build.mcpp

// build.mcpp — grpc-m build glue.
//
// Deliberately small. The source list is declarative in mcpp.toml (including
// the `ares` feature's own sources), so this program covers only the two
// things the manifest grammar genuinely cannot express:
//
//   1. PRIVATE include dirs. [build].include_dirs propagates to consumers, and
//      a consumer needs exactly one directory — third_party/grpc-1.83.0/include
//      for <grpcpp/grpcpp.h>. gRPC's own internal roots must NOT land on a
//      consumer's -I: the vendor root in particular contains src/, include/ and
//      third_party/, and putting it on a consumer's include path invites
//      collisions with their own headers. mcpp::include_dir() is private by
//      design (Cargo discipline, docs/07-build-mcpp.md), which is exactly the
//      scope needed.
//
//   2. The `ares` feature's OFF state. mcpp features are ADDITIVE — a feature
//      can add, never remove — so "no c-ares" cannot itself be a feature, and
//      writing -DGRPC_ARES=0 into [build].cxxflags could not be retracted when
//      the feature is on (both would reach the command line and the macro would
//      be redefined). The manifest therefore says nothing about GRPC_ARES:
//      include/grpc/support/port_platform.h sets it to 1 under #ifndef, so
//      silence IS the on-state, and this program defines 0 only when the
//      feature is absent. The 7 resolver TUs are handled the other way round —
//      declaratively, by [features.ares].sources.
//
// `import std;` rather than textual <cstdio>/<string>: a build program is
// ordinary C++23 and there is no reason for it to be the one place in a
// modular project that falls back to headers. mcpp builds the host std module
// for it, reusing the very BMI the project's own TUs import. If a host
// toolchain ships no std module, mcpp says so and names the fix.
//
// The mcpp module is bundled in the mcpp binary, so it always matches the
// directive protocol. Diagnostics go to stdout as non-directive lines — never
// stderr, which can interleave into the buffered directive stream.
import std;
import mcpp;

int main() {
    const std::string vendor = "third_party/grpc-1.83.0";

    // upb-gen / upbdefs-gen hold gRPC's CHECKED-IN generated upb code, which is
    // why this build needs no protoc. address_sorting and xxhash are the two
    // pieces gRPC really does vendor with content (unlike its empty submodule
    // placeholders for abseil/protobuf/re2/boringssl/zlib, which is what makes
    // a self-contained gRPC tarball impossible and this repo necessary).
    mcpp::include_dir(vendor.c_str());
    mcpp::include_dir((vendor + "/src/core/ext/upb-gen").c_str());
    mcpp::include_dir((vendor + "/src/core/ext/upbdefs-gen").c_str());
    mcpp::include_dir((vendor + "/third_party/address_sorting/include").c_str());
    mcpp::include_dir((vendor + "/third_party/xxhash").c_str());

    if (mcpp::has_feature("ares")) {
        std::printf("grpc-m: c-ares resolver ON (default)\n");
    } else {
        // Upstream's own spelling for this configuration (bazel
        // grpc_no_ares=true -> GRPC_ARES=0). gRPC falls back to its native
        // DNS resolver.
        mcpp::define("GRPC_ARES=0");
        std::printf("grpc-m: c-ares resolver OFF -> GRPC_ARES=0, native DNS resolver\n");
    }

    return 0;
}