systemd 2.35 or master

    nixpkgs.overlays = lib.singleton (self: super: {
      systemd = super.systemd.overrideAttrs (drv: rec {
        src = /root/systemd;
        patches = [];
        autoreconfPhase = ''
          touch autogen.sh
          chmod u+x autogen.sh
          ls -lathr
          sed -i -e 's|/bin/bash|${pkgs.bash}/bin/bash|g' configure
          sed -i -e 's|meson.*||g' configure
        '';
        buildInputs= [ pkgs.meson pkgs.ninja ] ++ super.buildInputs;
        #patches = [ ./d3070fbdf6077d7da9dbafa198fff8dea712d2ff.patch ];
        #patches = [ ./7246.patch ];
        #patches = [ https://patch-diff.githubusercontent.com/raw/systemd/systemd/pull/7246.patch ];
        #name = "systemd-${version}";
        #version = "232";
        #src = self.fetchFromGitHub {
        #  owner = "nixos";
        #  repo = "systemd";
        #  rev = "66e778e851440fde7f20cff0c24d23538144be8d";
        #  sha256 = "1valz8v2q4cj0ipz2b6mh5p0rjxpy3m88gg9xa2rcc4gcmscndzk";
        #};
        passthru = (drv.passthru or {}) // {
          udev.bin = self.systemd;
          udev.lib = self.libudev.out;
        };
      });
    });

instead of

    buildInputs= [ pkgs.meson pkgs.ninja ] ++ super.buildInputs;

one has to do

    buildInputs= [ pkgs.meson pkgs.ninja ] ++ drv.buildInputs;