I am observing attempt to subtract with overflow on this line:

self.lines.len() >= self.orphan_lines_count holds prior to entering this loop:

And real_len >= idx holds at the start of each iteration of that loop.

Those two facts causes me to squint at this if block:

In particular, commenting out that if block causes my panics to go away.

I don't completely understand the purpose of that if block, but maybe it can go away?

(indicatif is a great tool, BTW.)