nixos/fish: allow disabling completion generation (#418328)

nixos/modules/profiles/minimal.nix

···
     # The lessopen package pulls in Perl.
     less.lessopen = mkDefault null;
     command-not-found.enable = mkDefault false;
+    fish.generateCompletions = mkDefault false;
   };
 
   # This pulls in nixos-containers which depends on Perl.

nixos/modules/programs/fish.nix

···
         '';
       };
 
+      generateCompletions = lib.mkEnableOption "generating completion files from man pages" // {
+        default = true;
+        example = false;
+      };
+
       vendor.config.enable = lib.mkOption {
         type = lib.types.bool;
         default = true;
···
         '';
       }
 
-      {
+      (lib.mkIf cfg.generateCompletions {
         etc."fish/generated_completions".source =
           let
             patchedGenerator = pkgs.stdenv.mkDerivation {
···
             ignoreCollisions = true;
             paths = builtins.map generateCompletions config.environment.systemPackages;
           };
-      }
+      })
 
       # include programs that bring their own completions
       {
···
       }
     ];
 
-    programs.fish.interactiveShellInit = ''
-      # add completions generated by NixOS to $fish_complete_path
-      begin
-        # joins with null byte to accommodate all characters in paths, then respectively gets all paths before (exclusive) / after (inclusive) the first one including "generated_completions",
-        # splits by null byte, and then removes all empty lines produced by using 'string'
-        set -l prev (string join0 $fish_complete_path | string match --regex "^.*?(?=\x00[^\x00]*generated_completions.*)" | string split0 | string match -er ".")
-        set -l post (string join0 $fish_complete_path | string match --regex "[^\x00]*generated_completions.*" | string split0 | string match -er ".")
-        set fish_complete_path $prev "/etc/fish/generated_completions" $post
-      end
-      # prevent fish from generating completions on first run
-      if not test -d $__fish_user_data_dir/generated_completions
-        ${pkgs.coreutils}/bin/mkdir $__fish_user_data_dir/generated_completions
-      end
-    '';
+    programs.fish.interactiveShellInit =
+      lib.optionalString cfg.generateCompletions ''
+        # add completions generated by NixOS to $fish_complete_path
+        begin
+          # joins with null byte to accommodate all characters in paths, then respectively gets all paths before (exclusive) / after (inclusive) the first one including "generated_completions",
+          # splits by null byte, and then removes all empty lines produced by using 'string'
+          set -l prev (string join0 $fish_complete_path | string match --regex "^.*?(?=\x00[^\x00]*generated_completions.*)" | string split0 | string match -er ".")
+          set -l post (string join0 $fish_complete_path | string match --regex "[^\x00]*generated_completions.*" | string split0 | string match -er ".")
+          set fish_complete_path $prev "/etc/fish/generated_completions" $post
+        end
+      ''
+      + ''
+        # prevent fish from generating completions on first run
+        if not test -d $__fish_user_data_dir/generated_completions
+          ${pkgs.coreutils}/bin/mkdir $__fish_user_data_dir/generated_completions
+        end
+      '';
 
   };
   meta.maintainers = with lib.maintainers; [ sigmasquadron ];