Efficient GC requires tight integration with LLVM, and I assume that means also tight integration with the Rust compiler. I don't think that can be done as a library or even a plugin.
Rust doesn't need an efficient GC like the kind Java has. Most of the resources you need in Rust are freed when they get out of scope.
What Rust needs is just a better RC implementation. This is because only a small part of your data is reference counted, so it fits better with how Rust works, so an efficient implementation allows it to combine with ownership semantics to outperform even very advanced generational GCs.
