> Isearch could show in the mode line how many matches follow or precede 
> the currently highlighted one (or we could write "3 of 4 matches", as, 
> e.g., Firefox does).  For big files, it could be helpful to calculate 
> this information either lazily or in another thread.

Thanks to Drew, now we have the full-buffer lazy-highlighting
loop that can be reused for Isearch hit count.

Here is a minimal patch that implements this feature: