

[Mordante]

In general LGTM, I want to wait with approval after the CI is green. (I also like to see whether you think the weak-def version is a better solution.)

libcxx/docs/UsingLibcxx.rst

167

Sorry my bad I meant __assert instead of __debug.

170

The current version is ill-formed.

[ldionne]

Abandoning in favour of the third design I've tried: D121478

libcxx/CMakeLists.txt

 
 #===============================================================================
 # Setup CMake Options
 #===============================================================================
 include(CMakeDependentOption)
 include(HandleCompilerRT)
 
 # Basic options ---------------------------------------------------------------
-option(LIBCXX_ENABLE_ASSERTIONS "Enable assertions independent of build mode." OFF)
+option(LIBCXX_ENABLE_ASSERTIONS
+  "Enable assertions inside the compiled library, and at the same time make it the
+   default when compiling user code. Note that assertions can be enabled or disabled
+   by users in their own code regardless of this option." OFF)
 option(LIBCXX_ENABLE_SHARED "Build libc++ as a shared library." ON)
 option(LIBCXX_ENABLE_STATIC "Build libc++ as a static library." ON)
 option(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY "Build libc++experimental.a" ON)
 set(ENABLE_FILESYSTEM_DEFAULT ON)
 if (WIN32 AND NOT MINGW)
   # Filesystem is buildable for windows, but it requires __int128 helper
   # functions, that currently are provided by libgcc or compiler_rt builtins.
   # These are available in MinGW environments, but not currently in MSVC
if (APPLE)
     add_link_flags("-undefined dynamic_lookup")
   else()
     # Relax this restriction from HandleLLVMOptions
     string(REPLACE "-Wl,-z,defs" "" CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS}")
   endif()
 endif()
 
 # Assertion flags =============================================================
-define_if(LIBCXX_ENABLE_ASSERTIONS -UNDEBUG)
-define_if_not(LIBCXX_ENABLE_ASSERTIONS -DNDEBUG)
-define_if(LIBCXX_ENABLE_ASSERTIONS -D_LIBCPP_DEBUG=0)
 define_if(LIBCXX_DEBUG_BUILD -D_DEBUG)
 if (LIBCXX_ENABLE_ASSERTIONS AND NOT LIBCXX_DEBUG_BUILD)
   # MSVC doesn't like _DEBUG on release builds. See PR 4379.
   define_if_not(LIBCXX_TARGETING_MSVC -D_DEBUG)
 endif()
 
 # Modules flags ===============================================================
 # FIXME The libc++ sources are fundamentally non-modular. They need special
 # versions of the headers in order to provide C++03 and legacy ABI definitions.
 # NOTE: The public headers can be used with modules in all other contexts.
 function(cxx_add_module_flags target)
   if (LLVM_ENABLE_MODULES)
     # Ignore that the rest of the modules flags are now unused.

 config_define_if(LIBCXX_NO_VCRUNTIME _LIBCPP_NO_VCRUNTIME)
 config_define_if(LIBCXX_ENABLE_PARALLEL_ALGORITHMS _LIBCPP_HAS_PARALLEL_ALGORITHMS)
 config_define_if_not(LIBCXX_ENABLE_FILESYSTEM _LIBCPP_HAS_NO_FILESYSTEM_LIBRARY)
 config_define_if_not(LIBCXX_ENABLE_RANDOM_DEVICE _LIBCPP_HAS_NO_RANDOM_DEVICE)
 config_define_if_not(LIBCXX_ENABLE_LOCALIZATION _LIBCPP_HAS_NO_LOCALIZATION)
 config_define_if_not(LIBCXX_ENABLE_UNICODE _LIBCPP_HAS_NO_UNICODE)
 config_define_if_not(LIBCXX_ENABLE_WIDE_CHARACTERS _LIBCPP_HAS_NO_WIDE_CHARACTERS)
 config_define_if_not(LIBCXX_ENABLE_VENDOR_AVAILABILITY_ANNOTATIONS _LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS)
+if (LIBCXX_ENABLE_ASSERTIONS)
+  config_define(1 _LIBCPP_ENABLE_ASSERTIONS_DEFAULT)
+else()
+  config_define(0 _LIBCPP_ENABLE_ASSERTIONS_DEFAULT)
+endif()
 # Incomplete features get their own specific disabling flags. This makes it
 # easier to grep for target specific flags once the feature is complete.
 config_define_if_not(LIBCXX_ENABLE_INCOMPLETE_FEATURES _LIBCPP_HAS_NO_INCOMPLETE_FORMAT)
 config_define_if_not(LIBCXX_ENABLE_INCOMPLETE_FEATURES _LIBCPP_HAS_NO_INCOMPLETE_RANGES)
 
