Added Mozilla's Rust Nightly

rust-nightly.nix

@@ -0,0 +1,17 @@
+let
+  moz_overlay =
+    import (builtins.fetchTarball
+      "https://github.com/mozilla/nixpkgs-mozilla/archive/master.tar.gz");
+  nixpkgs = import <nixpkgs> {overlays = [moz_overlay];};
+in
+  with nixpkgs;
+    stdenv.mkDerivation {
+      name = "moz_overlay_shell";
+      buildInputs = [
+        # to use the latest nightly:
+        nixpkgs.latest.rustChannels.nightly.rust
+      ];
+      shellHook = ''
+        exec zsh
+      '';
+    }