starship: add xonsh shell configuration (#359614)

nixos/doc/manual/release-notes/rl-2505.section.md

@@ -515,6 +515,8 @@ Alongside many enhancements to NixOS modules and general system improvements, th
 
 - `bind.cacheNetworks` now only controls access for recursive queries, where it previously controlled access for all queries.
 
+- The [Starship](https://starship.rs) module now automatically loads the starship prompt when using [`xonsh`](https://xon.sh).
+
 - [`services.mongodb.enableAuth`](#opt-services.mongodb.enableAuth) now uses the newer [mongosh](https://github.com/mongodb-js/mongosh) shell instead of the legacy shell to configure the initial superuser. You can configure the mongosh package to use through the [`services.mongodb.mongoshPackage`](#opt-services.mongodb.mongoshPackage) option.
 
 - There is a new set of NixOS test tools for testing virtual Wi-Fi networks in many different topologies. See the {option}`services.vwifi` module, {option}`services.kismet` NixOS test, and [manual](https://nixos.org/manual/nixpkgs/unstable/#sec-nixos-test-wifi) for documentation and examples.

nixos/modules/programs/starship.nix

@@ -158,6 +158,18 @@ in
         eval "$(${cfg.package}/bin/starship init zsh)"
       fi
     '';
+
+    # use `config` instead of `${initOption}` because `programs.xonsh` doesn't have `shellInit` or `promptInit`
+    programs.xonsh.config = ''
+      if $TERM != "dumb":
+        # don't set STARSHIP_CONFIG automatically if there's a user-specified
+        # config file.  starship appears to use a hardcoded config location
+        # rather than one inside an XDG folder:
+        # https://github.com/starship/starship/blob/686bda1706e5b409129e6694639477a0f8a3f01b/src/configure.rs#L651
+        if not `$HOME/.config/starship.toml`:
+          $STARSHIP_CONFIG = ('${settingsFile}')
+        execx($(${cfg.package}/bin/starship init xonsh))
+    '';
   };
 
   meta.maintainers = pkgs.starship.meta.maintainers;