[FR]: Add example for cc_proto_library

[malt3]

What is the current behavior?

I can't quite get cc_proto_library to work and there is no end-to-end example for C++.

Describe the feature

I'd like to enable --incompatible_enable_proto_toolchain_resolution and use a precompiled protoc.
I got this mostly working:

MODULE.bazel:

bazel_dep(name = "rules_proto", version = "6.0.2")
bazel_dep(name = "toolchains_protoc", version = "0.3.1")

protoc = use_extension("@toolchains_protoc//protoc:extensions.bzl", "protoc")
protoc.toolchain(
    # Creates a repository to satisfy well-known-types dependencies such as
    # deps=["@com_google_protobuf//:any_proto"]
    google_protobuf = "com_google_protobuf",
    version = "LATEST",
)

use_repo(protoc, "com_google_protobuf", "toolchains_protoc_hub", "toolchains_protoc_hub.linux_x86_64")

register_toolchains("@toolchains_protoc_hub//:all")
register_toolchains("//toolchain:protoc_cc_toolchain")

toolchain/BUILD:

load("@rules_proto//proto:defs.bzl", "proto_lang_toolchain")

proto_lang_toolchain(
    name = "protoc_cc",
    runtime = "@toolchains_protoc_hub.linux_x86_64//:prebuilt_protoc_toolchain",
    command_line = "--cpp_out=$(OUT)",
    progress_message = "Generating C++ proto_library %{label}",
    toolchain_type = "@rules_cc//cc/proto:toolchain_type",
)

BUILD:

load("@rules_proto//proto:defs.bzl", "proto_library")

cc_proto_library(
    name = "foo_cc_proto",
    deps = [":foo_proto"],
)

proto_library(
    name = "foo_proto",
    srcs = ["foo.proto"],
    features = ["-asan"],
)

foo.proto:

syntax = "proto3";

package foo;

message Bar {
    uint64 baz = 1;
}

Output:

fatal error: google/protobuf/runtime_version.h: No such file or directory

edit

I think the runtime attribute of proto_lang_toolchain should be something like @com_google_protobuf//:protoc_lib.
I’ll experiment some more.

[alexeagle]

In theory, C++ doesn't have to be covered here because the official Protobuf documentation says that C++ developers ought to build it from source:
https://github.com/protocolbuffers/protobuf#protobuf-compiler-installation

If you author C++ code then it's reasonable to require every engineer on your team has a functioning local toolchain, or you've configured Bazel with a hermetic toolchain.

In practice of course it's ALWAYS annoying to compile protoc and watch its gcc warnings scrolling by, and I'd love to have C++ here too!

[alexeagle]

Tagging @thesayyn who might recall whether any work happened with cc_proto_library to make this possible