Cargo assumes you want to cache downloaded/built dependencies at the granularity of a unix user account. It's very easy to break things if you try to force current cargo cache dependencies at the per-project level and there's been no interest in caching things at a per-machine or shared-between-machines level. If duplicating compilation work is desirable to ensure some amount of noninterference, it should be supported to have a package cache per project. If we can treat Cargo as a pure function from inputs to outputs, we should be sharing these build results across the Internet, at least within the set of machines used by one person.

Cargo also refuses to follow the XDG directory specification for its per-unix-user configuration; it should put things in $XDG_CONFIG_HOME and $XDG_CACHE_HOME, but instead dumps both configuration and downloaded/compiled stuff in ~/.cargo.