 if (LIBCXX_ABI_DEFINES)
   set(abi_defines)
   foreach (abi_define ${LIBCXX_ABI_DEFINES})

libcxx/benchmarks/algorithms.bench.cpp

void fillValues(std::vector<std::string>& V, size_t N, Order O) {
   } else {
     while (V.size() < N)
       V.push_back(getRandomString(64));
   }
 }
 
 template <class T>
 void sortValues(T& V, Order O) {
-  assert(std::is_sorted(V.begin(), V.end()));
   switch (O) {
   case Order::Random: {
     std::random_device R;
     std::mt19937 M(R());
     std::shuffle(V.begin(), V.end(), M);
     break;
   }
   case Order::Ascending:

libcxx/cmake/caches/Generic-assertions.cmake

 set(LIBCXX_ENABLE_ASSERTIONS ON CACHE BOOL "")
+set(LIBCXX_TEST_PARAMS "enable_assertions=True" CACHE STRING "")
+set(LIBCXXABI_TEST_PARAMS "enable_assertions=True" CACHE STRING "")

libcxx/docs/BuildingLibcxx.rst

.. option:: LIBCXX_INSTALL_HEADERS:BOOL
   **Default**: ``ON``
 
   Toggle the installation of the libc++ headers.
 
 .. option:: LIBCXX_ENABLE_ASSERTIONS:BOOL
 
   **Default**: ``OFF``
 
-  Build libc++ with assertions enabled.
+  Build libc++ with assertions enabled in the compiled library, and enable assertions
+  by default when building user code as well. Assertions can be turned off by users
+  by defining ``_LIBCPP_ENABLE_ASSERTIONS=0``. For details, see
+  :ref:`the documentation <assertions-mode>`.
 
 .. option:: LIBCXX_ENABLE_SHARED:BOOL
 
   **Default**: ``ON``
 
   Build libc++ as a shared library. Either `LIBCXX_ENABLE_SHARED` or
   `LIBCXX_ENABLE_STATIC` has to be enabled.
 

libcxx/docs/ReleaseNotes.rst

- The integer distributions ``binomial_distribution``, ``discrete_distribution``,
   ``geometric_distribution``, ``negative_binomial_distribution``, ``poisson_distribution``,
   and ``uniform_int_distribution`` now conform to the Standard by rejecting
   template parameter types other than ``short``, ``int``, ``long``, ``long long``,
   (as an extension) ``__int128_t``, and the unsigned versions thereof.
   In particular, ``uniform_int_distribution<int8_t>`` is no longer supported.
 
 - The C++14 function ``std::quoted(const char*)`` is no longer supported in
   C++03 or C++11 modes.
 
