Function prune_unreachable_prelude_from 

pub fn prune_unreachable_prelude_from(
    user_source: &str,
    spliced: &str,
) -> String

prune_unreachable_prelude told which names the USER’s own (pre-prelude) source declares, so a name the stdlib also has is recognized as the user’s and kept. Callers that have the original source should prefer this; the bare form behaves as before.