Rust program to compute the trigram Jaccard similarity between two strings

stringsim.rs

//! The stringsim program prints out the trigram similarity of two strings
//! using what appears to be the same algorithm used by Postgres.
//! https://www.postgresql.org/docs/9.1/pgtrgm.html

use std::collections::HashSet;
use std::hash::Hash;

fn main() {
    let args: Vec<String> = ::std::env::args().collect();
    if args.len() != 1+2 {
        eprintln!("usage: stringsim s1 s2");
        ::std::process::exit(1);
    }
    let s1 = &args[1];
    let s2 = &args[2];
    let sim = similarity(&s1, &s2);
    println!("{}", sim);
}

fn similarity(a: &String, b: &String) -> f32 {
    let ta = trigrams(a);
    let tb = trigrams(b);
    return jaccard(ta, tb);
}

fn trigrams(s: &String) -> HashSet<String> {
    let mut ts = HashSet::new();
    let s = format!("{} ", s);
    let mut p1 = ' ';
    let mut p2 = ' ';
    for c in s.chars() {
        let v = vec![p1, p2, c];
        let t: String = v.into_iter().collect();
        ts.insert(t);
        p1 = p2;
        p2 = c;
    }
    ts
}

fn jaccard<T>(s1: HashSet<T>, s2: HashSet<T>) -> f32 where T: Hash+Eq {
    let i = s1.intersection(&s2).count() as f32;
    let u = s1.union(&s2).count() as f32;
    return i / u;
}