Remove all whitespace from a string

If you want to modify the String, use retain. This is likely the fastest way when available.

fn remove_whitespace(s: &mut String) {
    s.retain(|c| !c.is_whitespace());
}

If you cannot modify it because you still need it or only have a &str, then you can use filter and create a new String. This will, of course, have to allocate to make the String.

fn remove_whitespace(s: &str) -> String {
    s.chars().filter(|c| !c.is_whitespace()).collect()
}