emacs override + overrideAttrs

{ pkgs, ... }:

let
emacs26 = pkgs.emacs.override{withXwidgets = true;}.overrideAttrs (oldAttrs: {
      version = "26.1RC1";
      src = pkgs.fetchurl {
        url = ftp://alpha.gnu.org/gnu/emacs/pretest/emacs-26.1-rc1.tar.xz;
        sha256 = "6594e668de00b96e73ad4f168c897fe4bca7c55a4caf19ee20eac54b62a05758";
      };
      patches = [];
    };
  });
in
{
  environment.systemPackages = [
    emacs26
  ];
}