+- Libc++ now supports a variety of assertions that can be turned on to help catch
+  undefined behavior in user code. This new support is now separate from the old
+  (and incomplete) Debug Mode. Vendors can select whether the library they ship
+  should include assertions or not by default. For details, see
+  :ref:`the documentation <assertions-mode>` about this new feature.
+
 ABI Changes
 -----------
 
 - The ``_LIBCPP_ABI_USE_CXX03_NULLPTR_EMULATION`` macro controlling whether we use an
   emulation for ``std::nullptr_t`` in C++03 mode has been removed. After this change,
   ``_LIBCPP_ABI_USE_CXX03_NULLPTR_EMULATION`` will not be honoured anymore and there
   will be no way to opt back into the C++03 emulation of ``std::nullptr_t``.
 

libcxx/docs/UsingLibcxx.rst

 
 .. code-block:: bash
 
   $ gdb -ex "source <libcxx>/utils/gdb/libcxx/printers.py" \
         -ex "python register_libcxx_printer_loader()" \
         <args>
 
 
+.. _assertions-mode:
+
+Enabling the "safe libc++" mode
+===============================
+
+Libc++ contains a number of assertions whose goal is to catch undefined behavior in the
+library, usually caused by precondition violations. Those assertions do not aim to be
+exhaustive -- instead they aim to provide a good balance between safety and performance.
+In particular, these assertions do not change the complexity of algorithms. However, they
+might, in some cases, interfere with compiler optimizations.
+
+By default, these assertions are turned off. Vendors can decide to turn them on while building
+the compiled library by defining ``LIBCXX_ENABLE_ASSERTIONS=ON`` at CMake configuration time.
+When ``LIBCXX_ENABLE_ASSERTIONS`` is used, the compiled library will be built with assertions
+enabled, **and** user code will be built with assertions enabled by default. If ``LIBCXX_ENABLE_ASSERTIONS=OFF``
+at CMake configure time, the compiled library will not contain assertions and the default when
+building user code will be to have assertions disabled.
+
+Independently of any vendor-selected default, users can always control whether assertions
+are enabled in their code by defining ``_LIBCPP_ENABLE_ASSERTIONS=0|1`` before including any
+libc++ header. Note that if the compiled library was built by the vendor without assertions,
+functions compiled inside the static or shared library won't have assertions enabled even if
+the user defines ``_LIBCPP_ENABLE_ASSERTIONS=1`` (the same is true for the inverse case where
+the static or shared library was compiled **with** assertions but the user tries to disable
+them). However, most of the code in libc++ is in the headers, so the user-selected value for
+``_LIBCPP_ENABLE_ASSERTIONS`` (if any) will usually be respected.
+
+When an assertion fails, an assertion handler function is called. The library provides a default
+assertion handler that prints an error message and calls ``std::abort()``. Note that this assertion
+handler is provided by the static or shared library, so it is only available when deploying to a
+platform where the compiled library is sufficiently recent. However, users can also define their
+own assertion handler to replace the default one, which can be useful to provide custom behavior,
+or when deploying to older platforms where the default assertion handler isn't available.
+
+Replacing the default assertion handler is done by defining the ``_LIBCPP_ASSERTION_HANDLER`` macro,
+which is called with an instance of ``std::__libcpp_assertion_info``. Generally speaking,
+``_LIBCPP_ASSERTION_HANDLER`` should expand to a fully qualified function name. For example:
+
+.. code-block:: cpp
+
+  // In HelloWorldHandler.h
+  #define _LIBCPP_ASSERTION_HANDLER ::HelloWorldAssertionHandler
+  #include <__assert>
+
+  // This could also be defined in another translation unit, but we're defining it inline
+  // for simplicity in this example.
+  inline [[noreturn]] void HelloWorldAssertionHandler(std::__libcpp_assertion_info info) {
+      std::printf("Assertion %s failed at %s:%d, more info: %s",
+                  info.__predicate(), info.file(), info.__line(), info.__message());
+      std::abort();
+  }
+
+  // In HelloWorld.cpp
+  // Note: You must add `-include path/to/HelloWorldHandler.h` to your compiler flags.
+  #include <vector>
+
+  int main() {
+    std::vector<int> v;
+    int& x = v[0]; // Your assertion handler will be called here if _LIBCPP_ENABLE_ASSERTIONS=1
+  }
+
+Also note that the assertion handler should usually not return. Since the assertions in libc++
+catch undefined behavior, your code will proceed with undefined behavior if your assertion
+handler is called but does return.
+
+
 Libc++ Configuration Macros
 ===========================
 
 Libc++ provides a number of configuration macros which can be used to enable
 or disable extended libc++ behavior, including enabling "debug mode" or
 thread safety annotations.
 
