RE: Any objection to adding completing-read-function?

[Drew Adams]

> > Then that should have been the approach for 
> > `read-file-name-function' as well.
> 
> Yes.  Feel free to go back to travel back in time and fix it there.
> In the mean time we can do the right thing for new `foo-function's.

How about fixing `read-file-name-function' the same way now, so the two will be
symmetric?  In the vanilla Emacs sources only Ido uses `read-file-name-function'
at present.  3rd parties can adjust accordingly.