Re: vc-diff in non-VC buffer

["Eric S. Raymond" <esr@thyrsus.com>]

Richard Stallman <rms@gnu.org>:
>     C:\> emacs -Q -D
>     C-x v =
> 
>     => launches a traversal of the hard disk.
> 
> I think it should verify that the current buffer corresponds to a
> version-controlled directory or file.  Eric, do you see any
> argument against doing so?

Yes.  For that particular operation, and some other non-modifying ones
such as printing change-history logs, what users often want is a
fallback rule for computing selected filesets that says, in effect:
if the selected fileset is empty, look at all version-controlled 
files beneath the current directory.  

So, for example, you can go to a top-level project directory and just
do C-x v = to look at the state of all changes beneath it, regardless
of whether you're actually visiting a controlled file.  This is useful,
and I think many VC users would be (rightly) annoyed if they lost it.

What Juanma is pointing out is that this rule grinds your disk in what
is for him an unhelpful way when you do C-x v = from $HOME.  He's got a
point; aggregating diffs from the multiple projects that doubtless live
under there is not likely to be useful. 

I'm testing code that errors out if you (a) have an empty fileset, and
(b) no files are registered immediately under default-directory.  Juanma
thinks this will cover his use case.
-- 
		<a href="http://www.catb.org/~esr/">Eric S. Raymond</a>