[libc++][C++20 modules] Tests no except build.

[Mordante]

Summary

Depends on D158661

[ldionne]

LGTM but we should take a look at target_compile_features.

libcxx/modules/CMakeLists.txt.in

42

Can we use a feature with target_compile_features here instead? That would likely solve the MSVC issue as well.

[Mordante]

libcxx/modules/CMakeLists.txt.in

42

Based on https://cmake.org/cmake/help/latest/prop_gbl/CMAKE_CXX_KNOWN_FEATURES.html#prop_gbl:CMAKE_CXX_KNOWN_FEATURES there is no such feature. For now I keep it as is. What is the MSVC issue we're having? Is there a bug report?

Note the goal is to remove this file in the future and let the build system do the right thing.

libcxx/cmake/caches/Generic-no-exceptions.cmake

+set(LIBCXX_ENABLE_STD_MODULES ON CACHE BOOL "") # TODO MODULES Remove when enabled automatically.
 set(LIBCXX_ENABLE_EXCEPTIONS OFF CACHE BOOL "")
 set(LIBCXXABI_ENABLE_EXCEPTIONS OFF CACHE BOOL "")

libcxx/docs/Modules.rst

  * The following "parts disabled" configuration options are supported
 
    * ``LIBCXX_ENABLE_LOCALIZATION``
    * ``LIBCXX_ENABLE_WIDE_CHARACTERS``
    * ``LIBCXX_ENABLE_THREADS``
    * ``LIBCXX_ENABLE_FILESYSTEM``
    * ``LIBCXX_ENABLE_RANDOM_DEVICE``
    * ``LIBCXX_ENABLE_UNICODE``
+   * ``LIBCXX_ENABLE_EXCEPTIONS`` [#note-no-windows]_
 
  * A C++20 based extension
 
+.. note::
+
+   .. [#note-no-windows] This configuration will probably not work on Windows
+                         due to hard-coded compilation flags.
+
 Some of the current limitations
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
  * There is no official build system support, libc++ has experimental CMake support
  * Requires CMake 3.26 for C++20 support
  * Requires CMake 3.26 for C++23 support
  * Requires CMake 3.27 for C++26 support
  * Requires Ninja 1.11

libcxx/modules/CMakeLists.txt.in

macro(compile_define_if_not condition def)
   endif()
 endmacro()
 macro(compile_define_if condition def)
   if (${condition})
     target_compile_definitions(std PRIVATE ${def})
   endif()
 endmacro()
 
-if(NOT @LIBCXX_ENABLE_EXCEPTIONS@ OR NOT @LIBCXXABI_ENABLE_EXCEPTIONS@)
-  message(FATAL_ERROR "Modules without exception support is not yet implemented.")
-endif()
-
 add_library(std)
 target_sources(std
   PUBLIC FILE_SET cxx_modules TYPE CXX_MODULES FILES
     std.cppm
 )
 
 target_compile_definitions(std PRIVATE _LIBCPP_ENABLE_EXPERIMENTAL)
 target_include_directories(std SYSTEM PRIVATE @LIBCXX_CONFIGURED_INCLUDE_DIRS@)
 
+if (NOT @LIBCXX_ENABLE_EXCEPTIONS@)
+  target_compile_options(std PUBLIC -fno-exceptions)
+endif()
+
 target_compile_options(std
   PUBLIC
     -nostdinc++
     -Wno-reserved-module-identifier
     -Wno-reserved-user-defined-literal
     @LIBCXX_COMPILE_FLAGS@
 )
 set_target_properties(std
   PROPERTIES
     OUTPUT_NAME   "c++std"
 )

libcxx/utils/ci/buildkite-pipeline.yml

- label: "No experimental features"
       timeout_in_minutes: 120
 
     - label: "No exceptions"
       command: "libcxx/utils/ci/run-buildbot generic-no-exceptions"
       artifact_paths:
         - "**/test-results.xml"
         - "**/*.abilist"
       env:
-          CC: "clang-${LLVM_HEAD_VERSION}"
-          CXX: "clang++-${LLVM_HEAD_VERSION}"
+          # Note: Modules require and absolute path for clang-scan-deps
+          # https://github.com/llvm/llvm-project/issues/61006
+          CC: "/usr/lib/llvm-${LLVM_HEAD_VERSION}/bin/clang"
+          CXX: "/usr/lib/llvm-${LLVM_HEAD_VERSION}/bin/clang++"
+          CMAKE: "/opt/bin/cmake"
           ENABLE_CLANG_TIDY: "On"
       agents:
         queue: "libcxx-builders"
         os: "linux"
       retry:
         automatic:
           - exit_status: -1  # Agent was lost
             limit: 2