flake: add treefmt-nix (#562)

.github/workflows/treefmt.yml

@@ -0,0 +1,43 @@
+name: Check formatting with treefmt
+
+on:
+  [push, pull_request]
+
+jobs:
+  treefmt:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: cachix/install-nix-action@v22
+    - uses: actions/checkout@v3
+    - id: files
+      uses: tj-actions/changed-files@v44
+    - name: Run treefmt
+      run: nix fmt
+    - name: Check diff
+      run: |
+        unformatted_touched=()
+        unformatted_untouched=()
+        while IFS= read -r unformatted_file; do
+          matched=
+          for changed_file in ${{ steps.files.outputs.all }}; do
+            if [[ "$changed_file" == "$unformatted_file" ]]; then
+              unformatted_touched+=("$unformatted_file")
+              matched=1
+              break
+            fi
+          done
+          if [[ -z "$matched" ]]; then
+            unformatted_untouched+=("$unformatted_file")
+          fi
+        done <<< "$(git diff --name-only)"
+
+        if (( ${#unformatted_untouched[@]} )); then
+          echo "These files are not formatted, but out of scope of this PR:"
+          printf '%s\n' "${unformatted_untouched[@]}"
+        fi
+
+        if (( ${#unformatted_touched[@]} )); then
+          echo "These files are created/edited but not formatted:"
+          printf '%s\n' "${unformatted_touched[@]}"
+          exit 1
+        fi

flake.nix

@@ -5,9 +5,14 @@
     flake-parts.url = "github:hercules-ci/flake-parts";
 
     flake-root.url = "github:srid/flake-root";
+
+    treefmt-nix = {
+      url = "github:numtide/treefmt-nix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
   };
 
-  outputs = { nixpkgs, flake-parts, ... }@inputs: flake-parts.lib.mkFlake { inherit inputs; } {
+  outputs = { nixpkgs, flake-parts, treefmt-nix, ... }@inputs: flake-parts.lib.mkFlake { inherit inputs; } {
     imports = [
       inputs.flake-parts.flakeModules.easyOverlay
       inputs.flake-root.flakeModule
@@ -45,6 +50,7 @@
           '';
           hardeningDisable = [ "fortify" ];
         };
+        treefmtEval = treefmt-nix.lib.evalModule pkgs ./treefmt.nix;
       in
       {
         packages.default = nixd;
@@ -103,6 +109,7 @@
             (import ./nixd/docs/editors/vscodium.nix { inherit pkgs; })
           ];
         };
+        formatter = treefmtEval.config.build.wrapper;
       };
     systems = nixpkgs.lib.systems.flakeExposed;
   };

treefmt.nix

@@ -0,0 +1,10 @@
+{ ... }:
+
+{
+  projectRootFile = "flake.nix";
+  programs = {
+    clang-format.enable = true;
+    nixpkgs-fmt.enable = true;
+    black.enable = true;
+  };
+}