Bazel 9.0.0 LTS supports modules nativelly.

[unennhexium]

I spent an entire day to figure out that Bazel already supports modules.

Tip

Consider mention this fact in the README.

I found nothing about this in Bazel release notes, but here is confirmation from one of the contributors of this feature, and here is the demo repository by them demonstrating the usage.

I am successfully built the demo with Bazel 9.0.0 and Clang 22.0.0 from https://apt.llvm.org.

I also succeeded with building C++23 std module without overriding rules_cc with some "under construction"¹ stuff or precompiling std.pcm manually as implied when using this fork². Bellow are the instructions, maybe someone will find them useful.

Instructions

1. Find and symlink std.cppm's directory into source tree. On my system it is at /usr/lib/llvm-22/share/libc++/v1.

   $ pwd
   /home/user/repos/cpp_modules_demo
   $ ln -vs /usr/lib/llvm-22/share/libc++/v1 std
   'std' -> '/usr/lib/llvm-22/share/libc++/v1'
   $ tree -FL1 std
   std@
   ├── std/
   ├── std.compat/
   ├── std.compat.cppm
   └── std.cppm

2. Add cc_library³ rule building std module into BUILD.bazel file.

   # File: BUILD.bazel
   
   load("@rules_cc//cc:defs.bzl", "cc_binary", "cc_library")
   
   copts = [
       "-std=c++23",
       "-stdlib=libc++",
   ]
   
   linkopts = [
       "-stdlib=libc++"
   ]
   
   cc_library(
       name = "std",
       copts = copts,
       linkopts = linkopts,
       features = ["cpp_modules"],
       srcs = glob(["std/std/*.inc"]), # std.cppm `include`s these files.
       module_interfaces = ["std/std.cppm"],
   )

3. Add std target as dependency for each cc_binary/cc_library³ target that uses import std;.

   # File: BUILD.bazel
   
   load("@rules_cc//cc:defs.bzl", "cc_binary", "cc_library")
   
   # ...
   
   cc_binary(
       name = "main",
       copts = copts,
       linkopts = linkopts,
       features = ["cpp_modules"],
       srcs = ["main.cpp"],
       deps = ["//:std"],
   )

   // File: main.cpp
   
   import std;
   
   auto main() -> int {
     std::cout << "Hello, World!\n";
     return 0;
   }

4. Build and run the demo.

   $ cat .bazelrc
   build:std --repo_env=BAZEL_COMPILER=clang # Select Clang as compiler.
   build:std --experimental_cpp_modules      # Enable C++20 modules support.
   $ bazel --config=std run //:main
   INFO: Analyzed target //:main (0 packages loaded, 0 targets configured).
   INFO: Found 1 target...
   Target //:main up-to-date:
     bazel-bin/main
   INFO: Elapsed time: 0.079s, Critical Path: 0.00s
   INFO: 1 process: 1 internal.
   INFO: Build completed successfully, 1 total action
   INFO: Running command line: bazel-bin/main
   Hello, World!

P.S. Top 5 "c++ std module bazel" results

Footnotes

1. I failed to build with override with the error very similar to "bazel build //example/..." fails with error: "could not build module 'std_config'" #15 (__config is not found). ↩

2. Compiling std module outside Bazel is quite strange solution. ↩

3. Add bazel_dep(name = "rules_cc", version = "0.2.16", dev_dependency = True) into MODULE.bazel to use the same version I used. ↩ ↩²

[unennhexium]

@igormcoelho check this, please. Maybe it is worth to mention in your fork's README.

[igormcoelho]

@unennhexium thanks for pointing that out. Indeed some things (HOPEFULLY) changed in past 12 months, so we can finally see some advance in Bazel with C++23 import std (at least for clang!).
I tried your solution, and it's very interesting, it really worked with the symlink and Bazel 9!

Compiling std module outside Bazel is quite strange solution. ↩

I completely agree with you: manually doing stuff like that is very strange and error prone... however, my context was like I explained before: just needed to make things work and could not wait indefinitely from Bazel project to find a better way, so I pursued ANY WORKING SOLUTION. 😂
Compiling manually also generates crazy bugs on the compiler when flags differ slightly... I had to report many to GCC team last year during GCC 15 development, but the errors were really unlikely on practice, being caused by this crazy mix of bazel-built x manually built... so again, I fully agree.

@igormcoelho check this, please. Maybe it is worth to mention in your fork's README.

I've worked on this during this weekend, and I'm already working to update README and mention your solution there as a much better and more stable one for Bazel 9, ok?

One thing regarding your symlink approach... for me, this is also still quite manual! So, my interpretation is that we can create a local repository on MODULE.bzl and let bazel do this "symlink" automatically. What do you think of this solution?

Something like this (on Bazel 9!!!):

Create extensions.bzl file for a local package called local_libcxx:

load("@bazel_tools//tools/build_defs/repo:local.bzl", "new_local_repository")

def _libcxx_extension_impl(mctx):
    path = None
    std_version = None
    for mod in mctx.modules:
        for tag in mod.tags.configure:
            path = tag.path
            std_version = tag.std_version
    new_local_repository(
        name = "local_libcxx",
        path = path,
        build_file_content = '''
load("@rules_cc//cc:defs.bzl", "cc_library")
cc_library(
    name = "std",
    copts = [
        "-std=''' + std_version + '''",
        "-stdlib=libc++",
    ],
    linkopts = [
        "-stdlib=libc++",
        "-lc++",
        "-lc++abi",
    ],
    features = ["cpp_modules"],
    srcs = glob(["std/*.inc"]),
    module_interfaces = ["std.cppm"],
    visibility = ["//visibility:public"],
)
''',
    )

configure = tag_class(attrs = {
    "path": attr.string(mandatory = True),
    "std_version": attr.string(mandatory = True),
})

local_libcxx_extension = module_extension(
    implementation = _libcxx_extension_impl,
    tag_classes = {"configure": configure},
)

load it on MODULE:

local_libcxx = use_extension("//:extensions.bzl", "local_libcxx_extension")
local_libcxx.configure(
    path = "/usr/lib/llvm-21/share/libc++/v1",
    std_version = "c++23",
)
use_repo(local_libcxx, "local_libcxx")

Then use it on project:

# BUILD file
load("@rules_cc//cc:defs.bzl", "cc_binary", "cc_library")

cc_binary(
    name = "demo",
    srcs = ["main.cpp"],
    copts = ["-std=c++23", "-stdlib=libc++"],
    linkopts = ["-stdlib=libc++"]
)

cc_binary(
    name = "demo2",
    srcs = ["main2.cpp"],
    copts = [ "-std=c++23" ],
    linkopts = ["-stdlib=libc++"],
    features = ["cpp_modules"],
    #deps = [":std"]
    deps = ["@local_libcxx//:std"]
)

'''
cc_library(
    name = "std",
    copts = [
        "-std=c++23",
        "-stdlib=libc++",
    ],
    linkopts = [
        "-stdlib=libc++"
    ],
    features = ["cpp_modules"],
    srcs = glob(["std/std/*.inc"]),
    module_interfaces = ["std/std.cppm"],
)
'''

This prevents the symlink, so it seems less fragile. What are your thoughts on that?

Regards!

[igormcoelho]

@unennhexium thanks for the advice, I finally had time to create an improved version of the article: https://igormcoelho.medium.com/local-import-std-module-on-c-23-with-bazel-95b449a8e881
This time I tried to cover these new possibilities and modern support of CXX Modules on Bazel (v8 and v9) for Clang and GCC. Best regards!