Add formatter

flake.nix

@@ -7,6 +7,7 @@
     nur.url = "github:nix-community/NUR";
     nixos-hardware.url = "github:NixOS/nixos-hardware/master";
     stylix.url = "github:pizzapim/stylix/release-24.05";
+    treefmt-nix.url = "github:numtide/treefmt-nix";
 
     nix-index-database = {
       url = "github:nix-community/nix-index-database";
@@ -52,6 +53,23 @@
     , ...
     }@inputs:
     let
+      supportedSystems = [
+        "x86_64-linux"
+        "i686-linux"
+        "aarch64-linux"
+      ];
+      forAllSystems' = nixpkgs.lib.genAttrs;
+      forAllSystems = forAllSystems' supportedSystems;
+      pkgsForSystem =
+        system:
+        import nixpkgs {
+          inherit system;
+        };
+
+      treefmtEval = forAllSystems (
+        system: inputs.treefmt-nix.lib.evalModule (pkgsForSystem system) ./treefmt.nix
+      );
+
       mkNixosSystem = {extraModule, nixpkgs' ? nixpkgs, useDefaultConfiguration ? true}: nixpkgs'.lib.nixosSystem rec {
         system = "x86_64-linux";
 
@@ -66,6 +84,8 @@
       };
     in
     {
+      formatter = forAllSystems (system: (treefmtEval.${system}.config.build.wrapper));
+
       nixosConfigurations = {
         x260 = mkNixosSystem { extraModule = ({ pkgs, lib, ... }: {
             imports = [ inputs.nixos-hardware.nixosModules.lenovo-thinkpad-x260 ];