 **_LIBCPP_DEBUG**:

libcxx/include/__assert

 // -*- 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
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP___ASSERT
 #define _LIBCPP___ASSERT
 
+#include <__availability>
 #include <__config>
 #include <iosfwd> // for std::string
 
 #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.
 #if _LIBCPP_DEBUG_LEVEL >= 1
-#   define _LIBCPP_ASSERT(x, m) ((x) ? (void)0 : ::std::__libcpp_debug_function(::std::__libcpp_debug_info(__FILE__, __LINE__, #x, m)))
+# ifndef _LIBCPP_ENABLE_ASSERTIONS
+#   define _LIBCPP_ENABLE_ASSERTIONS 1
+# endif
+# ifndef _LIBCPP_ASSERTION_HANDLER
+#   define _LIBCPP_ASSERTION_HANDLER ::std::__libcpp_debug_function
+# endif
+#endif
+
+#ifndef _LIBCPP_ENABLE_ASSERTIONS
+# define _LIBCPP_ENABLE_ASSERTIONS _LIBCPP_ENABLE_ASSERTIONS_DEFAULT
+#endif
+
+#if _LIBCPP_ENABLE_ASSERTIONS != 0 && _LIBCPP_ENABLE_ASSERTIONS != 1
+# error "_LIBCPP_ENABLE_ASSERTIONS must be set to 0 or 1"
+#endif
+
+#ifndef _LIBCPP_ASSERTION_HANDLER
+# define _LIBCPP_ASSERTION_HANDLER ::std::__libcpp_default_assertion_handler
+#endif
+
+#if _LIBCPP_ENABLE_ASSERTIONS
+# define _LIBCPP_ASSERT(x, m) ((x) ? (void)0 : _LIBCPP_ASSERTION_HANDLER(::std::__libcpp_assertion_info(__FILE__, __LINE__, #x, m)))
 #else
-#   define _LIBCPP_ASSERT(x, m) ((void)0)
+# define _LIBCPP_ASSERT(x, m) ((void)0)
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+//////////////////////////////////////////////////////////////////
+// For backwards compatibility with clients of the Debug mode
 struct _LIBCPP_TEMPLATE_VIS __libcpp_debug_info {
   _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
   __libcpp_debug_info()
       : __file_(nullptr), __line_(-1), __pred_(nullptr), __msg_(nullptr) {}
   _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
   __libcpp_debug_info(const char* __f, int __l, const char* __p, const char* __m)
     : __file_(__f), __line_(__l), __pred_(__p), __msg_(__m) {}
 

 /// __libcpp_abort_debug_function - A debug handler that aborts when called.
 _LIBCPP_NORETURN _LIBCPP_FUNC_VIS
 void __libcpp_abort_debug_function(__libcpp_debug_info const&);
 
 /// __libcpp_set_debug_function - Set the debug handler to the specified
 ///    function.
 _LIBCPP_FUNC_VIS
 bool __libcpp_set_debug_function(__libcpp_debug_function_type __func);
+//////////////////////////////////////////////////////////////////
+
+struct __libcpp_assertion_info {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR
+  explicit __libcpp_assertion_info(char const* __file, int __line, char const* __predicate, char const* __message)
+    : __file_(__file), __line_(__line), __predicate_(__predicate), __message_(__message)
+  { }
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR
+  char const* __file() const { return __file_; }
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR
+  int __line() const { return __line_; }
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR
+  char const* __predicate() const { return __predicate_; }
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR
+  char const* __message() const { return __message_; }
+
+  // For backwards compatibility
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR
+  operator __libcpp_debug_info() const {
+    return __libcpp_debug_info(__file_, __line_, __predicate_, __message_);
+  }
+
+private:
+  char const* __file_;
+  int __line_;
+  char const* __predicate_;
+  char const* __message_;
+};
+
+_LIBCPP_NORETURN _LIBCPP_AVAILABILITY_ASSERTION_HANDLER _LIBCPP_EXPORTED_FROM_ABI
+void __libcpp_default_assertion_handler(__libcpp_assertion_info);
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___ASSERT

