flake: add a devShell

that includes the formatter

flake.nix

@@ -157,18 +157,7 @@
           lib.mapAttrs' renameTestPkg tests;
       in
       {
-        formatter = forAllPkgs (
-          pkgs:
-          pkgs.treefmt.withConfig {
-            runtimeInputs = with pkgs; [
-              nixfmt
-              deadnix
-              keep-sorted
-              nixf-diagnose
-            ];
-            settings = pkgs.lib.importTOML ./treefmt.toml;
-          }
-        );
+        formatter = forAllPkgs (pkgs: pkgs.callPackage ./home-manager/formatter.nix { });
 
         # TODO: increase buildbot testing scope
         buildbot = forCI (
@@ -214,6 +203,10 @@
           }
         );
 
+        devShells = forAllPkgs (pkgs: {
+          default = pkgs.callPackage ./home-manager/devShell.nix { };
+        });
+
         legacyPackages = forAllPkgs (
           pkgs:
           let

home-manager/devShell.nix

@@ -0,0 +1,11 @@
+{ pkgs, mkShell }:
+let
+  formatter = pkgs.callPackage ./formatter.nix { };
+in
+mkShell {
+  name = "devShell";
+  packages = [
+    pkgs.coreutils
+    formatter
+  ];
+}

home-manager/formatter.nix

@@ -0,0 +1,10 @@
+{ pkgs }:
+pkgs.treefmt.withConfig {
+  runtimeInputs = with pkgs; [
+    nixfmt
+    deadnix
+    keep-sorted
+    nixf-diagnose
+  ];
+  settings = pkgs.lib.importTOML ../treefmt.toml;
+}