C++23 stl modules

[Diyou]

This adds the libcxx/modules folder from https://github.com/emscripten-core/llvm-project needed for CMake to have the std and std.compat module interfaces generated during configuration.

At the moment emscripten only needs to roll the updated llvm headers to successfully compile those for c++23.
For c++26 we need to implement std::ranges::views::indices or comment out
https://github.com/Diyou/emscripten/blob/922e9aa8b84613afbb1651459054b1c60b55e69b/system/lib/libcxx/modules/std/ranges.inc#L120

Resolves #21143

[sbc100]

Nice!

To be clear are these files taken from the emscripten-libs-21 branch?

We normally use the system/lib/update_libcxx.py script to update these files. Perhaps you could try using that (maybe it needs modifying to include the modules directory somehow?)

[sbc100]

lgtm!

But maybe we could add a test for this ?

[Diyou]

But maybe we could add a test for this ?

Might need to get Clang-CXX-CXXImportStd working if the test environment doesn't support cmake 4.2 for CMAKE_CXX_STDLIB_MODULES_JSON .

In this case we'd need to patch emcc.py to allow using a relative search path for -print-file-name with CMAKE_CXX_COMPILER_ID_ARG1

[sbc100]

But maybe we could add a test for this ?

Might need to get Clang-CXX-CXXImportStd working if the test environment doesn't support cmake 4.2 for CMAKE_CXX_STDLIB_MODULES_JSON .

In this case we'd need to patch emcc.py to allow using a relative search path for -print-file-name with CMAKE_CXX_COMPILER_ID_ARG1

If we can get cmake 4.2 installed on at least one of the CI machines would that address the issue?

I would be fine landing this change standalone and following up with a test too.

[sbc100]

I think this should be referring to the installed version in the emscripten cache/sysroot not the version in source control.

[Diyou]

Yeah this implies relative paths to CMAKE_INSTALL_PREFIX = cache/sysroot.

[sbc100]

I think -print-file-name is already sysroot relative.

It looks like cmake expects clang -print-file-name=libc++.modules.json to print the file within the sysroot, like it does for libs.

It looks like libc++.modules.json is supposed to live in cache/sysroot/lib/wasm32-emscripten/ .. and the current code should then find it.

You might need to update install_system_headers so that the json files go in the right place.

[Diyou]

Yes it prints the correct path when installed in cache/sysroot/lib/wasm32-emscripten but CMake seems not to invoke the underlying function from Clang-CXX-CXXImportStd automatically for emscripten:

[cmake] CMake Error in CMakeLists.txt:
[cmake]   "The "CXX_MODULE_STD" property on target "HelloWorld" requires
[cmake]   CMAKE_CXX_STDLIB_MODULES_JSON be set, but it was not provided by the
[cmake]   toolchain.

So I guess the only alternative for CMake>3.30 would be using our own copy of the older Clang-CXX-CXXImportStd since they changed the underlying function.

[sbc100]

I'm not sure how Clang-CXX-CXXImportStd is supposed to be called.

Why don't we land this change (to add the new files) and then followup with another change to add test and figure out the Clang-CXX-CXXImportStd thing?

[Diyou]

I'm not sure how Clang-CXX-CXXImportStd is supposed to be called.

It can be explicitly called like:

include(Compiler/Clang-CXX-CXXImportStd)
_cmake_cxx_find_modules_json()     # >4.2 Sets CMAKE_CXX_STDLIB_MODULES_JSON
_cmake_cxx_import_std(23 to_eval)  # <4.2 Sets targets

But yeah lets figure that out later

[sbc100]

I think if you install libc++.modules.json in the correct place in the sysroot then CMake should be able to find it without setting CMAKE_CXX_STDLIB_MODULES_JSON like this ?