Add support for deprecated property

[yyoncho]

No description provided.

[dgutov]

Thanks!

[dgutov]

@yyoncho Looking at the gif you just posted: how about we apply the strike-through effect (and the deprecation face, I guess) only to the textual part of the completion item? Meaning, not the margins or the whitespace.

Like on VS Code's screenshot here:

https://code.visualstudio.com/assets/updates/1_38/css-deprecated-properties.png

[yyoncho]

I don't have a strong opinion on that.

[dgutov]

OK, I've done the change. Looks a bit more subtle.

Bonus news: elisp-completion-at-point now supports deprecated on master (Emacs 29).

[yyoncho]

Bonus news: elisp-completion-at-point now supports deprecated on master (Emacs 29).

Maybe it is time to put it in elpa

[dgutov]

Might be a pain to rewrite it in a way that works for Emacs 26.

If you really want this feature in Emacs 28, though, I can try to initiate a backport.

[yyoncho]

Is having elpa package that supports only emacs 27+(or even only emacs 28) an option? It is not big deal for me, but IMO it is very unfortunate that a handy feature like that will be available to the users after 2(?)+ years.

[dgutov]

Any kind of long-term support is additional maintenance burden. I think over time we will add more and more packages to "ELPA core", but I wish we get some better CI infrastructure by then, including running the tests of said packages in the oldest supported version.

Just over the recent months, I've managed to break project.el's and xref's Emacs 26 compatibility, and people are depending on it now. So that wasn't nice.

[dgutov]

A backport has much fewer risks, so if you try the new change in Emacs 29 for a couple of days and like it, doing that seems optimal.

The current implementation will say a variable is deprecated even if only a function with the same name is (or a face, etc). So either we verify it's no big deal, or it needs to be improved first.