Iterate Over Iterator or Rev

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2

Since they're both iterators yielding the same item type, you can ask the compiler to use dynamic dispatch:

fn parse_identifier(
    mut test_string: impl Iterator<Item = char> + DoubleEndedIterator,
    reverse: bool,
) -> Option<String> {
    let mut rev_iter;
    let test_string: &mut dyn Iterator<Item = char> = if reverse {
        rev_iter = test_string.rev();
        &mut rev_iter
    } else {
        &mut test_string
    };
    for c in test_string {
        // Do some parsing logic also slightly impacted by whether reverse is set
    }
    Some(String::new()) // New String built in the for loop
}

(Playground)

Edit: Removed unnecessary Option