Hmm, well, rust's brew seems a little outdated (0.10), so if you are intent on using Cargo soon, I would recommend on building rust yourself or grabbing a new binary from their website.
Stuff being either in Cask or Homebrew is just terrible. Homebrew also has a versions tap. Those two projects should combine efforts and remove ambiguity.
I 100% agree with you. It took me like an hour to figure out where I thought it made sense to put rust-nightly, and I'm still not really sure I did it right. But it works and is way better than the morning compilation cronjob I used to use.
