bug#66464: Vc mode-line

[Dmitry Gutov <dmitry@gutov.dev>]

On 10/11/2023 09:33, Juri Linkov wrote:
>>> It would be a pity not to add this handy feature.
>>> So here is a small patch that joins the project name and vc status
>>> on the mode line:
>>
>> Could you described the intended look?
> 
> Here is the screenshot how the mode line looks like:

Thanks.

>> Two mode-line entries one right by another, without a space between?
> 
> The format is "project-branch" instead of "backend-branch".

It might also be misread as a directory name. E.g. I have directories 
called "emacs" and "emacs-master" (thanks to git-worktree), and they get 
automatically assigned the corresponding project names. The new 
mode-line with your customization looks like "emacs-master-master". But 
that could just be my problem.

>> And with the backend name removed? Wouldn't you say we're losing some
>> useful information this way? Or perhaps the backend name is not essential.
> 
> I'm sure that showing the backend name in all file buffers is useless
> for most users and wastes the precious screen space.

Hmm, I suppose since we're in a situation where one VCS (Git) is used 
99% of the time, the indication might be unnecessary. But it's also a 
reminder that this mode-line item is for VC. Anyway...

> But I don't propose to change the default.
> 
>> In any case, it seems like
>>
>>    (const :tag "Show project name and status" with-project)
>>
>> describes the option inaccurately, since VC's mode line doesn't show the
>> project name.
> 
> Technically it's not vc-mode that shows the project name indeed.
> I already tried such things as calling '(project-mode-line-format)'
> from 'vc-mode-line-string'.  But it's an unnecessary complication.

That's probably an overkill indeed, also because the user would have to 
customize project-mode-line to nil, and then some projects might not be 
VC-controlled, which would result in no mode-line element at all.

>> And if you customize 'vc-display-status' to 'with-project' but don't
>> set project-mode-line to t, the project name won't be shown at all.
> 
> This is why the condition already handles this case:
> 
>    (and (eq vc-display-status 'with-project)
>         (bound-and-true-p project-mode-line))

What happens if mode-line-format doesn't contain 
project-mode-line-format, or has it in the wrong place? E.g. if the user 
(or third-party package) customized the mode-line.

It's probably not terrible not to support that (after all, that would be 
a combination of two non-default customizations).

Anyway, I've raised the questions, but I don't have strong objections to 
your patch. Perhaps consider changing the description from

   (const :tag "Show project name and status" with-project)

to

   (const :tag "Show only status next to project name" with-project)

and, maybe, check that project-mode-line-format precedes the vc-mode 
indicator in the current mode-line format. And otherwise keep the 
separating " ". If that makes sense to you.