Function rin::ecs::streaming_iterator::once_with

pub fn once_with<T, F>(gen: F) -> OnceWith<T, F> where
    F: FnOnce() -> T, 

Creates an iterator that returns exactly one item from a function call.

#[derive(Debug, PartialEq)]
struct Expensive(i32);
let mut streaming_iter = streaming_iterator::once_with(|| Expensive(1));
assert_eq!(streaming_iter.next(), Some(&Expensive(1)));
assert_eq!(streaming_iter.next(), None);