Invoking a tool that manages its own cache

[shaun-cox]

Let's say I'm authoring a new rule to invoke a tool. This tool produces not only a single default output artifact file for each target using the rule, but also a bunch of other cached artifacts that would be reused and shared by other targets using the same rule. My conceptual difficulty is in how to specify the location of the shared cache (relative to buck-out somewhere?) in the rule definition. Is this possible?