I'm re-reading the threads about Rust packaging and I realized there might be a solution to the build artifact reuse problem.
As I understand it, the problem is that crates can be compiled with any number of features enabled or disabled. If this and the compiler version are truly the only variables to consider, we could just lift the features to the package level, right? https://lists.gnu.org/archive/html/guix-devel/2021-02/msg00269.html