Creating a user-defined build setting doesn't require command-line flags. If you set flag = False, then it actually can't be set on the command line. You can use a user-defined transition to set it instead.
I think something like this will do what you're looking for (save it in extra_copts.bzl):
def _extra_copts_impl(ctx):
context = cc_common.create_compilation_context(
defines = depset(ctx.build_setting_value)
)
return [CcInfo(compilation_context = context)]
extra_copts = rule(
implementation = _extra_copts_impl,
build_setting = config.string_list(flag = False),
)
def _use_extra_copts_implementation(ctx):
return [ctx.attr._copts[CcInfo]]
use_extra_copts = rule(
implementation = _use_extra_copts_implementation,
attrs = "_copts": attr.label(default = "//:extra_copts")},
)
def _add_copts_impl(settings, attr):
return {"//:extra_copts": ["MY_DEFINE"]}
_add_copts = transition(
implementation = _add_copts_impl,
inputs = [],
outputs = ["//:extra_copts"],
)
def _with_extra_copts_implementation(ctx):
infos = [d[CcInfo] for d in ctx.attr.deps]
return [cc_common.merge_cc_infos(cc_infos = infos)]
with_extra_copts = rule(
implementation = _with_extra_copts_implementation,
attrs = {
"deps": attr.label_list(cfg = _add_copts),
"_allowlist_function_transition": attr.label(
default = "@bazel_tools//tools/allowlists/function_transition_allowlist"
)
},
)
and then in the BUILD file:
load("//:extra_copts.bzl", "extra_copts", "use_extra_copts", "with_extra_copts")
extra_copts(name = "extra_copts", build_setting_default = [])
use_extra_copts(name = "use_extra_copts")
cc_library(
name = "G",
deps = [":use_extra_copts"],
)
with_extra_copts(
name = "P_deps",
deps = [":G"],
)
cc_library(
name = "P",
deps = [":P_deps"],
)
extra_copts is the build setting. It returns a CcInfo directly, which means it's straightforward to do any other C++ library swapping with the same approach. Its default is effectively an "empty" CcInfo which won't do anything to libraries that depend on it.
with_extra_copts wraps a set of dependencies, configured to use a different CcInfo. This is the rule that actually changes the value, to create the second version of G with different flags.
_add_copts is the transition which with_extra_copts uses to change the value of the extra_copts build setting. It could examine attr to do something more sophisticated than adding a hard-coded list.
use_extra_copts pulls the CcInfo out of extra_copts so a cc_library can use them.
To avoid rewriting the builtin C++ rules, this uses wrapper rules to pull the copts out and do the transition. You might want to create macros to bundle the wrapper rules along with the corresponding cc_library. Alternatively, you could use rules_cc's my_c_archive as a starting point to create custom rules that reuse the core implementation of the builtin C++ rules while integrating the transition and use of the build setting into a single rule.
