core/pkgs/by-name/fo/fontconfig/make-fonts-conf.nix

{
  runCommand,
  stdenv,
  lib,
  libxslt,
  fontconfig,
  dejavu_fonts,
}:

let
  fontconfig_ = fontconfig;
in
{
  fontconfig ? fontconfig_,
  # an array of fonts, e.g. `[ pkgs.dejavu_fonts.minimal ]`
  fontDirectories,
  impureFontDirectories ?
    [
      # nix user profile
      "~/.nix-profile/lib/X11/fonts"
      "~/.nix-profile/share/fonts"
    ]
    ++ lib.optional stdenv.isDarwin "~/Library/Fonts"
    ++ [
      # FHS paths for non-NixOS platforms
      "/usr/share/fonts"
      "/usr/local/share/fonts"
    ]
    # darwin paths
    ++ lib.optionals stdenv.isDarwin [
      "/Library/Fonts"
      "/System/Library/Fonts"
    ]
    # nix default profile
    ++ [
      "/nix/var/nix/profiles/default/lib/X11/fonts"
      "/nix/var/nix/profiles/default/share/fonts"
    ],
}:

runCommand "fonts.conf"
  {
    nativeBuildInputs = [ libxslt ];
    buildInputs = [ fontconfig ];
    inherit fontDirectories;
    # Add a default font for non-nixos systems, <1MB and in nixos defaults.
    impureFontDirectories = impureFontDirectories ++ [ dejavu_fonts.minimal ];
  }
  ''
    xsltproc --stringparam fontDirectories "$fontDirectories" \
      --stringparam impureFontDirectories "$impureFontDirectories" \
      --path ${fontconfig.out}/share/xml/fontconfig \
      ${./make-fonts-conf.xsl} ${fontconfig.out}/etc/fonts/fonts.conf \
      > $out
  ''