A bit more fun…
Running nix shell nixpkgs#rustc creates a working environment where rustc --version can run.
Running nix shell nixpkgs#rustc nixpkgs#rustup creates an environment where rustc fails to run.
Running nix shell nixpkgs#rustup nixpkgs#rustc creates an environment where rustc and rustup can run.
It seems like nixpkgs#rustup shadows nixpkgs#rustc. Likely it does so for some of its other symlinks; although nixpkgs#cargo always seems to work.