@hergertme omg, several days after I started trying to do this, I have a mostly-working gnome-builder, hooray! I had to replace nixpkgs' g-i patch with https://gitlab.gnome.org/GNOME/gobject-introspection/commit/a41abe1868a693387cd5cf85567cf2e0fd6c62df but strip out the meson.build portion of that patch because the last released version didn't use meson yet 😅
Now, why can't cargo find things like pkg-config that are on my PATH when I build inside Builder? 🤔 Is that safe_path setting I saw used here? I don't have /bin or /usr/bin...