Function prune_unreachable_prelude_mapped_with 

pub fn prune_unreachable_prelude_mapped_with(
    source: &str,
    user_declared: &[&str],
) -> (String, Vec<(usize, usize)>)

prune_unreachable_prelude_mapped with the names the user’s own source declares, which are never eligible for pruning (see there).