Thanks, builtins.trace is exactly what I was looking for.
I just wanted to poke around to see if old was the top-level pkgs set, or something else.
I’m using rPackageOverrides because I’m trying to modify the jqr derivation produced by r-modules by leveraging the overrides argument that r-modules/default.nix accepts. In top-level/all-packages.nix, rPackages is set like:
I can’t figure out how to actually accomplish this (referencing old.rPackages.jqr leads to infinite recursion), but builtins.trace is an invaluable lesson. Thanks!