lib: add functions for creating NixOS-like environments

lib/env-specific/nixos.nix

@@ -11,4 +11,34 @@
   # Checks if the NixOS config is being built for a particular format.
   isFormat = config: format:
     (config.formatAttr or "") == format;
+
+  # Create a separate environment similar to NixOS `system.path`. This is
+  # typically used to create isolated environments for custom desktop sessions
+  # which makes it possible to have them installed side-by-side with their own
+  # set of applications and everything (except for overlapping NixOS services
+  # that will just add them into the NixOS environment itself).
+  mkNixoslikeEnvironment = config: args:
+    pkgs.buildEnv {
+      inherit (args) paths name;
+      inherit (config.environment) pathsToLink extraOutputsToInstall;
+      ignoreCollisions = true;
+      postBuild =
+       ''
+         # Remove wrapped binaries, they shouldn't be accessible via PATH.
+         find $out/bin -maxdepth 1 -name ".*-wrapped" -type l -delete
+
+         if [ -x $out/bin/glib-compile-schemas -a -w $out/share/glib-2.0/schemas ]; then
+             $out/bin/glib-compile-schemas $out/share/glib-2.0/schemas
+         fi
+
+         ${config.environment.extraSetup}
+       '';
+    };
+
+  # Given an environment (built with `pkgs.buildEnv`), create a systemd
+  # environment attrset meant to be used as part of the desktop service.
+  mkSystemdDesktopEnvironment = env: {
+    XDG_DATA_DIRS = "${env}/share\${XDG_DATA_DIRS:+:$XDG_DATA_DIRS}";
+    XDG_CONFIG_DIRS = "${env}/etc/xdg\${XDG_CONFIG_DIRS:+:$XDG_CONFIG_DIRS}";
+  };
 }