I need it with library evolution disabled, so non-resilient modules, because resilient modules have all sorts of performance disadvantages. I need this to work in this very specific way, which is the most hard to implement. As for your example, do you build your module with library evolution on or off? Is memory layout of public structures in your project affected by size of a type from the DeviceKit? I was under an impression that a certain combination of these answers (and in fact, my current situation) is where suddenly it turns out that, yeah, we need @_implementationOnly import in non-resilient modules to improve compilation times without hurting anything else.
Anyway, this is not a new topic for me, and in this thread I didn't want to troubleshoot, but to raise awareness of [Pitch] Allow non-resilient modules to hide dependencies to clients.