To setup to analyze and browse with my rust-analyzer & Emacs config:

Generate the "build" directory needed by ~/.config/emacs/my/lib/my-rust-lang-proj.el:
```
./x check --build-dir /var/tmp/$HOME/rust-lang/build-for-rust-analyzer
```
Apparently, this doesn't require entering the nix-shell environment of ./shell.nix. (But doing ./x build would.)