Support C++20 standard modules in the rules-based toolchain#654
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Drive Bazel's C++20 modules pipeline (clang-scan-deps -> .ddi -> aggregate-ddi -> generate-modmap -> module compile) through the rules-based cc/toolchains API. Toolchain: - Add tools/internal/deps_scanner.c, a wrapper that runs `clang-scan-deps -format=p1689 -- clang++ <args>` and writes the P1689 JSON to DEPS_SCANNER_OUTPUT_FILE, mirroring the other internal tool wrappers. Wire it as the cpp_module_deps_scanning tool in both the prebuilt (toolchain/llvm/llvm.bzl) and bootstrap (toolchain/bootstrap/declare_toolchains.bzl) toolchains. - Add cc_args for the compiler-specific C++20 module flags only: module output (-fmodule-output), the modmap (@ModMap) and the dependency scanner output env. The generic -c / -o / -MD -MF / copt flags are emitted by rules_cc's legacy compile features, which #750 extends to the C++20 module actions, so every rules_cc toolchain benefits. Gate -fno-cxx-modules on the new cpp_modules feature being disabled. - Add the cpp_modules feature and extend sysroot / header search-path / resource-dir args to cover cpp20_module_actions. rules_cc currently requires bazelbuild/rules_cc PRs #750 (rules-based module build variables/actions, plus extending the legacy compile features -- compiler_input_flags, compiler_output_flags, dependency_file, user_compile_flags -- to the C++20 module actions) and #775 (compile.bzl analysis fix). Until a release ships them, they are applied to rules_cc 0.2.20 as checked-in patches (3rd_party/rules_cc) via single_version_override. The e2e/rules_cc module is a separate root module, so it carries its own single_version_override and symlinks the patch files from the parent. e2e: add //:cpp20_modules_test exercising an exported `math` module (math.cppm) imported by modules_main.cc, built with --features=cpp_modules and --experimental_cpp_modules.
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Drives Bazel's C++20 modules pipeline (
clang-scan-deps→.ddi→ aggregate-ddi → generate-modmap → module compile) through the rules-basedcc/toolchainsAPI.Toolchain
tools/internal/deps_scanner.c, a compiled wrapper that runsclang-scan-deps -format=p1689 -- clang++ <args>and writes the P1689 JSON toDEPS_SCANNER_OUTPUT_FILE, mirroring the other internal tool wrappers. Wire it as thecpp_module_deps_scanningtool in both the prebuilt (toolchain/llvm/llvm.bzl) and bootstrap (toolchain/bootstrap/declare_toolchains.bzl) toolchains.cc_argsfor the C++20 module actions: module output (-fmodule-output), modmap (@modmap), scanner output env, source/output flags, dependency file and user compile flags. Gate-fno-cxx-moduleson the newcpp_modulesfeature being disabled.cpp_modulesfeature and extend the sysroot / header search-path / resource-dir args to covercpp20_module_actions.rules_cc dependency
This currently requires two unreleased rules_cc PRs, applied to
rules_cc0.2.20 as checked-in patches (3rd_party/rules_cc) viasingle_version_override:cpp_module_output_file,cpp_module_modmap_file) and thecpp20_module_actionsaction set.cc/private/compile/compile.bzlanalysis fix.The
e2e/rules_ccmodule is a separate root module, so it carries its ownsingle_version_overrideand symlinks the patch files from the parent (single_version_overrideonly accepts main-repo patch labels).Draft because of the above: it should land once those PRs are merged and a
rules_ccrelease includes them, at which point the patches and overrides are dropped and therules_ccversion is bumped.e2e
Adds
//:cpp20_modules_testine2e/rules_cc, exercising an exportedmathmodule (math.cppm) imported bymodules_main.cc, built with--features=cpp_modulesand--experimental_cpp_modules. Passes on macOS arm64.🤖 Generated with Claude Code