How to add a shortcode function that returns the taxonomy slug of the actual post within the loop

Circumstances: I have a CPT called “designs” and a taxonomy related to it, called “project_category” so that I can assign a category to each design. I’m building the template for each ‘design’ and I need a shortcode that returns the assigned project category SLUG of the current design within the loop.

It’s important that I retrieve the slug instead of the ID because I’ll be using the shortcode to wrap the whole template and assign it an #ID name so I can then target it with a link.

Example: I have a design post called “New logo for Pepsi” which has “Visual Identity” as it’s project category. So I’d need the shortcode to return “visual_identity” instead of “Visual Identity”.

Facts to consider: each design will have only one category assigned, so I don’t need the shortcode to return an array, only a single slug.

I guess it should be something like this:

add_shortcode( 'return_taxonomy_slug', 'my_shortcode_return_taxonomy_slug' );

function my_shortcode_return_taxonomy_slug() {
    return get_the_terms( $designs, $project_category->slug );
}

…or maybe something like this to work-around:

add_shortcode( 'return_taxonomy_slug', 'my_shortcode_return_taxonomy_slug' );

$terms = wp_get_post_terms( get_the_ID(), 'project_category');

function my_shortcode_return_taxonomy_slug() {
    return $terms->slug;
}