Rust: What is the difference between size_hint and ExactSizeIterator?

size_hint — bounds estimate. ExactSizeIterator — exact length.

size_hint:

• Returns (min, max).
• Approximate.
• Optional.

ExactSizeIterator:

• Returns exact length.
• Requires Iterator.
• For known sizes.

1let v = vec![1, 2, 3];
2let iter = v.iter();
3
4// size_hint: (3, Some(3))
5println!("{:?}", iter.size_hint());
6
7// ExactSizeIterator
8fn count_exact<I: ExactSizeIterator>(iter: I) -> usize {
9    iter.len()
10}

Key: size_hint for bounds, ExactSizeIterator for exact count.