build(nix): add treefmt-nix

currently still bare but can be extended with a number of different
formatters (rustfmt, clang-format, ...). For full list see
https://flake.parts/options/treefmt-nix

Signed-off-by: Thomas Mühlbacher <tmuehlbacher@posteo.net>

2 files changed, 42 insertions, 4 deletions

diff --git a/flake.lock b/flake.lock
index 2b883b6e..58b8eeb5 100644
--- a/flake.lock
+++ b/flake.lock
@@ -66,7 +66,28 @@
       "inputs": {
         "flake-compat": "flake-compat",
         "flake-parts": "flake-parts",
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": "nixpkgs",
+        "treefmt-nix": "treefmt-nix"
+      }
+    },
+    "treefmt-nix": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1715940852,
+        "narHash": "sha256-wJqHMg/K6X3JGAE9YLM0LsuKrKb4XiBeVaoeMNlReZg=",
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "rev": "2fba33a182602b9d49f0b2440513e5ee091d838b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "type": "github"
       }
     }
   },
diff --git a/flake.nix b/flake.nix
index 4a2d3c4c..1070466e 100644
--- a/flake.nix
+++ b/flake.nix
@@ -6,6 +6,11 @@
 
     flake-parts.url = "github:hercules-ci/flake-parts";
 
+    treefmt-nix = {
+      url = "github:numtide/treefmt-nix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+
     flake-compat = {
       url = "github:edolstra/flake-compat";
       flake = false;
@@ -17,10 +22,13 @@
       self,
       nixpkgs,
       flake-parts,
+      treefmt-nix,
       flake-compat,
       ...
     }:
     flake-parts.lib.mkFlake { inherit inputs; } {
+      imports = [ inputs.treefmt-nix.flakeModule ];
+
       # can be extended, but these have proper binary cache support in nixpkgs
       # as of writing.
       systems = [
@@ -41,13 +49,22 @@
 
           packages.bcachefs-tools-fuse = config.packages.bcachefs-tools.override { fuseSupport = true; };
 
-          formatter = pkgs.nixfmt-rfc-style;
-
           devShells.default = pkgs.mkShell {
-            inputsFrom = [ config.packages.default ];
+            inputsFrom = [
+              config.packages.default
+              config.treefmt.build.devShell
+            ];
 
             LIBCLANG_PATH = "${pkgs.clang.cc.lib}/lib";
           };
+
+          treefmt.config = {
+            projectRootFile = "flake.nix";
+
+            programs = {
+              nixfmt-rfc-style.enable = true;
+            };
+          };
         };
     };
 }