Rustaceans at the border

At the very least they probably don't wanna allow multiple versions of packages, which is very common in rust projects (but people don't realize it, because they are hidden behind an up-to-date dependency).

Cargo doesn't really like being the passenger in the build process, it likes to drive things itself, and that's probably not gunna fly in the kernel either, which means they need to interact with the rust compiler itself, and that interface is not well documented.

At a higher level I think this is the sort of thing that could cause rust to stabilize their ABI unintentionally, without the core rust developers having a choice in the matter.