This crate implements a backend for Cocoa-based widgets, such as NSView, NSTextField, and so on. Note that while it's under development currently, the fate of AppKit is still kind of a gray area. If Apple ends up pushing Marzipan as "the" solution, it's possible this might become obsolete, or would run in tandem with the iOS crate for iOS/Marzipan.
Open an issue, or hit me up on Twitter.