Editing Iterator (section)

===Rust===
Rust makes use of external iterators throughout the standard library, including in its <code>for</code> loop, which implicitly calls the <code>next()</code> method of an iterator until it is consumed. The most basic <code>for</code> loop for example iterates over a <code>Range</code> type:

<syntaxhighlight lang="rust">
for i in 0..42 {
    println!("{}", i);
}
// Prints the numbers 0 to 41
</syntaxhighlight>

Specifically, the <code>for</code> loop will call a value's <code>into_iter()</code> method, which returns an iterator that in turn yields the elements to the loop. The <code>for</code> loop (or indeed, any method that consumes the iterator), proceeds until the <code>next()</code> method returns a <code>None</code> value (iterations yielding elements return a <code>Some(T)</code> value, where <code>T</code> is the element type).

All collections provided by the standard library implement the <code>IntoIterator</code> trait (meaning they define the <code>into_iter()</code> method). Iterators themselves implement the <code>Iterator</code> trait, which requires defining the <code>next()</code> method. Furthermore, any type implementing <code>Iterator</code> is automatically provided an implementation for <code>IntoIterator</code> that returns itself.

Iterators support various adapters (<code>map()</code>, <code>filter()</code>, <code>skip()</code>, <code>take()</code>, etc.) as methods provided automatically by the <code>Iterator</code> trait.

Users can create custom iterators by creating a type implementing the <code>Iterator</code> trait. Custom collections can implement the <code>IntoIterator</code> trait and return an associated iterator type for their elements, enabling their use directly in <code>for</code> loops. Below, the <code>Fibonacci</code> type implements a custom, unbounded iterator:
<syntaxhighlight lang="rust">
struct Fibonacci(u64, u64);

impl Fibonacci {
    pub fn new() -> Self {
        Self(0, 1)
    }
}

impl Iterator for Fibonacci {
    type Item = u64;

    fn next(&mut self) -> Option<Self::Item> {
        let next = self.0;
        self.0 = self.1;
        self.1 = self.0 + next;

        Some(next)
    }
}

let fib = Fibonacci::new();
for n in fib.skip(1).step_by(2).take(4) {
    println!("{n}");
}
// Prints 1, 2, 5, and 13
</syntaxhighlight>

[[File:UML class diagram Rust iterators.svg|UML class diagram depecting the iterator-related structs and traits in Rust]]