s = "the quick (brown (fox) jumps) over"
(f, t) = (18, 21)
ss = s[:f] + s[t-1:f-1:-1] + s[t:]

let s = String::from("the quick (brown (fox) jumps) over");
let sc: Vec<_> = s.chars().collect();
let (f, t) = (18, 21); // result of searching through sc
let mut new_s: Vec<_> = sc[..f].into_iter().collect();
new_s.extend(&sc[f..t].into_iter().rev().collect::<Vec<_>>());
new_s.extend(&sc[t..].into_iter().collect::<Vec<_>>());
let mut ss = String::with_capacity(new_s.len());
for c in new_s.iter() {ss.push(**c);}