RE: Fontification of the interactive prompt string?

[Drew Adams <drew.adams@oracle.com>]

> I have an (interactive "cprompt\n") call, and I'd like the prompt to
> list possible characters (they define a few options for my function,
> and I have good reasons to implement it this way and not with C-u etc.)
> I'd like to have the relevant characters highlighted, like in:
> "file: _n_ew, _o_pen, _c_lose" where the underscores mean that
> something is e.g. in a different color.
> 
> Is it possible?

Yes.

The value of variable `minibuffer-prompt-properties' is applied to
the prompt.  And by default the value is this, which applies a face:
`(read-only t face minibuffer-prompt)'.

So you will want to bind that variable to a value that does not
have any entry for property `face' - e.g., to just `(read-only t)'.

Then you can use whatever propertized prompt string you like.

Here is something quick-&-dirty, to give you an idea what I mean:

;; Substitute for `completing-read'.  It just binds
;; `minibuffer-prompt-properties', to stop it from imposing its
;; single default face.
;;
(defun my-compl-read (prompt collection &optional predicate
                             require-match initial-input hist def
                             inherit-input-method)
  (let ((minibuffer-prompt-properties  '(read-only t)))
    (completing-read prompt collection predicate require-match
                     initial-input hist def inherit-input-method)))

(defun my-prompt (string.face-alist)
  (apply #'concat (mapcar (lambda (s.f)
                            (if (cdr s.f)
                                (propertize (car s.f) 'face (cdr s.f))
                              (car s.f)))
                          string.face-alist)))

(my-compl-read (my-prompt '(("Plain and ")
                            ("highlighted " . font-lock-comment-face)
                            ("text ")
                            ("IS " . font-lock-keyword-face)
                            ("possible: ")))
                          '(a b c d e))

Obviously, you can construct the prompt string in other ways.

And instead of defining a function like `my-compl-read' you could
use variable `completing-read-function' or advise `completing-read'.