Rust: What is the difference between Peekable and FusedIterator?

Peekable — lookahead. FusedIterator — guaranteed termination.

Peekable:

• Peek at next item.
• Don't consume.
• For lookahead.

FusedIterator:

• Iterator fused.
• Once None, always None.
• Safe to call next.

1let v = vec![1, 2, 3];
2let mut iter = v.iter().peekable();
3
4if let Some(&&first) = iter.peek() {
5    println!("First: {}", first);
6}
7
8// FusedIterator
9let mut fused = v.iter().fuse();
10while let Some(item) = fused.next() {
11    println!("{}", item);
12}

Key: Peekable for lookahead, FusedIterator for safe iteration.