add treefmt

flake.nix

@@ -95,6 +95,10 @@
           in
           {
             home-manager = self.legacyPackages.${system}.homeConfigurations.sops.activation-script;
+            treefmt =
+              (pkgs.callPackage ./formatter.nix {
+                inputs = privateInputs;
+              }).config.build.check;
           }
           // (suffix-stable packages-stable)
           // nixpkgs.lib.optionalAttrs pkgs.stdenv.isLinux tests
@@ -131,6 +135,13 @@
           }
         );
 
+        formatter = eachSystem (
+          { pkgs, ... }:
+          (pkgs.callPackage ./formatter.nix {
+            inputs = privateInputs;
+          }).config.build.wrapper
+        );
+
         apps = eachSystem (
           { pkgs, ... }:
           {

formatter.nix

@@ -0,0 +1,27 @@
+{ pkgs, inputs, ... }:
+inputs.treefmt-nix.lib.evalModule pkgs {
+  projectRootFile = ".git/config";
+
+  programs = {
+    nixfmt.enable = true;
+
+    deadnix.enable = true;
+    deno.enable = true;
+  };
+
+  settings = {
+    global.excludes = [
+      "./pkgs/sops-install-secrets/test-assets/*"
+      "*.narHash"
+      # unsupported extensions
+      "*.{asc,pub,gpg}"
+      "*/secrets.{bin,json,ini,yaml}"
+    ];
+
+    formatter = {
+      deadnix = {
+        priority = 1;
+      };
+    };
+  };
+}