Deprecate string_concat

[Mattias Loverot]

Continuing from #26866 I'd like to deprecate django.utils.translation.string_concat in favor of django.utils.text.format_lazy. string_concat can easily be replaced by format_lazy in Django source code.

The only use case I can think of where string_concat is still needed is when concatenating an arbitrary amount of list elements. My guess is that this use case rarely exists - and those that do exist might be able to use django.utils.text.get_text_list ?

(An alternative approach would be to move django.utils.translation.string_concat to django.utils.text.concat_lazy - since django.utils.text is more appropriate)

[Mattias Loverot]

Just realized string_concat could be directly replaced by:

format_lazy('{}'*len(params), *params)

Maybe not the prettiest but should work for those rare cases.

[Claude Paroz]

I'm fully for deprecating string_concat if we have a nice replacement. I guess the first step to demonstrate the feasibility is to do the replacement for Django usage itself.

[Mattias Loverot]

​https://github.com/lovmat/django/tree/ticket_27067
Todo: Documentation
Depends on: #26866 / PR 7087

[Claude Paroz]

I think I'm totally convinced now :-)

[Claude Paroz]

​https://docs.djangoproject.com/en/1.10/topics/i18n/translation/#joining-strings-string-concat will also need an update.

[Tim Graham <timograham@…>]

In 23151140:

Fixed #27067 -- Deprecated string_concat() in favor of format_lazy().

[Tim Graham <timograham@…>]

In 87d2240e:

Refs #27067 -- Removed django.utils.translation.string_concat() per deprecation timeline.

[Tim Graham <timograham@…>]

In 3fa0a82:

Refs #27067 -- Removed string_concat in django.utils.translation.all.

Undefined since 87d2240e6cc594a3bf28dfdb2ec023c54fb76ff7.