Function prune_unreachable_prelude_mapped_from 

pub fn prune_unreachable_prelude_mapped_from(
    user_source: &str,
    spliced: &str,
) -> (String, Vec<(usize, usize)>)

prune_unreachable_prelude_from with the byte ranges of spliced it kept, as prune_unreachable_prelude_mapped reports them — for a caller that maps offsets in the pruned text back, and must still prune exactly as a build does.