24 sep. 2019 kl. 21.47 skrev Eli Zaretskii <eliz@gnu.org>:
> 
>> It was responsible for a sizeable portion of local consing in some (non-public) code I was profiling.
> 
> "Sizable portion" meaning what, in quantitative terms?

I no longer have the data, maybe 25-40% in the part of the code that I was looking at. It's not uncommon to use 'regexp-quote' just in case.

> I think it should be in NEWS and in the ELisp manual.  Doc string
> doesn't necessarily have to say that (although it isn't a catastrophe
> if it does), given its current language.

Thanks -- the updated patch includes NEWS and manual amendments, as well as a slightly simplified condition. The manual didn't actually say anything about a new string being returned (in contrast to 'format' before 3db388b0bf), but a note about it won't hurt.