Unique character check in Rust

unique_chars.rs

use std::os;

fn unique_chars(s: &str) -> bool {
    let v: Vec<char> = s.chars().collect();
    let mut y = v.clone();

    y.dedup();
    v.len() == y.len()
}

fn main() -> () {
    let args = os::args();
    let sentence = args.get(1);

    match unique_chars(sentence.as_slice()) {
        true => println!("String contains only unique characters"),
        false => println!("String doesn't contain only unique characters")
    }
}

This will work only for sorted input.
According to official documentation dedup() deletes duplicates only consecutive repeated elements.

let mut vec = vec![1, 2, 2, 3, 2];

vec.dedup();

assert_eq!(vec, [1, 2, 3, 2]);