fn func<T: iter::Iterator<Item = char>>(i: T) {
  //...
}

fn option_iter(input: Option<char>) {
    let i: Box<iter::Iterator<Item = char>> =
    if let Some(input) = input {
        Box::new(iter::once(input))
    } else {
        Box::new(iter::empty())
    };
    func(i);
}

let i = if let Some(input) = input {
    iter::once(input)
} else {
    iter::empty()
};
func(i);