[libc++] Add a persistent way to disable availability

[smeenai]

Summary

Add _LIBCPP_DISABLE_AVAILABILITY to the site config options, and add a
cmake option LIBCXX_DISABLE_AVAILABILITY to control the site config.
This is similar to other options which influence headers and therefore
shold have some way to be set persistently.

[mehdi_amini]

Seems fine to me. Adding @dexonsmith (I don't know who maintain libc++ at Apple right now)

[dexonsmith]

LGTM.

[mclow.lists]

What's the use case here? What are you trying to accomplish?

[smeenai]

In D36713#841478, @mclow.lists wrote:

What's the use case here? What are you trying to accomplish?

The _LIBCPP_DISABLE_AVAILABILITY macro already exists, but since it influences headers, it's a lot more useful to have it as a site config option, so that it can be set consistently.

I have an internal codebase where the availability macros cause issues, for complicated reasons that I unfortunately can't go into detail about. Rather than having to add _LIBCPP_DISABLE_AVAILABILITY to every single build using the libc++ headers, it would be really nice to be able to do the disabling via the site config. I know that's not a very satisfying explanation, but this seemed like a small and generally useful change, and it's consistent with how we're handling other macros that influence headers.

[smeenai]

@mclow.lists does this seem reasonable to you?

[smeenai]

Ping.

[smeenai]

@mclow.lists, any final verdict here? I ended up doing this differently for my internal use case, so if you think this isn't generally useful, I'm happy to abandon.

[smeenai]

I ended up handling this differently internally (via a custom site config). If someone else ends up needing the same functionality, they can revive it.

CMakeLists.txt

 config_define_if_not(LIBCXX_ENABLE_THREADS _LIBCPP_HAS_NO_THREADS)
 config_define_if_not(LIBCXX_ENABLE_MONOTONIC_CLOCK _LIBCPP_HAS_NO_MONOTONIC_CLOCK)
 config_define_if_not(LIBCXX_ENABLE_THREAD_UNSAFE_C_FUNCTIONS _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS)
 
 config_define_if(LIBCXX_HAS_PTHREAD_API _LIBCPP_HAS_THREAD_API_PTHREAD)
 config_define_if(LIBCXX_HAS_EXTERNAL_THREAD_API _LIBCPP_HAS_THREAD_API_EXTERNAL)
 config_define_if(LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY _LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL)
 config_define_if(LIBCXX_HAS_MUSL_LIBC _LIBCPP_HAS_MUSL_LIBC)
+config_define_if(LIBCXX_DISABLE_AVAILABILITY _LIBCPP_DISABLE_AVAILABILITY)
 
 # By default libc++ on Windows expects to use a shared library, which requires
 # the headers to use DLL import/export semantics. However when building a
 # static library only we modify the headers to disable DLL import/export.
 if (DEFINED WIN32 AND LIBCXX_ENABLE_STATIC AND NOT LIBCXX_ENABLE_SHARED)
   message(STATUS "Generating custom __config for non-DLL Windows build")
   config_define(ON _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS)
 endif()

docs/UsingLibcxx.rst

 
 **_LIBCPP_DISABLE_ADDITIONAL_DIAGNOSTICS**:
   This macro disables the additional diagnostics generated by libc++ using the
   `diagnose_if` attribute. These additional diagnostics include checks for:
 
     * Giving `set`, `map`, `multiset`, `multimap` a comparator which is not
       const callable.
 
+**LIBCXX_DISABLE_AVAILABILITY**:
+  This macro disables availability annotations in the libc++ headers.
+
 C++17 Specific Configuration Macros
 -----------------------------------
 **_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES**:
   This macro is used to re-enable all the features removed in C++17. The effect
   is equivalent to manually defining each macro listed below.
 
 **_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS**:
   This macro is used to re-enable the `set_unexpected`, `get_unexpected`, and
   `unexpected` functions, which were removed in C++17.
 
 **_LIBCPP_ENABLE_CXX17_REMOVED_AUTO_PTR**:
   This macro is used to re-enable `std::auto_ptr` in C++17.

include/__config_site.in

 #cmakedefine _LIBCPP_HAS_NO_STDOUT
 #cmakedefine _LIBCPP_HAS_NO_THREADS
 #cmakedefine _LIBCPP_HAS_NO_MONOTONIC_CLOCK
 #cmakedefine _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
 #cmakedefine _LIBCPP_HAS_MUSL_LIBC
 #cmakedefine _LIBCPP_HAS_THREAD_API_PTHREAD
 #cmakedefine _LIBCPP_HAS_THREAD_API_EXTERNAL
 #cmakedefine _LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL
+#cmakedefine _LIBCPP_DISABLE_AVAILABILITY
 #cmakedefine _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS
 
 #endif // _LIBCPP_CONFIG_SITE

utils/libcxx/test/config.py

def parse_config_site_and_add_features(self, header):
         # Transform each macro name into the feature name used in the tests.
         # Ex. _LIBCPP_HAS_NO_THREADS -> libcpp-has-no-threads
         for m in feature_macros:
             if modules_enabled:
                 define = '-D%s' % m
                 if feature_macros[m]:
                     define += '=%s' % (feature_macros[m])
                 self.cxx.compile_flags += [define]
+            if m == '_LIBCPP_DISABLE_AVAILABILITY':
+                assert not self.with_availability, \
+                    'Availability is disabled in the site configuration'
+                continue
             if m == '_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS':
                 continue
             if m == '_LIBCPP_ABI_VERSION':
                 self.config.available_features.add('libcpp-abi-version-v%s'
                     % feature_macros[m])
                 continue
             assert m.startswith('_LIBCPP_HAS_') or m == '_LIBCPP_ABI_UNSTABLE'
             m = m.lower()[1:].replace('_', '-')