Implement a `select`-like operator to resolve `config_common.FeatureFlagInfo` or similar provider

[alexander-shiryaev]

Description of the feature request:

I would like to pass a computed string to an arbitrary rule. If the string can only take a finite number of values, the problem can be solved by implementing a rule that returns config_common.FeatureFlagInfo and a set of config_setting, one for each of the allowed values (see resolve_feature_flag_emulation).

def resolve_feature_flag_emulation(allowed_values):
    def _(feature_flag_name):
        cases = {}

        for index, feature_flag_value in enumerate(allowed_values):
            config_setting_name = "{}-{}".format(feature_flag_name, index)

            native.config_setting(
                name = config_setting_name,
                flag_values = {feature_flag_name: feature_flag_value},
                visibility = ["//visibility:private"],
            )

            cases[config_setting_name] = feature_flag_value

        return select(cases, no_match_error = "unable to resolve feature flag: allowed_values = {}".format(allowed_values))

    return _

However, if the string can have an unlimited number of values, there is currently no way to achieve the desired functionality.

In this example, you can see how the value of the shared_lib_name attribute should be computed based on the given string_flag.

My suggestion is to implement the resolve_feature_flag operator, which would function similarly to select and convert the value from config_common.FeatureFlagInfo to a string, similar to how resolve_feature_flag_emulation works. However, it should support an unlimited set of values.

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

I want to have an ability to pass computed values to the rule attributes.

What is the output of bazel info release?

release 6.1.0

Have you found anything relevant by searching the web?

No.

Any other information, logs, or outputs that you want to share?

Code for this proposal: https://github.com/alexander-shiryaev/bazel_issue/tree/main/feature_request/feature_flag_value