I used DUMMY_SP for generating dummy span of max_width_error in rewrite_pair, rewrite_all_pairs and its callees. There might be a better way to manually create an appropriate span for PairPart, and I need to give it more thought.

@ding-young When I took a quick look at the code I was surprised to see DUMMY_SP being used. I'd really like to avoid using DUMMY_SP if possible because it would mean that we can't guarantee that the spans in RewriteError::ExceedsMaxWidth are even valid. Could you look into how hard it would be to update the bounds on rewrite_pairs_multiline and rewrite_pair to add a bound for our Spanned trait.

If it's not possible, or just too much effort to add the Spanned bound right now, then I'd probable want to add another RewriteError variant that captures the fact that "this is a max_width issue, but we can't accurately report the span".

I removed DUMMY_SP by updating PairList and RangeOperand to hold corresponding span as follows. I'll mark the code line ranges.