Re: Difficulties with `purecopy' discarding text properties from strings

[Daniel Colascione <dancol@dancol.org>]

On 05/19/2017 11:55 AM, Alan Mackenzie wrote:
> Hello, Emacs.
>
> The context is my implementing of new mode-line %-constructs.  I want a
> customisable user option whose values will be things like ("%o"), with
> text properties on the string (for things like help strings and mouse
> action key bindings).
>
> I naively constructed such a defcustom, with text properties on the
> value option strings.  After some debugging, I discover that defcustom
> strips those text properties off the strings.  It does this by using
> `purecopy' on these strings (in the function `custom-declare-variable').
>
> (Yes, the variable I'm trying to declare is in bindings.el along with
> all the other mode-line stuff, and this is preloaded stuff.)
>
> So, I'm a little bit stuck.
>
> Presumably there are good reasons for discarding text properties when
> purecopying strings.  Or, maybe, there was just no particular reason to
> amend the workings of `purecopy' when text properties came into
> existence.
>
> How does this sound for a solution to the problem?  In `defcustom',
> introduce a new keyword `impure:' which, when placed early enough in the
> defcustom would inhibit the `purecopy' action.
>
> Or should I work at amending `purecopy' also to copy text properties?

When we move to the new dumper (whatever it is), I don't think we'll 
have a conventional purespace at all anymore. In my system, text 
properties are just automatically preserved along with everything else 
and don't get special treatment.