fib.rs

extern crate num;

use num::{BigUint, Zero, One};
use std::mem::replace;

struct Fib {
	f1: BigUint,
	f2: BigUint
}

impl Iterator for Fib {
	type Item = BigUint;
	fn next(&mut self) -> Option<BigUint> {
		let f0 = replace(&mut self.f1, self.f2.clone());
		self.f2 = f0 + &self.f1;
		return Some(self.f2.clone());
	}
}

fn main() {
	let f = Fib {
		f1: BigUint::zero(),
		f2: BigUint::one()
	};
	for n in f {
		println!("{}", n);
	}
}

Without the first self.f2.clone():

src/bin/fib.rs:14:34: 14:38 error: cannot move out of borrowed content [E0507]
src/bin/fib.rs:14       let f0 = replace(&mut self.f1, self.f2);
                                                       ^~~~