It really isn’t so bad, this is just a rust-specific papercut. This post describes your exact issue, why it is the way it is, and how you achieve what you want: Is it possible to override cargoSha256 in buildRustPackage?
It’s not off-the-beaten-path, you’ve found one of the many thoroughly beaten paths that aren’t very well-documented.
Unfortunately, the software ecosystem has lots of idiosyncratic sub-ecosystems, and nixpkgs faces the rather difficult problem of trying to standardize them all.
Be glad you’re not trying to override a gradle dep 