move treefmt to tests flake

this way we don't polute our consumers flake.lock with treefmt

Author: Mic92

.github/workflows/test.yml

@@ -10,7 +10,7 @@ jobs:
     steps:
     - uses: actions/checkout@v4
     - uses: cachix/install-nix-action@v31
-    - run: nix fmt .
+    - run: cd tests && nix fmt .. -- --fail-on-change
   tests:
     needs: nixfmt
     runs-on: ubuntu-latest

flake.lock

@@ -1,34 +1,9 @@
 {
   description = "nixos-hardware";
 
-  inputs = {
-    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.05-small";
-    treefmt-nix.url = "github:numtide/treefmt-nix";
-  };
-
   outputs =
+    { ... }:
     {
-      nixpkgs,
-      self,
-      treefmt-nix,
-    }:
-    let
-      systems = [
-        "aarch64-darwin"
-        "aarch64-linux"
-        "x86_64-darwin"
-        "x86_64-linux"
-      ];
-      forEachSystem =
-        function: nixpkgs.lib.genAttrs (systems) (system: function nixpkgs.legacyPackages.${system});
-
-      treefmtEval = forEachSystem (pkgs: treefmt-nix.lib.evalModule pkgs ./treefmt.nix);
-    in
-    {
-      checks = forEachSystem (pkgs: {
-        formatting = treefmtEval.${pkgs.system}.config.build.check self;
-      });
-      formatter = forEachSystem (pkgs: treefmtEval.${pkgs.system}.config.build.wrapper);
 
       nixosModules =
         let

flake.nix

@@ -8,11 +8,16 @@
     nixos-hardware.url = "github:NixOS/nixos-hardware";
     flake-parts.url = "github:hercules-ci/flake-parts";
     flake-parts.inputs.nixpkgs-lib.follows = "nixos-unstable-small";
+    treefmt-nix.url = "github:numtide/treefmt-nix";
+    treefmt-nix.inputs.nixpkgs.follows = "nixos-unstable-small";
   };
 
   outputs =
     inputs@{ flake-parts, ... }:
     flake-parts.lib.mkFlake { inherit inputs; } {
+      imports = [
+        inputs.treefmt-nix.flakeModule
+      ];
       systems = [
         "aarch64-linux"
         "x86_64-linux"
@@ -85,6 +90,24 @@
         in
         {
           _module.args.pkgs = nixpkgsUnstable;
+
+          treefmt = {
+            projectRootFile = "COPYING";
+            programs = {
+              deadnix = {
+                enable = true;
+                no-lambda-pattern-names = true;
+              };
+              nixfmt = {
+                enable = true;
+                package = pkgs.nixfmt-rfc-style;
+              };
+            };
+            settings = {
+              on-unmatched = "info";
+            };
+          };
+
           checks =
             checksForNixpkgs "nixos-unstable" nixpkgsUnstable
             // checksForNixpkgs "nixos-stable" nixpkgsStable;