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

[Nikolay Kudryavtsev <nikolay.kudryavtsev@gmail.com>]

> I didn't find a definite answer in the rest of the email, so I'll 
> continue this thread here. 
Lets restate the answer: VC should be available to the user regardless 
of the current project backend. When it is so, you can use whatever 
defaults you deem reasonable, with ignores being respected probably 
being a good choice.

> So you want to add a new backend based on GTAGS which also lists files 
> by calling 'global -P' or something.
> I want all of those capabilities to be reachable, but we also need to 
> decide which project backends configuration is on by default.
The question of default project backends is pretty irrelevant here. I'm 
not suggesting adding anything there. I'm just saying that when the user 
adds something, it should not be reduced to the second class citizenship.

The reason I've jumped in against your patch in particular is because 
that's what it does. It makes a distinction between the (possibly) full 
fledged function backends and the inferior marker file backends, which 
get their own little ghetto. While I believe that we should treat such 
marker file backends as first class citizens that are equal to any other 
backend.

> If the user wants to choose what to act on (whole project or current 
> module), the information about modules needs to be discovered as a 
> separate semantic unit.
  I don't think so. We have two options:

1. Nestable projects. In this paradigm any project can have however many 
other projects in each subdirectories. You can go from child project to 
a parent project pretty easily. Going from parent project to child 
projects is a little bit harder, but still possible. So I can 
compile(use custom action on) the parent projects from a child one, or a 
particular(or all) child projects from parent.

2. Project units. Not gonna use the word "module" here, because we've 
already used it for a different thing in this discussion. So one project 
can have 0 or more units. Units cannot exist outside of a project and 
are in turn are not projects themselves. Except that they can completely 
look like projects, in cases like Maven or Makefiles.

Option 1 is simpler because it has only 1 concept. Option 2 introduces a 
second separate semantic unit, but I don't see any benefit from 
introducing it.

> Only if you intend to use the 'tags tool' as a build tool, e.g. list 
> the available tasks or invoke one (for example, to rebuild the index). 
> Then said tags tool can be put into build-tools-functions, with 
> appropriate implementations for 'list tasks' and 'run task'.
I don't like such over-engineering, due to conceptually assuming that 
all projects are build tool projects, all actions being connected to 1 
particular tool and each tool necessarily having multiple actions. I 
believe all of this is better left to users and backend developers. With 
just actions and maybe action groups, if we want them.

> Do we really want to ask everyone to use a separate set of commands to 
> apply the 'ignores' config from the current VCS repo? 
No? Separate commands is just another advantage of VC being always 
there(if it's there). Maybe I'm maintaining a certain part of the 
repository, be it a module or whatever, and that's why I'm treating that 
part as a project, since that's the scope I normally care about. But 
then I notice a buggy function and use those commands to look whether 
anything else apart from my code uses it in the wider repo.

P.S. Sorry for the terrible amount of typing errors in my previous 
letter, too much writing for me last week.