$ cat WORKSPACE
load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

http_archive(
    name = "com_google_protobuf",
    sha256 = "7892a35d979304a404400a101c46ce90e85ec9e2a766a86041bb361f626247f5",
    strip_prefix = "protobuf-3.16.0",
    url = "https://github.com/protocolbuffers/protobuf/archive/v3.16.0.tar.gz",
)

load("@com_google_protobuf//:protobuf_deps.bzl", "protobuf_deps")

protobuf_deps()

$ cat BUILD.bazel
load(":rules.bzl", "exec_with_library_dep", "apply_transition")

proto_library(
    name = "proto",
    srcs = ["empty.proto"],
    deps = [],
)

cc_proto_library(
    name = "cc_proto",
    deps = [":proto"],
)

apply_transition(
    name = "cc_proto_with_transition",
    native_target = ":cc_proto",
)

exec_with_library_dep(
    name = "error",
    out = "dummy.gen",
    library = ":cc_proto_with_transition",
)

$ touch empty.proto

$ cat rules.bzl
_COPT_BUILD_SETTING = "//command_line_option:copt"

def _set_foobar_copt(settings, attr):
    return {_COPT_BUILD_SETTING: ["-DFOOBAR"]}

set_foobar_copt = transition(
    implementation = _set_foobar_copt,
    inputs = [],
    outputs = [_COPT_BUILD_SETTING],
)

def _apply_transition_impl(ctx):
    pass

apply_transition = rule(
        implementation = _apply_transition_impl,
        attrs = {
            "native_target": attr.label(
                cfg = set_foobar_copt,
            ),
            "_allowlist_function_transition": attr.label(
                default = "@bazel_tools//tools/whitelists/function_transition_whitelist",
            ),
        },
    )


def _exec_with_library_dep_impl(ctx):
    ctx.actions.write(ctx.outputs.out, "")

exec_with_library_dep = rule(
    _exec_with_library_dep_impl,
    attrs = {
        "library": attr.label(
            mandatory = True,
            cfg = "exec",
        ),
        "out": attr.output(
            mandatory = True,
        ),
    },
)

$ bazel build //:error