feat: add vimdiffAlias option to provide nvim -d as vimdiff alias

- Introduce vimdiffAlias option to modules and wrappers for enabling a
  vimdiff alias that runs nvim -d
- Add vimdiffPackage output and wrapper script for vimdiff alias
- Update tests to verify vimdiff alias behavior when enabled or disabled
- Add shell aliases for bash, fish, and zsh when vimdiffAlias is enabled

Author: takeokunn

modules/top-level/output.nix

@@ -29,6 +29,14 @@ in
       '';
     };
 
+    vimdiffAlias = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Create a `vimdiff` alias that runs `nvim -d`.
+      '';
+    };
+
     waylandSupport = mkOption {
       type = types.bool;
       default = lib.meta.availableOn pkgs.stdenv.hostPlatform pkgs.wayland;
@@ -175,6 +183,15 @@ in
         visible = false;
       };
 
+      vimdiffPackage = mkOption {
+        type = types.package;
+        description = ''
+          A wrapper script that runs `nvim -d` as `vimdiff`.
+        '';
+        readOnly = true;
+        visible = false;
+      };
+
       manDocsPackage = mkOption {
         type = types.package;
         defaultText = lib.literalMD "`packages.<system>.man-docs` from Nixvim's flake";
@@ -318,6 +335,9 @@ in
             ]
             ++ lib.optionals config.enablePrintInit [
               printInitPackage
+            ]
+            ++ lib.optionals config.vimdiffAlias [
+              vimdiffPackage
             ];
           meta.mainProgram = "nvim";
           passthru = {
@@ -336,6 +356,10 @@ in
           '';
         };
 
+        vimdiffPackage = pkgs.writeShellScriptBin "vimdiff" ''
+          exec "${config.build.nvimPackage}/bin/nvim" -d "$@"
+        '';
+
         manDocsPackage = config.flake.packages.${system}.man-docs;
       };
 

tests/test-sources/modules/output.nix

@@ -161,4 +161,50 @@
         end
       '';
     };
+
+  vimdiffAlias-enabled =
+    { config, pkgs, ... }:
+    {
+      vimdiffAlias = true;
+
+      extraPlugins = [
+        {
+          plugin = pkgs.runCommandLocal "vimdiff-enabled-test" { } ''
+            # Test that vimdiff binary exists
+            if [ ! -x "${config.build.package}/bin/vimdiff" ]; then
+              echo "vimdiff binary should exist when vimdiffAlias is enabled" >&2
+              exit 1
+            fi
+
+            # Test that vimdiff script contains nvim -d
+            if ! grep -q 'nvim.*-d' "${config.build.package}/bin/vimdiff"; then
+              echo "vimdiff script should invoke nvim with -d flag" >&2
+              exit 1
+            fi
+
+            touch $out
+          '';
+        }
+      ];
+    };
+
+  vimdiffAlias-disabled =
+    { config, pkgs, ... }:
+    {
+      vimdiffAlias = false;
+
+      extraPlugins = [
+        {
+          plugin = pkgs.runCommandLocal "vimdiff-disabled-test" { } ''
+            # Test that vimdiff binary does NOT exist
+            if [ -e "${config.build.package}/bin/vimdiff" ]; then
+              echo "vimdiff binary should NOT exist when vimdiffAlias is disabled" >&2
+              exit 1
+            fi
+
+            touch $out
+          '';
+        }
+      ];
+    };
 }

wrappers/hm.nix

@@ -42,11 +42,5 @@ in
       EDITOR = "nvim";
       VISUAL = "nvim";
     };
-
-    programs = mkIf cfg.vimdiffAlias {
-      bash.shellAliases.vimdiff = "nvim -d";
-      fish.shellAliases.vimdiff = "nvim -d";
-      zsh.shellAliases.vimdiff = "nvim -d";
-    };
   };
 }

wrappers/modules/hm.nix

@@ -2,14 +2,6 @@
 {
   options = {
     defaultEditor = lib.mkEnableOption "nixvim as the default editor";
-
-    vimdiffAlias = lib.mkOption {
-      type = lib.types.bool;
-      default = false;
-      description = ''
-        Alias `vimdiff` to `nvim -d`.
-      '';
-    };
   };
 
   imports = [ ./shared.nix ];