bug#64799: [PATCH] Add 'project-prompt-key' face

[Dmitry Gutov <dmitry@gutov.dev>]

On 24/07/2023 08:53, Protesilaos Stavrou wrote:
>> From: Philip Kaludercic <philipk@posteo.net>
>> Date: Sun, 23 Jul 2023 13:59:41 +0000
> 
>> [... 14 lines elided]
> 
>>>> Does this even need a separate face, or shouldn't `help-key-binding' be
>>>> reused directly?
>>>
>>> That would be my choice, though also with the removal of the square
>>> brackets around it.  The proposed patch is for backward-compatibility.
>>
>> Backwards-compatible to what?
> 
> Just so that existing users do not complain.
> 
>> Can someone depend on this specific behaviour?
> 
> No, but the appearance still comes with certain expectations.  Plus,
> there is this at the top of the file:
> 
>      ;; This is a GNU ELPA :core package.  Avoid using functionality that
>      ;; not compatible with the version of Emacs recorded above
> 
> The required Emacs version is 26.1, while that of 'help-key-binding' is
> 28.1.

Indeed, so if we switch to it in project.el, it will need some polyfill 
for Emacs 26-27.

> Perhaps a user option for the whole 'format' would be better?

We can add a user option. But while we could also apply 'face' property 
on that option's string value, that doesn't seem like a very 
user-friendly approach toward someone who will want to change the face 
used (it will require some Lisp knowledge from the user).

But a user option to change how the prompt text is constructed -- why 
not. E.g. one defaulting to "[%s] %s".