RE: Omitting :group [was: master 7936c8a: * lisp/mb-depth.el (minibuffer-depth-indicator): New face.]

[Drew Adams]

> > > This change causes a byte-compilation warning:
> > >   In toplevel form:
> > >   mb-depth.el:38:1: Warning: defface for
> > >   `minibuffer-depth-indicator' fails to specify
> > >   containing group
> >
> > IMHO, we should adopt and promote a guideline
> > that it's a good, not a bad, idea to always
> > include an explicit `:group'.
> 
> Aren't you confusing :group in defcustom with :group in defface?

Perhaps I am (am I)?  What's the difference?

(elisp) `Common Keywords' suggests that its
description of :group applies to "the next few
sections-'defcustom', 'defgroup', etc."

And node `Group Definitions' suggests that a
group defined by `defgroup' applies to all of
the group's members, and that such members are
declared using `:group'.

`:group' is specified for use in many constructs,
in many nodes of the Elisp manual.  Shouldn't its
behavior (and usage guidelines) be the same
everywhere?

FWIW, I don't see any special description of 
`:group' in the doc about `defface'.  I do see,
in other nodes, cross-references to nodes that
introduce/define customization groups - but I
don't see any such xref in the node for `defface'.