[libc++] Disables transitive includes in library.

[Mordante]

Summary

I noticed this wile investigating https://llvm.org/PR61314

[philnik]

libcxx/include/__config

278–279 ↗

(On Diff #504158)

I don't understand. Why would this make a difference whether a user sets it or not? We should be able to set this when building the library, just like we set _LIBCPP_BUILDING_LIBRARY (and I think we should set it in CMake).

[Mordante]

libcxx/include/__config

278–279 ↗

(On Diff #504158)

I'm not sure whether that would break some unexpected cases where vendors do things we don't expect.
For example I know Chromium uses this option and they build their own libc++.
If @ldionne agrees with you I'm happy to move it to the CMakeLists,txt directly below where we define
_LIBCPP_BUILDING_LIBRARY.

[ldionne]

LGTM once set within CMake

libcxx/include/__config

278–279 ↗

(On Diff #504158)

Hmm yes, I think this would be the better approach:

target_compile_definitions(${target} PRIVATE -D_LIBCPP_BUILDING_LIBRARY)

// Make sure the library can be build without transitive includes. This makes
// it easier to upgrade the library to a newer language standard without build
// errors.
target_compile_definitions(${target} PRIVATE -D_LIBCPP_REMOVE_TRANSITIVE_INCLUDES)

[Mordante]

libcxx/include/__config

278–279 ↗

(On Diff #504158)

Thanks both for the feedback. I was doubting between these two options.

[Mordante]

Addresses review comments. Give it a CI run before landing.

libcxx/CMakeLists.txt

function(cxx_add_basic_build_flags target)
   if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
     target_add_compile_flags_if_supported(${target} PRIVATE -fvisibility=hidden)
   endif()
 
   # Let the library headers know they are currently being used to build the
   # library.
   target_compile_definitions(${target} PRIVATE -D_LIBCPP_BUILDING_LIBRARY)
 
+  # Make sure the library can be build without transitive includes. This makes
+  # it easier to upgrade the library to a newer language standard without build
+  # errors.
+  target_compile_definitions(${target} PRIVATE -D_LIBCPP_REMOVE_TRANSITIVE_INCLUDES)
+
   if (NOT LIBCXX_ENABLE_NEW_DELETE_DEFINITIONS)
     target_compile_definitions(${target} PRIVATE -D_LIBCPP_DISABLE_NEW_DELETE_DEFINITIONS)
   endif()
 
   if (C_SUPPORTS_COMMENT_LIB_PRAGMA)
     if (LIBCXX_HAS_PTHREAD_LIB)
       target_compile_definitions(${target} PRIVATE -D_LIBCPP_LINK_PTHREAD_LIB)
     endif()