From 908e055e157a0b35466faf4125d7e7410ff56160 Mon Sep 17 00:00:00 2001
From: Manuel <2084639+tennox@users.noreply.github.com>
Date: Mon, 24 Mar 2025 20:04:19 +0545
Subject: [PATCH] git: option to use difftastic as difftool (#5335)

I want `git diff` to stay the same, but `git difftool` to use
difftastic, as described in the difftastic docs:
https://difftastic.wilfred.me.uk/git.html#regular-usage
---
 modules/programs/git.nix | 41 ++++++++++++++++++++++++++++------------
 1 file changed, 29 insertions(+), 12 deletions(-)

diff --git a/modules/programs/git.nix b/modules/programs/git.nix
index 16fc6a0fa..a4d528d4d 100644
--- a/modules/programs/git.nix
+++ b/modules/programs/git.nix
@@ -283,6 +283,13 @@ in {
 
         package = mkPackageOption pkgs "difftastic" { };
 
+        enableAsDifftool = mkEnableOption "" // {
+          description = ''
+            Enable the {command}`difftastic` syntax highlighter as a git difftool.
+            See <https://github.com/Wilfred/difftastic>.
+          '';
+        };
+
         background = mkOption {
           type = types.enum [ "light" "dark" ];
           default = "light";
@@ -656,18 +663,28 @@ in {
       };
     })
 
-    (mkIf cfg.difftastic.enable {
-      home.packages = [ cfg.difftastic.package ];
-
-      programs.git.iniContent = let
-        difftCommand = concatStringsSep " " [
-          "${getExe cfg.difftastic.package}"
-          "--color ${cfg.difftastic.color}"
-          "--background ${cfg.difftastic.background}"
-          "--display ${cfg.difftastic.display}"
-        ];
-      in { diff.external = difftCommand; };
-    })
+    (let
+      difftCommand = concatStringsSep " " [
+        "${getExe cfg.difftastic.package}"
+        "--color ${cfg.difftastic.color}"
+        "--background ${cfg.difftastic.background}"
+        "--display ${cfg.difftastic.display}"
+      ];
+    in (lib.mkMerge [
+      (mkIf cfg.difftastic.enable {
+        home.packages = [ cfg.difftastic.package ];
+        programs.git.iniContent = { diff.external = difftCommand; };
+      })
+      (mkIf cfg.difftastic.enableAsDifftool {
+        home.packages = [ cfg.difftastic.package ];
+        programs.git.iniContent = {
+          diff = { tool = lib.mkDefault "difftastic"; };
+          difftool = {
+            difftastic = { cmd = "${difftCommand} $LOCAL $REMOTE"; };
+          };
+        };
+      })
+    ]))
 
     (let
       deltaPackage = cfg.delta.package;