Reflection will make it trivial to generate tracing code automatically, so roots are the only real issue. If you do like me and require that the roots be explicitly passed to collect, then the programmer can just make the shadow stack by hand.

Though of course this will still be a much simpler beast than the sort of concurrent GC you see in managed languages. Supporting that sort of GC is a whole other ballgame, needing safepoints inserted into functions and things like that.

YAGNI imo.