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WIP: support std module #522

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WIP: support std module #522

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2 changes: 2 additions & 0 deletions cc/private/toolchain/BUILD.tpl
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Original file line number Diff line number Diff line change
Expand Up @@ -26,6 +26,8 @@ licenses(["notice"]) # Apache 2.0

cc_library(name = "empty_lib")

%{std_module}

# Label flag for extra libraries to be linked into every binary.
# TODO(bazel-team): Support passing flag multiple times to build a list.
label_flag(
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16 changes: 16 additions & 0 deletions cc/private/toolchain/unix_cc_configure.bzl
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Expand Up @@ -141,6 +141,9 @@ def _get_cxx_include_directories(repository_ctx, print_resource_dir_supported, c

return inc_directories

def _get_share_libcxx_v1_dir(repository_ctx, cc, additional_flags = []):
return repository_ctx.execute([cc, "-print-resource-dir"] + additional_flags).stdout.strip() + "/../../../share/libc++/v1"

def _is_compiler_option_supported(repository_ctx, cc, option):
"""Checks that `option` is supported by the C compiler. Doesn't %-escape the option."""
result = repository_ctx.execute([
Expand Down Expand Up @@ -608,6 +611,11 @@ def configure_unix_toolchain(repository_ctx, cpu_value, overridden_tools):
extra_flags_per_feature["use_module_maps"] = ["-Xclang", "-fno-cxx-modules"]

write_builtin_include_directory_paths(repository_ctx, cc, builtin_include_directories)
if is_clang:
libcxx_v1_dir = _get_share_libcxx_v1_dir(repository_ctx, cc)
files = repository_ctx.execute(["ls", libcxx_v1_dir]).stdout.strip().split("\n")
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@fmeum fmeum Nov 10, 2025

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You can use repository_ctx.path(...).readdir() instead to list files within Starlark.

for file in files:
repository_ctx.symlink(libcxx_v1_dir + "/" + file, file)
repository_ctx.template(
"BUILD",
paths["@rules_cc//cc/private/toolchain:BUILD.tpl"],
Expand Down Expand Up @@ -767,5 +775,13 @@ def configure_unix_toolchain(repository_ctx, cpu_value, overridden_tools):
"-D__TIME__=\\\"redacted\\\"",
],
),
"%{std_module}": """
cc_library(
name = "std_modules",
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@fmeum fmeum Nov 10, 2025

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I think that the std module has to be passed into the cc_toolchain in some way so that users can get the module for their current toolchain, not the host toolchain. That's why exposing this target directly wouldn't work well.

If it's common for folks to import std, could we add this dependency automatically to all module_interfaces? Or should this be toggleable on the toolchain? Requiring an explicit dep on std could become tedious.

srcs = glob(["std/**"]),
module_interfaces = ["std.cppm"],
features = ["cpp_modules"],
)
""",
},
)