Currently there is no way to represent dependencies that are specific to a library under runtimes/{rid}/lib/{tfm}. This made since when RID was only used for architecture, now that RID also represents OS platform it does not. There are entire packages/technologies that will only exist on a certain OS and folks will need to build fat-packages that depend on these.

I met with @davidfowl, @yishaigalatzer, @anurse, @Eilon, and @lodejard on 10/6 and we came up with the following:
Sounds like a good feature. Doesn’t replace runtime.json, solves a different problem.

A couple different options for representation:

• targetRuntime on a dependency group
• Separate runtime dependency section that supports both targetFramework and targetRuntime.
  • Has the benefit of also supporting another scenario which is “private dependencies”.

We’d need to make sure that it has the right interaction with runtime.json if that is also in play.