Updated shell.nix

shell.nix

@@ -1,11 +1,22 @@
+{ nixpkgs ? import <nixpkgs> { }}:
 let
- pkgs = import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/f155f0cf4ea43c4e3c8918d2d327d44777b6cad4.tar.gz") {};
+  # If you set hash to an empty string, you'll get
+  # an error with the correct hash.
+  pinnedPkgs = nixpkgs.fetchFromGitHub {
+    owner  = "NixOS";
+    repo   = "nixpkgs";
+    rev    = "4ecab3273592f27479a583fb6d975d4aba3486fe";
+    sha256 = "btHN1czJ6rzteeCuE/PNrdssqYD2nIA4w48miQAFloM=";
+  };
+  pkgs = import pinnedPkgs {};
+
 in pkgs.mkShell {
   buildInputs = with pkgs; [
     cargo
     rustc
     rustfmt
-    m4
+    rust-analyzer
+    wasm-pack
    ];
 }