libcxx/include/__availability

#if defined(_LIBCPP_BUILDING_LIBRARY) ||                                        \
     !__has_extension(pragma_clang_attribute_external_declaration)
 #   if !defined(_LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS)
 #       define _LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS
 #   endif
 #endif
 
 #if defined(_LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS)
 
+    // This controls whether the default assertion handler is provided by the library.
+    // When the default assertion handler is not provided, users either can't enable
+    // assertions in the library, or must provide their own assertion handler.
+#   define _LIBCPP_AVAILABILITY_ASSERTION_HANDLER
+
     // This controls the availability of std::shared_mutex and std::shared_timed_mutex,
     // which were added to the dylib later.
 #   define _LIBCPP_AVAILABILITY_SHARED_MUTEX
 // #   define _LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_shared_mutex
 // #   define _LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_shared_timed_mutex
 
     // These macros control the availability of std::bad_optional_access and
     // other exception types. These were put in the shared library to prevent
// #   define _LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_semaphore
     // The library is in development and not ABI stable yet. P2216 is
     // retroactively accepted in C++20. This paper contains ABI breaking
     // changes.
 #   define _LIBCPP_AVAILABILITY_FORMAT
 // #   define _LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_format
 
 #elif defined(__APPLE__)
 
+#   define _LIBCPP_AVAILABILITY_ASSERTION_HANDLER                               \
+        __attribute__((unavailable))
+
 #   define _LIBCPP_AVAILABILITY_SHARED_MUTEX                                    \
         __attribute__((availability(macosx,strict,introduced=10.12)))           \
         __attribute__((availability(ios,strict,introduced=10.0)))               \
         __attribute__((availability(tvos,strict,introduced=10.0)))              \
         __attribute__((availability(watchos,strict,introduced=3.0)))
 #   if (defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ < 101200) ||    \
         (defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__ < 100000) || \
         (defined(__ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__ < 100000) ||         \

libcxx/include/__config_site.in

 #cmakedefine _LIBCPP_TYPEINFO_COMPARISON_IMPLEMENTATION @_LIBCPP_TYPEINFO_COMPARISON_IMPLEMENTATION@
 #cmakedefine _LIBCPP_HAS_NO_FILESYSTEM_LIBRARY
 #cmakedefine _LIBCPP_HAS_PARALLEL_ALGORITHMS
 #cmakedefine _LIBCPP_HAS_NO_RANDOM_DEVICE
 #cmakedefine _LIBCPP_HAS_NO_LOCALIZATION
 #cmakedefine _LIBCPP_HAS_NO_WIDE_CHARACTERS
 #cmakedefine _LIBCPP_HAS_NO_INCOMPLETE_FORMAT
 #cmakedefine _LIBCPP_HAS_NO_INCOMPLETE_RANGES
+#cmakedefine01 _LIBCPP_ENABLE_ASSERTIONS_DEFAULT
 
 // __USE_MINGW_ANSI_STDIO gets redefined on MinGW
 #ifdef __clang__
 #  pragma clang diagnostic push
 #  pragma clang diagnostic ignored "-Wmacro-redefined"
 #endif
 
 @_LIBCPP_ABI_DEFINES@
 @_LIBCPP_EXTRA_SITE_DEFINES@
 
 #ifdef __clang__
 #  pragma clang diagnostic pop
 #endif
 
 #endif // _LIBCPP_CONFIG_SITE

libcxx/lib/abi/arm64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.debug.incomplete.abilist

