nix fmt: automatically expose the default nix formatter

flake.lock

@@ -1,6 +1,26 @@
 {
   "nodes": {
-    "root": {}
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1763966396,
+        "narHash": "sha256-6eeL1YPcY1MV3DDStIDIdy/zZCDKgHdkCmsrLJFiZf0=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "5ae3b07d8d6527c42f17c876e404993199144b6a",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
   },
   "root": "root",
   "version": 7

flake.nix

@@ -1,3 +1,6 @@
 {
+  # only used for nixfmt-tree
+  inputs.nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+
   outputs = { self, ... }@inputs: import ./lib/flake.nix inputs ./.;
 }

lib/flake.nix

@@ -11,7 +11,9 @@ let
     convert = attr: value: value self inputs imports;
 
     result = (builtins.mapAttrs convert schema.exports);
-  in result // { lib = lib // (result.lib or {}); };
+  in {
+    formatter = builtins.mapAttrs (_: pkgs: pkgs.nixfmt-tree) inputs.nixpkgs.legacyPackages;
+  } // result // { lib = lib // (result.lib or {}); };
 
   flake =  { imports?{}, exports?{}, base?[] }@schema'old: { ... }@inputs'old: { ... }@imports'old: { ... }@inputs'new: base: let
     schema = {