"Extension Methods" in Rust

ext.rs

use std::collections::HashSet;
use std::hash::{Hash, Hasher};
use std::collections::hash_state::HashState;

fn main() {
    if let Some(content) = {
        let args = std::os::args();
        if args.len() == 2 {
            Some(args[1].to_string())
        } else {
            None
        }
    } {
        let mut set = HashSet::new();
        let mut dup = Vec::new();

        for c in content.chars().filter(|&c| c != ' ') {
            if !set.add_unique(c) {
                dup.add_unique(c.to_string());
            }
        }

        println!("{}", dup.concat());
    }
}

trait UniqueCollection<T> {
    fn add_unique(&mut self, item: T) -> bool;
}

impl<T, S, H> UniqueCollection<T> for HashSet<T, S>
    where T: Eq + Hash<H>,
    S: HashState<Hasher=H>,
    H: Hasher<Output=u64>
{
    fn add_unique(&mut self, item: T) -> bool {
        if !self.contains(&item) {
            self.insert(item);
            true
        } else {
            false
        }
    }
}

impl<T> UniqueCollection<T> for Vec<T>
    where T: Eq
{
    fn add_unique(&mut self, item: T) -> bool {
        if !self.iter().any(|i| *i == item) {
            self.push(item);
            true
        } else {
            false
        }
    }
}