How do you implement custom iterators in Rust?

Custom iterators in Rust: • Implement the Iterator trait • Define associated Item type • Implement next() method • Lazy evaluation by default • Chainable with adapter methods • Zero-cost abstractions Iterator trait: trait Iterator { type Item; fn next(&mut self) -> Option; } Example - Counter iterator: struct Counter { current: usize, max: usize, } impl Counter { fn new(max: usize) -> Counter { Counter { current: 0, max } } } impl Iterator for Counter { type Item = usize; fn next(&mut self) -> Option { if self.current < self.max { let current = self.current; self.current += 1; Some(current) } else { None } } } // Usage let counter = Counter::new(5); for num in counter { println!("{}", num); // 0, 1, 2, 3, 4 } // With iterator adapters let sum: usize = Counter::new(5) .map(|x| x * x) .filter(|&x| x % 2 == 0) .sum(); println!("Sum of even squares: {}", sum);