bug#46859: 28.0.50; [PATCH]: Add option to truncate long lines in xref.el

[Gregory Heytings]

> No.  byte-to-position works for text in an Emacs buffer, whereas we are 
> talking about the text in its original file on disk.  Unless that file 
> is encoded in UTF-8, byte-to-position will give you wrong results.  You 
> need to use filepos-to-bufferpos, and you will need to specify the 
> file's encoding.  And it's relatively slow for non-UTF-8 encoded files.

Thank you, I was not aware of that subtlety.

But you provide the solution: when an xref is followed, the file is opened 
in a buffer, at which point buffer-file-coding-system is set.  So it seems 
that it suffices to do (goto-char (filepos-to-bufferpos 
(get-byte-position))).

I just did a filepos-to-bufferpos for one of the last bytes of a 6 MB 
Latin-1 file, and it took only ~2 ms.