build(flake): Add treefmt

This only adds this as nix module. So running `nix fmt` will now format
all files under the repo root.

If you want to run this as non-nix user feel free to take a look at the
generated `treefmt.toml`.

flake.lock

@@ -174,7 +174,8 @@
         "flake-utils": "flake-utils",
         "nixpkgs": "nixpkgs",
         "rust-overlay": "rust-overlay",
-        "systems": "systems"
+        "systems": "systems",
+        "treefmt-nix": "treefmt-nix"
       }
     },
     "rust-overlay": {
@@ -247,6 +248,26 @@
         "repo": "default",
         "type": "github"
       }
+    },
+    "treefmt-nix": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1708335038,
+        "narHash": "sha256-ETLZNFBVCabo7lJrpjD6cAbnE11eDOjaQnznmg/6hAE=",
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "rev": "e504621290a1fd896631ddbc5e9c16f4366c9f65",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "type": "github"
+      }
     }
   },
   "root": "root",

flake.nix

@@ -16,7 +16,6 @@
 # You should have received a copy of the GNU General Public License
 # and the Lesser GNU General Public License along with this program.
 # If not, see <https://www.gnu.org/licenses/>.
-
 {
   description = "A rust crate used to generate multi-language apis for your
         application";
@@ -29,6 +28,13 @@
       flake = false;
     };
 
+    treefmt-nix = {
+      url = "github:numtide/treefmt-nix";
+      inputs = {
+        nixpkgs.follows = "nixpkgs";
+      };
+    };
+
     systems = {
       url = "github:nix-systems/default";
     };
@@ -64,11 +70,13 @@
     };
   };
   outputs = {
+    self,
     nixpkgs,
     crane,
     flake-utils,
     rust-overlay,
     ebnf2pdf,
+    treefmt-nix,
     ...
   }:
     flake-utils.lib.eachDefaultSystem (system: let
@@ -112,18 +120,52 @@
         doCheck = true;
         inherit nativeBuildInputs buildInputs;
       };
+      treefmtEval = treefmt-nix.lib.evalModule pkgs (
+        {pkgs, ...}: {
+          # Used to find the project root
+          projectRootFile = "flake.nix";
+
+          programs = {
+            alejandra.enable = true;
+            rustfmt.enable = true;
+            clang-format.enable = true;
+            mdformat.enable = true;
+            shfmt = {
+              enable = true;
+              indent_size = 4;
+            };
+            shellcheck.enable = true;
+          };
+
+          settings.formatter = {
+            clang-format = {
+              options = ["--style" "GNU"];
+            };
+            rustfmt = {
+              # also add the Trixy files
+              includes = ["*.tri"];
+            };
+          };
+        }
+      );
     in {
       checks.default = craneBuild;
       packages = {
         default = craneBuild;
       };
 
+      formatter = treefmtEval.config.build.wrapper;
+      checks = {
+        formatting = treefmtEval.config.build.check self;
+      };
+
       devShells.default = pkgs.mkShell {
         packages = with pkgs; [
           alejandra
 
           cocogitto
           licensure
+          treefmt
 
           rust
           cargo-edit