Function sample_single_from_known_length 

pub fn sample_single_from_known_length<I, R, T>(
    rng: &mut R,
    iter: I,
) -> Option<T>
where
    R: Rng,
    I: Iterator<Item = T>,

Samples one element uniformly at random from an iterator whose length is known at runtime.

The caller must ensure that (len, Some(len)) == iter.size_hint(), i.e. the iterator reports its exact length via size_hint. We do not require ExactSizeIterator because that is a compile-time guarantee, whereas our requirement is a runtime condition.

The implementation selects a random index and uses Iterator::nth. For iterators with O(1) nth (e.g., randomly indexable structures), this is very efficient. The selected value is cloned.

The iterator need only support iteration; random indexing is not required. This function is intended for use when the result set is indexed and its length is known.