Also, I've been changing how rust crates work in antioxidant[0]
(not yet merged in Guix, not all build failures have been fixed yet).
Antioxidant makes Rust packaging simpler and faster, so maybe you could
give it a try.
To fix some of the resulting build failures, some crates have been
updated there (e.g., rust-clap, rust-clap-derive), so if you are
interested and don't mind the extra time required until it's ready for
merging back into Guix, maybe you could give [0] a try to reduce
duplicated effort a little?