Accept config_settings in target_compatible_with

[chsigg]

Description of the feature request:

bazel 4.0 adds a target_compatible_with common attribute that accepts a list constraint_values. It would be great if it could also accept config_settings.

What underlying problem are you trying to solve with this feature?

I would like to mark a target to be compatible only if :my_config_setting is true, so that the target itself and all inverse dependencies don't build otherwise.

I can currently work around this by translating a config_setting into a constraint_value by select()ing the constraint_setting's default_constraint_value. But apparently constraint_settings are supposed to be configuration independent, so that workaround is bad style:

config_setting(
  name = "my_config",
  ...
)
constraint_setting(
  name = "my_constraint",
  default_constraint_value = select({
    ":my_config": ":my_value",
    "//conditions_default": None,
  }),
)
constraint_value(
  name = "my_value",
  constraint_setting = ":my_constraint",
)
cc_binary(
  ...
  target_compatible_with = [
    # feature request: allow ":my_config" here instead
    ":my_value",
  ],
)

[gregestren]

Interesting. What conditions are you interested in?

CC @AustinSchuh @philsc

[gregestren]

We've also had some interesting discussion on compatibility based on toolchain properties (like compiler type) from the advent of this feature. So maybe there's conceptual overlap?

[chsigg]

What conditions are you interested in?

I hope I understand your question correctly, that you are referring to the attributes of config_setting. I would probably only need config_settings that use the flag_values attribute.

[AustinSchuh]

@gregestren, This is in our original proposal, but was deferred until a second phase so we could get something working first. It was how I've been thinking about implementing toolchain property based skipping. So yea, sounds great!

[philsc]

Just so I understand properly. Is this functionally equivalent to creating a helper like this?

def config_compatible(config_setting_target):
    return select({
        config_setting_target: [],
        "//conditions:default": ["@platforms//:incompatible"],
    })

and then using it like so:

cc_binary(
  ...
  target_compatible_with = config_compatible(":my_value"),
)

Pretty sure you can do the above today.

Again, so I'm clear exactly what we're talking about, you'd like to get rid of the need for a helper function like config_compatible() above?

[chsigg]

Ha! Thanks a lot for thinking of this Philipp.

Yes, selecting on the target_compatible_with would be a perfectly fine alternative. I don't mind the helper function.

In fact, that's what I used before I realized that you can (currently, and I hear it might go away) select on the default_constraint_value. But by the time I wrote this issue I had already forgotten :-(

I'm happy with Philipp's approach and as far as I'm concerned you can close this issue.

Thanks for your help!

[gregestren]

Closing as acknowledged.

To be clear, my preference is to try to limit built-in Bazel support to constraint_value and the mentioned to-be-implemented toolchain properties.

I like that a reasonable user-space approach was identified here, so you can get what you need without stretching Bazel's core API complexity.