This example has five files:

• custom_settings/BUILD - This defines a custom Starlark flag called //custom_settings:mycopts which defines the set of possible features and stores whatever features the cc_binary sets.

  cc_library uses a select to read this flag and set its copts accordingly. This file also declares the config_settings the select uses.

• defs.bzl - This defines a custom Starlark rule transition_rule which defines an attribute set_features that sets the desired feature and actual_binary which declares the cc_binary this should apply to.

  Because cc_binary is a native (not Starlark-defined) rule, we can't add set_features directly to it. For Starlark-defined rules, we could omit transition_rule and just add the functionality directly.

  transition_rule applies a Starlark transition called _copt_transition that reads the value of set_features and sets //custom-settings:mycopts accordingly.

  Finally, this file declares a macro (also) called cc_binary that automates away all this extra abstraction: the new macro cc_binary simply instantiates a transition_rule with the desired set_features then passes all other attributes directly to the native cc_binary. To the end user this makes it look like cc_binary magically supports a new attribute.

• BUILD - This defines three versions of a cc_binary all depending on the same cc_library: one uses feature, the other uses feature2, and the third fails to build because it "forgets" to set any feature.

• main.cc and lib.cc for the actual C++ code.

To see it in action, cd to this directory and try the following commands:

$ bazel run :app_with_feature1
...
Running my app!
Building lib with feature 1!

$ bazel run :app_with_feature2
...
Running my app!
Building lib with feature 2!

$ bazel run :app_forgets_to_set_features 
ERROR: $(MYWORKSPACE)/cc_binary_selectable_copts/BUILD:32:13: Configurable attribute "copts"
doesn't match this configuration: You must explicitly set which features you want!

This example relies on select and involves some duplication of values (e.g. "feature1" is defined in //custom_settings:mycopts and //:lib separately sets "-Dfeature1"). You could write more Starlark macros to make the BUILD API even simpler. For example: not requiring any changes to cc_library at all.