I do not have enough know-how to properly review your patch, but at least I can verify it seems to work.

I was able to build rustc and cargo succesfully with your patches, and was able to build and run a trivial hello world program written in rust.

When this patch is merged,  I think we can also close http://lists.gnu.org/archive/html/bug-guix/2017-12/msg00006.html, as it will no longer be relevant.