bug#66260: 29.0.92; project.el should support copying a file path relative to the project root

[Dmitry Gutov <dmitry@gutov.dev>]

On 29/09/2023 02:26, sbaugh@catern.com wrote:
> When communicating with others, it is often useful to copy a file path
> to the current file which is relative to the project root.  For example
> I'd rather send someone the path lisp/progmodes/project.el than
> ~/src/emacs/trunk/lisp/progmodes/project.el.
> 
> project.el should have a way to copy this file path.
> 
> Almost good enough is:
> 
>   C-x p f       ;; project-find-file
>   M-n	       ;; next-history-element
>   C-k	       ;; kill-line
> 
> but with project--read-file-cpd-relative, if all the files in
> project-files start with a common-parent-directory which is a
> subdirectory of project-root, the resulting file path will not be
> relative to the project-root but to the subdirectory.

A problem with modifying the above function's behavior like that is that 
sometimes it is passed a wider range of file names, where not all of 
them are inside the project root (M-x project-or-external-find-file).

> Maybe the thing to do is just make project--read-file-cpd-relative use
> project-root instead of calculating a common-parent-directory?  I would
> be happy to make that patch if that's acceptable.  I think that's
> probably the best option.
> 
> Alternatively, it might be cool if there was a variation on:
> 
>   C-x C-j     ;; dired-jump
>   M-0	     ;; digit-argument
>   w	     ;; dired-copy-filename-as-kill
> 
> (which copies an absolute path) to instead copy a project-root-relative
> path.

This is unfortunately less discoverable, but sounds good otherwise.

Regarding the potential binding (discussed further downthread), I have 
no opinion: copying the relative name doesn't happen too often for me, 
and when it does, I can usually do that from the VC-Dir buffer (where 
the file in question would usually be 'edited'). Should we add 'w' to 
vc-dir-mode too?

'C-x p w' does make sense as such, but it would be unfortunate if we 
find a more important command later which fits the 'w' mnemonics. Maybe 
'C-x p W' would be a more conservative choice.