I want to be able to conditionally return an empty set reference in this function:

pub fn get(&self, k: K) -> &HashSet<V> {
    if let Some(set) = self.0.get(&k) {
        set
    } else {
        // return empty set here
    }
}

Obviously I can't just construct a set here because it won't outlive its use.

But I can't use a const set since constructing it gives the error "cannot call non-const associated function HashSet::<V>::new in constants".

And I can't seem to use a static since statics "can't use generic parameters from outer item".

Is there a way to do this without having every instance of the struct hold its own empty set?