There is an example now. You can change the version of the rust-libc package and then run guix refresh -u rust-libc on it.
- [PATCH 1/7] build-system: Add cargo build system. David Craven
- [PATCH 2/7] import: Add importer for rust crates. David Craven
- Re: [PATCH 2/7] import: Add importer for rust cr... Ludovic Courtès
- [PATCH 6/7] gnu: Add cargo-bootstrap. David Craven
- Re: [PATCH 6/7] gnu: Add cargo-bootstrap. Ludovic Courtès
- [PATCH 4/7] upstream: Use a the first url from urls ... David Craven
- Re: [PATCH 4/7] upstream: Use a the first url fr... David Craven
- Re: [PATCH 4/7] upstream: Use a the first ur... Ludovic Courtès
- [PATCH 7/7] gnu: Add rust-libc. David Craven
- [PATCH 3/7] import: crate: Add crate updater. David Craven
- Re: [PATCH 3/7] import: crate: Add crate updater... Ludovic Courtès
- [PATCH 5/7] gnu: Add rustc-bootstrap. David Craven
- Re: [PATCH 5/7] gnu: Add rustc-bootstrap. David Craven
- Re: [PATCH 5/7] gnu: Add rustc-bootstrap. David Craven
- Re: [PATCH 5/7] gnu: Add rustc-bootstrap. Ludovic Courtès
- Re: [PATCH 5/7] gnu: Add rustc-bootstrap. ng0
- Re: [PATCH 5/7] gnu: Add rustc-bootstrap... Ludovic Courtès
