What is tools:mockup, tools:mockup_crop and tools:mockup_opacity

I just updated Android Studio to the version 2.3 (canary version) and to the last build tools 'com.android.tools.build:gradle:2.3.0-alpha1' and when I open a layout and write tools: and press ctrl + space to autocomplete it I found new tools namespaces :

tools:mockup
tools:mockup_crop
tools:mockup_opacity

I never see them before and search in google but I have found nothing. What are the use of them?

It's for a feature we're working on but which isn't part of Studio yet. This part was accidentally not put behind a flag (which normally removes it during compilation.) We just fixed it; next canary won't show these items -- until the feature is enabled when it's done.