[klmr]

If you save the offset (and you are, if you use a borrowed string) you can retrieve the original source code by re-reading the input file on the fly. This is expensive (due to IO, but especially also since you need to re-lex and recompute all offsets up to the point of the error), but it only needs to happen in case a parse error is generated, so the cost is likely negligible.

I’ve done this in a parser where I wanted to avoid storing line and column numbers instead of a single byte offset, but still wanted to provide the more user-friendly line and column numbers in the error message.