Function detect_pre_dec_end_offset 

fn detect_pre_dec_end_offset<'tcx>(
    tcx: TyCtxt<'tcx>,
    body: &Body<'tcx>,
) -> Option<u64>

Detect a pre_dec_end(offset) call on the receiver (arg 0) and return its constant offset. next_back_unchecked calls self.pre_dec_end(1) before returning the end_or_len field, so the returned pointer must be adjusted by offset elements.