Add Rust env

rust.nix

@@ -0,0 +1,12 @@
+let
+  # Pinned nixpkgs, deterministic. Last updated: 2/12/21.
+  #pkgs = import (fetchTarball("https://github.com/NixOS/nixpkgs/archive/a58a0b5098f0c2a389ee70eb69422a052982d990.tar.gz")) {};
+
+  # Rolling updates, not deterministic.
+  pkgs = import (fetchTarball("channel:nixpkgs-unstable")) {};
+in pkgs.mkShell {
+  buildInputs = [ pkgs.cargo pkgs.rustc pkgs.zsh pkgs.bacon ];
+  shellHook = ''
+    exec zsh
+  '';
+}