Obsolescence warnings (was: [Emacs-diffs] master 9227b5c: last-chance: new utility lib for dangling deterrence)

Obsolescence warnings (was: [Emacs-diffs] master 9227b5c: last-chance: new utility lib for dangling deterrence)

[Stefan Monnier]

> +;; [Insert "nobody reads ChangeLog files" lament, here.  --ttn]

I think the problem is older, since it was made obsolete a long
time ago.  IOW, the problem really is that there's no warning ever given
when we use an obsolete face.

In general, I think we're currently not proactive enough in warning
about uses of obsolete thingies: for functions and variables, we handle
that during byte-compilation, so those are mostly covered, but not so
for:
- faces (as evidenced here).
- user-variables or hooks (since the users most likely don't
  byte-compile their ~/.emacs file).
- more generally code that isn't byte-compiled (it's very common for
  users to use Elisp code without ever byte-compiling it).
- text-properties.
- I'm sure there are more cases.

I think it would be good to introduce a mechanism to tell the user
about uses of obsolete features.  And by "user" here I mean "end user",
not because I prefer warning the end-user than the developer, but
because in many of those cases they're one and the same, and because in
other cases that's the best we can do.

To the extent that it affects the end-user and is caught during
execution, this mechanism needs to be very careful not to get in the
way, although it should be visible enough that we can reasonably expect
that the user will likely see those messages.


        Stefan

Re: Obsolescence warnings

[Thien-Thi Nguyen]

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() Stefan Monnier <monnier@iro.umontreal.ca>
() Sat, 24 Dec 2016 00:02:03 -0500

   To the extent that it affects the end-user and is caught
   during execution, this mechanism needs to be very careful not
   to get in the way, although it should be visible enough that
   we can reasonably expect that the user will likely see those
   messages.

Here's an idea:

(defun message-with-properties (plist fmt &rest args)
  "Like ‘message’ for FMT and ARGS, and also propertize w/ PLIST.
Return the propertized, formatted string."
  (with-current-buffer (get-buffer "*Messages*")
    (let* ((beg (goto-char (point-max)))
           (str (apply #'message fmt args))
           (end (+ beg (length str) 1))
           (inhibit-read-only t))
      (add-text-properties beg end plist)
      (buffer-substring beg end))))

(message-with-properties
 '(face font-lock-warning-face)
 "obsolete: %S" 'obsolete-thing)

We can then tell people to look for easily visible "obsolete" in
*Messages*, and they can do that at their own convenience, or we
can programmatically collect such blurbs for a more concentrated
presentation, later.  In the meantime, this can be tweaked a bit
for other (ab)uses, such as ‘C-u C-h e’ doing filtering based on
various properties, etc.  WDYT?

-- 
Thien-Thi Nguyen -----------------------------------------------
 (defun responsep (query)
   (pcase (context query)
     (`(technical mailing-list) t)
     ...))                              748E A0E8 1CB8 A748 9BFA
--------------------------------------- 6CE4 6703 2224 4C80 7502


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Re: Obsolescence warnings

[Stefan Monnier]

> (message-with-properties
>  '(face font-lock-warning-face)
>  "obsolete: %S" 'obsolete-thing)

> We can then tell people to look for easily visible "obsolete" in
> *Messages*, and they can do that at their own convenience, or we
> can programmatically collect such blurbs for a more concentrated
> presentation, later.

I think this is the easy part.  The harder parts are:
- perform the obsolescence checks (we don't do that currently for faces,
  and we don't do it for (setq <obsolete-var> <val>) in the user's
  ~/.emacs either).
- perform those checks efficiently.
- display them in an effective and friendly way.

For the last part, for example, I use locally

       (advice-add obsolete-name :before
                   (lambda (&rest _)
                     (message "Warning: %s is obsolete!" obsolete-name))
                   '((name . obsolescence-warning)))

in `make-obsolete`.  It kinda works.  But for some obsolete functions, it
leads occasionally to a deluge of messages which can be not just
annoying but can also hide the useful info.  So it's not friendly.

In other cases, the obsolescence message is actually never shown because
it's immediately replaced with another message.  Another problem with it
is that it doesn't tell me where that obsoleted function was called, so
I then have to chase down the source: you can't expect an end user to do
that, so his bug-report will often be rather useless.
So it's not really effective either.

I guess we could live with something like that where we only activate it
in the master branch (i.e. version numbers of the form NN.50.OO), but
even then people will get annoyed and will want to turn it off.


        Stefan