feat: add treefmt

nix/default.nix

@@ -6,5 +6,6 @@
     ./tests
 
     ./hydra.nix
+    ./fmt.nix
   ];
 }

nix/fmt.nix

@@ -0,0 +1,37 @@
+{ inputs, ... }:
+{
+  # load the flake module from upstream
+  imports = [ inputs.treefmt-nix.flakeModule ];
+
+  perSystem =
+    { self', lib, ... }:
+    {
+      treefmt = {
+        # repo root as project root
+        projectRoot = inputs.self;
+
+        # the formatters
+        programs = {
+          nixfmt.enable = true;
+          typos = {
+            enable = true;
+            configFile = "${inputs.self}/.typos.toml";
+          };
+          taplo = {
+            enable = true;
+            settings = lib.importTOML "${inputs.self}/taplo.toml";
+          };
+        };
+
+        settings.formatter.rustfmt = {
+          command = "${lib.getExe' self'.packages.dev-toolchain "rustfmt"}";
+          includes = [ "**/*.rs" ];
+          options = [
+            "--unstable-features"
+            "--edition=2024"
+            "--config-path=${inputs.self}/rustfmt.toml"
+          ];
+        };
+      };
+    };
+}