Use treefmt-nix and Alejandro

flake.nix

@@ -17,6 +17,12 @@
     deploy-rs.url = "github:serokell/deploy-rs";
     nixos-hardware.url = "github:NixOS/nixos-hardware/master";
     flake-utils.url = "github:numtide/flake-utils";
+    treefmt-nix.url = "github:numtide/treefmt-nix";
+
+    pre-commit-hooks = {
+      url = "github:cachix/git-hooks.nix";
+      inputs.nixpkgs.follows = "nixpkgs-unstable";
+    };
 
     disko = {
       url = "github:nix-community/disko";
@@ -58,7 +64,6 @@
       ./shell.nix
       ./utils
       ./machines
-    ] // (flake-utils.lib.eachDefaultSystem (system: {
-      formatter = nixpkgs.legacyPackages.${system}.nixfmt;
-    }));
+      ./formatter.nix
+    ];
 }

formatter.nix

@@ -0,0 +1,9 @@
+{ nixpkgs, treefmt-nix, flake-utils, ...}: flake-utils.lib.eachDefaultSystem (system:
+let
+  pkgs = nixpkgs.legacyPackages.${system};
+  treefmtEval = treefmt-nix.lib.evalModule pkgs ./treefmt.nix;
+in
+{
+  formatter = treefmtEval.config.build.wrapper;
+}
+)

treefmt.nix

@@ -0,0 +1,4 @@
+{...}: {
+  projectRootFile = "flake.nix";
+  programs.alejandra.enable = true;
+}