Drupal views: text search in country name
You might find this module even better than you expect:

Views Selective Exposed Filters (views_filters_selective)

"Restrict exposed filter values to those present in the result set."

Whenever you're looking at a selection (or subselection), it'll filter the select options to only those options that are in the result set. So you never have the issue of pointlessly offering filter options that will give the visitor an empty result.

https://www.drupal.org/project/views_selective_filters

[thanks Aidan]