bug#41572: 28.0.50; [PATCH] Support plain project marked with file .emacs-project

[Dmitry Gutov <dgutov@yandex.ru>]

On 28.05.2020 18:46, Zihao Zhu wrote:
> If I have a project.el based plugin,  and  I wanna use them in a 
> directory not under VC. Add an mark to force project.el work is easier 
> than modify the source of plugin or initialize VC system.

The problem with that, is that next time we'll get a report that these 
projects are too slow. Or that people who added .emacs-project file in 
the middle of a VC repository suddenly get significantly worse file 
listing performance, without expecting it.

So we'd have to add caching to the file list, and then some 
invalidation, probably. And I'm not a fan of having manual invalidation 
commands.

That's why I'm wary of adding such a separate project type by default, 
especially when the initial proposal doesn't add any of the advanced 
features described above, or explains how they won't be necessary.

But opinions welcome.

> And this  can also be used as a side solution to use project.el in 
> unsupported VCed project in Emacs (AFAIK, P4(Perforce) is not supported 
> by vc.el).

Perhaps we could add Perforce support to project-vc instead?

There was a vc-p4 packages somewhere out there. But if it's entirely 
dead, we could add such support to project--vc-list-files directly.

Or, better yet, release it as a project-p4 package on GNU ELPA.

That all depends, of course, on whether the p4 command line utility also 
has the means to quickly list repository files and add ignores.

> IMO, a plain project is like a transient project.

One difference is, nobody really expects much from "transient" projects. 
And this type of project is only applied when the directory is not 
covered by any other kind of project.