GNU bug report logs - #79426
diagnostic with empty range => preceding char is highlighted

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From: Eli Zaretskii <eliz <at> gnu.org>
To: Alan Donovan <adonovan <at> google.com>, João Távora <joaotavora <at> gmail.com>
Cc: 79426 <at> debbugs.gnu.org
Subject: bug#79426: diagnostic with empty range => preceding char is highlighted
Date: Thu, 11 Sep 2025 07:49:52 +0300

> Date: Wed, 10 Sep 2025 18:07:56 -0400
> From:  Alan Donovan via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs <at> gnu.org>
> 
> (from https://github.com/joaotavora/eglot/issues/27#issuecomment-3275571619)
> 
> The fix for issue #27 (zero-width ranges) included this code:
> 
>                        ;; Fallback to `flymake-diag-region' if server
>                        ;; botched the range
>                        (when (= beg end)
> (if-let* ((st (plist-get range :start))
>   (diag-region
>    (flymake-diag-region
>     (current-buffer) (1+ (plist-get st :line))
>     (plist-get st :character))))
>     (setq beg (car diag-region) end (cdr diag-region))
>   (eglot--widening
>    (goto-char (point-min))
>    (setq beg
>  (eglot--bol
>   (1+ (plist-get (plist-get range :start) :line))))
>    (setq end
>  (line-end-position
>   (1+ (plist-get (plist-get range :end) :line)))))))
> 
> This causes Emacs to highlight the character before the indicated
> range, which is often an irrelevant blank space, instead of the
> character after, which is the token of interest. (Gopls' nilness
> analyzers publishes zero-width diagnostics.)
> 
> Why is that the appropriate fix, as opposed to something as simple as this?
> 
>                        (when (= beg end)
>                          (setq end (1+ end)))
> 
> (Aside: "botched" seems like an unnecessarily pejorative: an empty
> range is what it is, and sometimes all the information the server has
> is the start point of a token.)
> 
> There have been two minor changes to that logic since #27 was fixed,
> but neither seems relevant to this question:
> - 3d9a6f1 Handle case when diagnostic :character is out of range
> - 5b62d00 Enable lsp project-wide diagnostics via flymake
> 
> I couldn't check for related issues because
> https://lists.gnu.org/archive/html/bug-gnu-emacs/ appears to be down.

Thanks, I added João to the discussion.

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