iter.rs

pub struct MatchIter<'s, S, I> {
    world: &'s World<S>,
    iter: I,
}

impl<'s, S, I> Iterator for MatchIter<'s, S, I>
where
    I: Iterator,
{
    type Item = I::Item;
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }
    #[inline]
    fn next(&mut self) -> Option<Self::Item> {
        self.iter.next()
    }
}
...
    pub fn matcher<'s, Q>(
        &'s self,
    ) -> impl Iterator<...> + 's {
        let iter = unsafe {
            self.storages
                .iter()
                .filter(|&storage| Q::is_match(storage))
                .map(|storage| Q::query(storage))
                .flat_map(|iter| iter)
        };
        iter // <= fast
        //MatchIter { world: self, iter } // <= 3x slower
    }