let xs = [1, 2, 3];

let mut iter = xs.iter().peekable();

// peek() lets us see into the future
assert_eq!(iter.peek(), Some(&&1));
assert_eq!(iter.next(), Some(&1));

assert_eq!(iter.next(), Some(&2));

// The iterator does not advance even if we `peek` multiple times
assert_eq!(iter.peek(), Some(&&3));
assert_eq!(iter.peek(), Some(&&3));

assert_eq!(iter.next(), Some(&3));

// After the iterator is finished, so is `peek()`
assert_eq!(iter.peek(), None);
assert_eq!(iter.next(), None);

#![feature(peekable_peek_mut)]
let mut iter = [1, 2, 3].iter().peekable();

// Like with `peek()`, we can see into the future without advancing the iterator.
assert_eq!(iter.peek_mut(), Some(&mut &1));
assert_eq!(iter.peek_mut(), Some(&mut &1));
assert_eq!(iter.next(), Some(&1));

// Peek into the iterator and set the value behind the mutable reference.
if let Some(p) = iter.peek_mut() {
    assert_eq!(*p, &2);
    *p = &5;
}

// The value we put in reappears as the iterator continues.
assert_eq!(iter.collect::<Vec<_>>(), vec![&5, &3]);

let mut iter = (0..5).peekable();
// The first item of the iterator is 0; consume it.
assert_eq!(iter.next_if(|&x| x == 0), Some(0));
// The next item returned is now 1, so `consume` will return `false`.
assert_eq!(iter.next_if(|&x| x == 0), None);
// `next_if` saves the value of the next item if it was not equal to `expected`.
assert_eq!(iter.next(), Some(1));

let mut iter = (1..20).peekable();
// Consume all numbers less than 10
while iter.next_if(|&x| x < 10).is_some() {}
// The next value returned will be 10
assert_eq!(iter.next(), Some(10));

let mut iter = (0..5).peekable();
// The first item of the iterator is 0; consume it.
assert_eq!(iter.next_if_eq(&0), Some(0));
// The next item returned is now 1, so `consume` will return `false`.
assert_eq!(iter.next_if_eq(&0), None);
// `next_if_eq` saves the value of the next item if it was not equal to `expected`.
assert_eq!(iter.next(), Some(1));