or_else_searcher.rs

trait PrioritySearcher<'a, T> {
    fn test(&mut self, item: &'a T) -> bool;
    fn next_best(self) -> Option<&'a T>;
}

impl<'a, F, T> PrioritySearcher<'a, T> for F
    where F: FnMut(&'a T) -> bool
{
    fn test(&mut self, item: &'a T) -> bool {
        (self)(item)
    }
    
    fn next_best(self) -> Option<&'a T> {
        None
    }
}

struct OrElseSearcher<'a, T, A, B> {
    primary: A,
    secondary: B,
    secondary_best: Option<&'a T>,
}
impl<'a, T, A, B> PrioritySearcher<'a, T> for OrElseSearcher<'a, T, A, B>
where
    A: PrioritySearcher<'a, T>,
    B: PrioritySearcher<'a, T>,
{
    fn test(&mut self, item: &'a T) -> bool {
        if self.primary.test(item) {
            return true;
        }
        if self.secondary_best.is_none() && self.secondary.test(item) {
            self.secondary_best = Some(item);
        }
        false
    }
    fn next_best(self) -> Option<&'a T> {
        self.primary
            .next_best()
            .or(self.secondary_best)
            .or_else(move || self.secondary.next_best())
    }
}