Using X without a Display Manager

To run X11 as a regular user, without services.xserver.enable = true; in configuration.nix, do the following:

First, install packages; you need X11 itself, some X11 input modules (e.g. xf86-input-evdev, xf86-input-synaptics, xf86-input-libinput), and possibly video modules as well (xf86-video-intel, xf86-video-ati, xf86-video-nouveau).

You probably want to use DRI acceleration for X; enable it and OpenGL in configuration.nix: hardware.opengl.enable = true; and hardware.opengl.driSupport = true;.

Then, it's just necessary to gather X configuration files into one directory and create a config file that also points X at the correct module paths.

This script does that (but you will need to add or remove to the pkgs and fontpkgs arrays according to your preferences):

#!/bin/sh
#generate unprivileged user xorg.conf for nixOS
#before running:
#    install any desired packages with `nix-env -iA ...`
#    update by running `nix-channel --update` and `nix-env --upgrade --leq`

#TODO: obey XDG guidelines rather than assuming ~/.config
mkdir -p ~/.config/xorg.conf.d
cd ~/.config/xorg.conf.d

#failed glob expansions become empty, not literal 'foo/*'
shopt -s nullglob

get_pkg_path() {
	attr=$1
	nix show-derivation -f '<nixpkgs>' "$attr" | jq -r '.[].env.out' 
}

#add to this set according to your driver needs
pkgs="
	xorg.xf86inputevdev
	xorg.xf86videointel
	xorg.xf86inputsynaptics
	xorg.xorgserver
"

echo 'Section "Files"' > 00-nix-module-paths.conf
for pkg in $pkgs; do
	pkg_path=$(get_pkg_path $pkg)
	for conf in "$pkg_path"/share/X11/xorg.conf.d/*; do 
		ln -s "$conf" ./
	done
	echo '	ModulePath "'"$pkg_path"'/lib/xorg/modules/"' >> 00-nix-module-paths.conf
done

#add to this set according to your font preferences
fontpkgs="
	xorg.fontmiscmisc
	ucsFonts
"

for pkg in $fontpkgs; do
	pkg_path=$(get_pkg_path $pkg)
	path="$pkg_path"'/share/fonts/'
	[ -d "$path" ] && echo '	FontPath "'"$path"'"' >> 00-nix-module-paths.conf
	path="$pkg_path"'/lib/X11/fonts/misc/'
	[ -d "$path" ] && echo '	FontPath "'"$path"'"' >> 00-nix-module-paths.conf
done

echo 'EndSection' >> 00-nix-module-paths.conf

You can now start X11 by running:

startx -- :0 -configdir ~/.config/xorg.conf.d