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 No newline at end of file

libcxx/lib/abi/arm64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.nodebug.noincomplete.abilist

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 No newline at end of file

libcxx/lib/abi/x86_64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.debug.incomplete.abilist

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 No newline at end of file

libcxx/lib/abi/x86_64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.nodebug.noincomplete.abilist

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 No newline at end of file

libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.nonew.debug.incomplete.abilist

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 {'is_defined': True, 'name': '_ZTv0_n24_NSt3__19strstreamD1Ev', 'type': 'FUNC'}
 No newline at end of file

libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.nonew.debug.noincomplete.abilist

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 No newline at end of file

libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.nonew.nodebug.incomplete.abilist

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+{'is_defined': True, 'name': '_ZNSt3__134__libcpp_default_assertion_handlerENS_23__libcpp_assertion_infoE', 'type': 'FUNC'}
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 No newline at end of file

libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.noexceptions.nonew.debug.incomplete.abilist

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 {'is_defined': True, 'name': '_ZNSt3__134__construct_barrier_algorithm_baseERl', 'type': 'FUNC'}
+{'is_defined': True, 'name': '_ZNSt3__134__libcpp_default_assertion_handlerENS_23__libcpp_assertion_infoE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__13cinE', 'size': 168, 'type': 'OBJECT'}
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 {'is_defined': True, 'name': '_ZTv0_n24_NSt3__114basic_iostreamIcNS_11char_traitsIcEEED0Ev', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZTv0_n24_NSt3__114basic_iostreamIcNS_11char_traitsIcEEED1Ev', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZTv0_n24_NSt3__19strstreamD0Ev', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZTv0_n24_NSt3__19strstreamD1Ev', 'type': 'FUNC'}
 No newline at end of file

libcxx/src/CMakeLists.txt

 set(LIBCXX_LIB_CMAKEFILES_DIR "${CMAKE_CURRENT_BINARY_DIR}${CMAKE_FILES_DIRECTORY}"  PARENT_SCOPE)
 
 # Get sources
 set(LIBCXX_SOURCES
   algorithm.cpp
   any.cpp
+  assert.cpp
   atomic.cpp
   barrier.cpp
   bind.cpp
   charconv.cpp
   chrono.cpp
   condition_variable.cpp
   condition_variable_destructor.cpp
   exception.cpp
set(LIBCXX_SOURCES
   utility.cpp
   valarray.cpp
   variant.cpp
   vector.cpp
   )
 
 if (LIBCXX_ENABLE_DEBUG_MODE_SUPPORT)
   list(APPEND LIBCXX_SOURCES
-    assert.cpp
     debug.cpp
     )
 endif()
 
 if (LIBCXX_ENABLE_RANDOM_DEVICE)
   list(APPEND LIBCXX_SOURCES
     random.cpp
     )

libcxx/src/assert.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
 //
 //===----------------------------------------------------------------------===//
 
 #include <__assert>
 #include <__config>
 #include <cstdio>
 #include <cstdlib>
-#include <string>
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-std::string __libcpp_debug_info::what() const {
-  string msg = __file_;
-  msg += ":" + std::to_string(__line_) + ": _LIBCPP_ASSERT '";
-  msg += __pred_;
-  msg += "' failed. ";
-  msg += __msg_;
-  return msg;
-}
-
-_LIBCPP_NORETURN void __libcpp_abort_debug_function(__libcpp_debug_info const& info) {
-    std::fprintf(stderr, "%s\n", info.what().c_str());
-    std::abort();
+void __libcpp_default_assertion_handler(__libcpp_assertion_info __info) {
+  std::fprintf(stderr, "%s:%d: libc++ assertion '%s' failed. %s\n",
+                      __info.__file(), __info.__line(), __info.__predicate(), __info.__message());
 }
 
