Writing a GC in rust without just dropping the whole business into unsafe is really annoying.

Jason Orendorff has an implementation of a GC in rust called "cell-gc" that seemed like only one I've seen so far that seemed to "get" how to marry rust to the requirements of a GC implementation: https://github.com/jorendorff/cell-gc

Still has a lot of unsafe code and macro helpers, but it's laid out well and documented pretty well. Not sure if you've run across it yet.