bug#34873: 27.0.50; Creating unibyte string in emacs modules

[Eli Zaretskii]

> From: lg.zevlg@gmail.com
> Date: Sat, 16 Mar 2019 16:28:05 +0100
> Cc: 34873@debbugs.gnu.org
> 
> wand-mode does many trickery on images in very fast fasion, and it needs 
> method to display previews/results in Emacs buffer. Result of image 
> manipulation is raw rgb(a) data that seats in user-ptr available from 
> imagemagick without any copying. I want `create-image’ with :format 
> ‘image/x-rgb to accept this user-ptr as data to make image creation fast
> 
> Method proposed by Stefan with make_string/encode-coding-string works, but 
> its speed is far from being usable on big images.  There is no method just to 
> use data as-is to make things fast

I'd prefer to provide a way to use user-ptr objects for this purpose,
rather than allow unibyte strings to be created by modules.  The
latter is a slippery slope which I think we should rather avoid.

Can you propose a patch that uses user-ptr instead?

Thanks.