bug#73172: [PATCH] Move to start of current header in diff-{file, hunk}-prev

[Spencer Baugh via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Dmitry Gutov <dmitry@gutov.dev> writes:
> Hi Spencer and Stefan,
>
> On 10/09/2024 21:40, Spencer Baugh via Bug reports for GNU Emacs, the
> Swiss army knife of text editors wrote:
>
>> (The following change is split across two patches; the first one, "move
>> easy-mmode", fixes an unrelated FIXME, which makes the diff in the
>> second patch simpler)
>> If point was after a file or hunk header, the diff-file-prev and
>> diff-hunk-prev commands would move to the start of that header.
>> But if point was *within* the header, they would not move, and
>> would report "No previous file" or "No previous hunk".  This
>> differs from the behavior of most other movement commands,
>> e.g. backward-sexp or backward-sentence.
>> This commit fixes diff-file-prev and diff-hunk-prev, as well as
>> other easy-mmode-define-navigation BASE-prev commands.  Now
>> these commands move to the start of the containing "thing" just
>> like other movement commands.
>> * lisp/emacs-lisp/easy-mmode.el (easy-mmode--prev): Move to
>> start of current match first.
>> Also discussed here:
>> https://lists.gnu.org/archive/html/help-gnu-emacs/2024-08/msg00367.html
>
> Patch#1 seems unequivocally a good thing (easier code iteration)

BTW, if we want to go ahead and install that patch on its own, that
would be fine with me.

> and patch#2 seems good on balance.
>
> It does introduce some backward incompatibility in rare cases where I
> have probably internalized the current behavior already -- for example
> in the vc-print-root-log output pressing 'p' while on the first line
> somewhere between the initial '*' and the end of the date dings with
> "No previous log message", and how will move to bol.
>
> But it might be more consistent anyway, given that the there is no
> ding already if you start out inside the summary text.

Yes.  It's definitely a change in behavior.

But, if someone has internalized:

  diff-file-prev usually acts like "backward-sexp for files" (it moves
  point to the start of the current file), except that if point is in
  the header of the first file in the buffer then diff-file-prev does
  nothing instead.

Then I think they'll be happy to be able to discard that knowledge
in favor of:

  diff-file-prev always acts like "backward-sexp for files" (it moves
  point to the start of the current file).

Especially because "does nothing in one weird corner case" is not useful
or deliberate behavior (it's just a bug).

> There aren't many in-tree callers of easy-mmode-define-navigation
> (diff-mode, log-view, smerge and, uh, cvs-status), and few 3rd party
> callers, so this seems low-impact. diff-hl-mode is not affected either
> because it only uses diff-hunk-next, not diff-hunk-prev.

Yep.  I think it will be a nice improvement for those few other commands
as well.