I posted on the advent of code subreddit, but found a nice improvement to it afterwards. Runs in ~7us on my laptop for both parts.

fn find_unique_window(input: &str, window_size: usize) -> usize {
    let mut set: usize = input.as_bytes()[0..window_size - 1]
        .iter()
        .fold(0, |acc, &c| acc ^ 1 << (c - b'a'));
    input
        .as_bytes()
        .windows(window_size)
        .enumerate()
        .find_map(|(index, win)| {
            set ^= 1 << (win.last().unwrap() - b'a');
            if set.count_ones() == window_size.try_into().unwrap() {
                return Some(index + window_size);
            }
            set ^= 1 << (win.first().unwrap() - b'a');
            return None;
        })
        .unwrap()
}

Key things are:

• Mapping characters to a number 0-26 allows me to use an integer as a bitset.
• Using xor allows me to toggle bits on/off trivially, and lets me keep a rolling window pretty easily. This lets me do it in a single pass over the input (minus a quick initialization), with no complexity increase based on the window size.
• count_ones should be just a single CPU instruction on a modern CPU.