I'm mostly happy with fetchCargoVendor, though I worry about the impact on the cache:
every time a dependency of fetchCargoVendor changes, the non-FOD part needs to be re-cached, which can accumulate, and take up quite a lot of storage.

I wish there was a way to force hydra to not cache certain packages.

There is meta.hydraPlatforms = [], but AIUI it would only disable building it if it was directly built by hydra, but since it's a dependency of a package that is not disabled on hydra, it will get built and cached anyway...

I guess this could also be handled by content-addressed derivations...

Note: there are >2000 rust-based packages in nixpkgs currently

Question 1: What do you all think, how bad is this problem?
Question 2: How does one check how much storage a certain derivation takes up cached?