Description of changes

XeTeX relies on the system's font managent tooling to locate fonts. This means that XeTeX was unable to find the fonts distributed with texlive as these weren't installed in paths searched by the system's font configuration.

This PR fixes this for linux by wrapping xetex such that the FONTCONFIG_FILE environment value is set to a suitable fontconfig file generated by makeFontsConf. This file is almost identical to the one from the fontconfig package and
/etc/fonts/fonts.conf on NixOS -- it only adds the texlive font directories. In particular this means that all the actual
configuration files in /etc/fonts/conf.d are still included and there also is a /usr/share/fonts font directory entry for use with non-NixOS linuxes, so it's unlikely that this breaks someone's setup.

Darwin remains broken since XeTeX doesn't use fontconfig there.

This also exposes texlive's font directory as a passthru-attribute, which makes adding texlive's fonts to the fonts.fonts. nixos-option (or symlinking the directory to ~/Library/fonts on MacOS with home-manager) a lot easier.

As the wrapper uses --set-default, devShells can easily override the font directories to add more fonts. For example in the shell

{ pkgs ? import <nixpkgs> {} }:
let
  texlive = pkgs.texlive.combined.scheme-full;
  systemfontdirs = with pkgs; map toString [ comic-mono crimson ];
in pkgs.mkShell {
    nativeBuildInputs = [ texlive ];
    # used by xetex and mtx-fonts (context)
    FONTCONFIG_FILE = pkgs.makeFontsConf { fontDirectories = systemfontdirs ++ [ texlive.fonts ]; };
    # used by luaotfload (lualatex)
    OSFONTDIR = pkgs.lib.concatStringsSep "//:" systemfontdirs;
}

the document

\documentclass{article}
\usepackage{mwe,fontspec}
\begin{document}
\fontspec{Crimson} \blindtext % "system" font
\fontspec{STIX Two Text} \blindtext % texlive font
\end{document}

compiles with both lualatex and xelatex. (It's a bit unfortunate that luaotfload hardcodes the fontconfig location to /etc/fonts/fonts.conf and /usr/local/etc/fonts/fonts.conf, so a separate environment variable has to be used, but at least it works).

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.05 Release Notes (or backporting 22.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.