Add rust example

shell.nix

@@ -1,6 +1,8 @@
 
-with import <nixpkgs> { };
-
+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;
 mkShell {
   LANG="en_US.UTF-8";
   nativeBuildInputs = [
@@ -13,5 +15,8 @@ mkShell {
     wasm3
     wasmer
     kakoune
+    #(rustChannelOf { rustToolchain = ./rust-toolchain; }).rust
+    #(rustChannelOf { date = "2022-04-10"; channel = "nightly"; targets = [ "wasm32-wasi" ]; }).rust
+    (latest.rustChannels.nightly.rust.override  {  targets = [ "wasm32-wasi" ]; })
   ];
 }