From b3233405381b50c7f7eb87d7962b25a06d16be44 Mon Sep 17 00:00:00 2001
From: Rebecca Turner <rbt@sent.as>
Date: Thu, 4 Apr 2024 16:11:37 -0700
Subject: [PATCH] Add `nixfmt`

Change-Id: I7f21695e3971cfd02b2cce0dd016ff6eb3389905
---
 flake.nix              | 11 +++++--
 nix-support/nixfmt.nix | 65 ++++++++++++++++++++++++++++++++++++++++++
 treefmt.toml           |  4 +++
 3 files changed, 78 insertions(+), 2 deletions(-)
 create mode 100644 nix-support/nixfmt.nix

diff --git a/flake.nix b/flake.nix
index 26540a995..77e34fa7f 100644
--- a/flake.nix
+++ b/flake.nix
@@ -71,6 +71,9 @@
             };
             overlays = [
               (overlayFor (p: p.${stdenv}))
+              (final: prev: {
+                nixfmt = final.callPackage ./nix-support/nixfmt.nix {};
+              })
             ];
 
             config.permittedInsecurePackages = [ "nix-2.13.6" ];
@@ -296,7 +299,11 @@
             };
             treefmt = {
               enable = true;
-              settings.formatters = [ ];
+              settings.formatters =
+                let
+                  pkgs = nixpkgsFor.${system}.native;
+                in
+                [ pkgs.nixfmt ];
             };
           };
         }) pre-commit-hooks.lib;
@@ -366,7 +373,7 @@
             }).overrideAttrs (prev: {
               # Required for clang-tidy checks
               buildInputs = prev.buildInputs
-                ++ [ pkgs.just ]
+                ++ [ pkgs.just pkgs.nixfmt ]
                 ++ lib.optional (pre-commit ? enabledPackages) pre-commit.enabledPackages
                 ++ lib.optionals (stdenv.cc.isClang) [ pkgs.llvmPackages.llvm pkgs.llvmPackages.clang-unwrapped.dev ];
               nativeBuildInputs = prev.nativeBuildInputs
diff --git a/nix-support/nixfmt.nix b/nix-support/nixfmt.nix
new file mode 100644
index 000000000..b51e26295
--- /dev/null
+++ b/nix-support/nixfmt.nix
@@ -0,0 +1,65 @@
+# Copy of `nixfmt-rfc-style` vendored from `nixpkgs` master:
+# https://github.com/NixOS/nixpkgs/blob/ab6071eb54cc9b66dda436111d4f569e4e56cbf4/pkgs/by-name/ni/nixfmt-rfc-style/package.nix
+{
+  haskell,
+  haskellPackages,
+  fetchFromGitHub,
+}:
+let
+  inherit (haskell.lib.compose) justStaticExecutables;
+  raw-pkg = haskellPackages.callPackage (
+    {
+      mkDerivation,
+      base,
+      cmdargs,
+      directory,
+      fetchzip,
+      filepath,
+      lib,
+      megaparsec,
+      mtl,
+      parser-combinators,
+      safe-exceptions,
+      scientific,
+      text,
+      transformers,
+      unix,
+    }:
+    mkDerivation {
+      pname = "nixfmt";
+      version = "0.6.0-unstable-2024-03-14";
+      src = fetchFromGitHub {
+        owner = "serokell";
+        repo = "nixfmt";
+        rev = "8d13b593fa8d8d6e5075f541f3231222a08e84df";
+        hash = "sha256-HtXvzmfN4wk45qiKZ7V+/5WBV7jnTHfd7iBwF4XGl64=";
+      };
+      isLibrary = true;
+      isExecutable = true;
+      libraryHaskellDepends = [
+        base
+        megaparsec
+        mtl
+        parser-combinators
+        scientific
+        text
+        transformers
+      ];
+      executableHaskellDepends = [
+        base
+        cmdargs
+        directory
+        filepath
+        safe-exceptions
+        text
+        unix
+      ];
+      jailbreak = true;
+      homepage = "https://github.com/serokell/nixfmt";
+      description = "An opinionated formatter for Nix";
+      license = lib.licenses.mpl20;
+      mainProgram = "nixfmt";
+    }
+  ) { };
+in
+justStaticExecutables raw-pkg
diff --git a/treefmt.toml b/treefmt.toml
index 81df72cc2..ff7d972be 100644
--- a/treefmt.toml
+++ b/treefmt.toml
@@ -1 +1,5 @@
 [formatter]
+# [formatter.nix]
+# command = "nixfmt"
+# includes = ["*.nix"]
+# excludes = ["tests/**"]