nix: use rustoverlay; thanks nullcube <3

flake.nix

@@ -3,9 +3,27 @@
 
   inputs = {
     nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
+    rust-overlay = {
+      url = "github:oxalica/rust-overlay";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
   };
 
-  outputs = { self, nixpkgs }: {
+  outputs = { nixpkgs, rust-overlay, ... }:
-    devShells.x86_64-linux.default = import ./shell.nix { pkgs = nixpkgs.legacyPackages.x86_64-linux; };
+    let
+      # System types to support.
+      supportedSystems = [
+        "x86_64-linux"
+        "aarch64-linux"
+      ];
+
+      forAllSystems = function: nixpkgs.lib.genAttrs supportedSystems
+        (system: function (import nixpkgs {
+          inherit system;
+          overlays = [ rust-overlay.overlays.default ];
+        }));
+    in
+    {
+      devShells = forAllSystems (pkgs: import ./shell.nix { inherit pkgs; });
     };
 }