use rand::Rng;
const LETTERS: &[u8] = b"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
pub fn rand_str(n: usize) -> String {
let mut rng = rand::thread_rng();
let mut b = vec![0u8; n];
for i in 0..n {
b[i] = LETTERS[rng.gen_range(0..LETTERS.len())];
}
String::from_utf8(b).unwrap()
}