bug#66317: Project mode-line

[Juri Linkov]

>>>>>> Agreed.  No need to support it in older versions.  Moreover,
>>>>>> I'm trying to add another option to join project and vc indicators
>>>>>> on the mode-line that won't be possible to support in older versions.
>>>>> Then that needs to be explained in the docstring. project.el is 
>>>>> installable
>>>>> in older versions.
>>>> Hopefully I explained this sufficiently well in the docstring
>>>> both for users of older versions and users who customized the
>>>> default mode line.
>>> Not sure: I imagine that most users don't customize mode-line-format
>>> themselves, and thus don't know which contents it has.
>>>
>>> I would append some clarification that in Emacs 30 the mode-line has that
>>> element by default.
>> I'm not sure if this helps because a user might run Emacs 30
>> and still using some package that heavily modifies the mode line.
>> This will puzzle the user: why it doesn't work in Emacs 30
>> as documented.
>
> Hence "by default".
>
> Mode-line package authors might be helped out by this explanation as well.

Maybe this would be sufficient:

diff --git a/lisp/progmodes/project.el b/lisp/progmodes/project.el
index 2475d7c403e..1d6c9c1cf1b 100644
--- a/lisp/progmodes/project.el
+++ b/lisp/progmodes/project.el
@@ -2109,7 +2109,8 @@ project-uniquify-dirname-transform
 (defcustom project-mode-line nil
   "Show the current project name with the menu on the mode line.
 This feature requires the presence of the following item in
-`mode-line-format': `(project-mode-line project-mode-line-format)'."
+`mode-line-format': `(project-mode-line project-mode-line-format)'
+that already exists by default in Emacs 30."
   :type 'boolean
   :group 'project
   :version "30.1")