-constinit __libcpp_debug_function_type __libcpp_debug_function = __libcpp_abort_debug_function;
-
-bool __libcpp_set_debug_function(__libcpp_debug_function_type __func) {
-  __libcpp_debug_function = __func;
-  return true;
-}
-
 _LIBCPP_END_NAMESPACE_STD

libcxx/src/debug.cpp

 #  include <mutex>
 #  if defined(__ELF__) && defined(_LIBCPP_LINK_PTHREAD_LIB)
 #    pragma comment(lib, "pthread")
 #  endif
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+std::string __libcpp_debug_info::what() const {
+  string msg = __file_;
+  msg += ":" + std::to_string(__line_) + ": _LIBCPP_ASSERT '";
+  msg += __pred_;
+  msg += "' failed. ";
+  msg += __msg_;
+  return msg;
+}
+
+_LIBCPP_NORETURN void __libcpp_abort_debug_function(__libcpp_debug_info const& info) {
+    std::fprintf(stderr, "%s\n", info.what().c_str());
+    std::abort();
+}
+
+constinit __libcpp_debug_function_type __libcpp_debug_function = __libcpp_abort_debug_function;
+
+bool __libcpp_set_debug_function(__libcpp_debug_function_type __func) {
+  __libcpp_debug_function = __func;
+  return true;
+}
+
 _LIBCPP_FUNC_VIS
 __libcpp_db*
 __get_db()
 {
     static _LIBCPP_NO_DESTROY __libcpp_db db;
     return &db;
 }
 

libcxx/test/libcxx/assertion_handler.legacy_debug_mode.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
+//
+//===----------------------------------------------------------------------===//
+
+// Test that we still honor any legacy debug handler set in user code.
+
+#define _LIBCPP_DEBUG 0
+#include <cassert>
+#include <vector> // any random header that definitely includes assertions
+
+bool handler_called = false;
+inline void HelloWorldDebugHandler(std::__libcpp_debug_info const&) {
+  handler_called = true;
+}
+
+int main(int, char**) {
+  std::__libcpp_set_debug_function(&HelloWorldDebugHandler);
+  _LIBCPP_ASSERT(false, "message");
+  assert(handler_called);
+  return 0;
+}

libcxx/test/libcxx/assertion_handler.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
+//
+//===----------------------------------------------------------------------===//
+
+// Test that we can set a custom assertion handler.
+
+#define _LIBCPP_ENABLE_ASSERTIONS 1
+#define _LIBCPP_ASSERTION_HANDLER ::HelloWorldAssertionHandler
+#include <__assert>
+
+bool handler_called = false;
+inline void HelloWorldAssertionHandler(std::__libcpp_assertion_info) {
+  handler_called = true;
+}
+
+#include <cassert>
+
+int main(int, char**) {
+  _LIBCPP_ASSERT(false, "message");
+  assert(handler_called);
+  return 0;
+}

libcxx/utils/libcxx/test/params.py

Parameter(name='long_tests', choices=[True, False], type=bool, default=True,
             ]),
 
   Parameter(name='enable_debug_tests', choices=[True, False], type=bool, default=True,
             help="Whether to enable tests that exercise the libc++ debugging mode.",
             actions=lambda enabled: [] if enabled else [
               AddFeature('libcxx-no-debug-mode')
             ]),
 
+  Parameter(name='enable_assertions', choices=[True, False], type=bool, default=False,
+            help="Whether to enable assertions when compiling the test suite. This is only meaningful when "
+                 "running the tests against libc++.",
+            actions=lambda assertions: [AddCompileFlag('-D_LIBCPP_ENABLE_ASSERTIONS=1')] if assertions else []),
+
   Parameter(name='additional_features', type=list, default=[],
             help="A comma-delimited list of additional features that will be enabled when running the tests. "
                  "This should be used sparingly since specifying ad-hoc features manually is error-prone and "
                  "brittle in the long run as changes are made to the test suite.",
             actions=lambda features: [AddFeature(f) for f in features]),
 ]
 
 DEFAULT_PARAMETERS += [