Rust simple rate limiter

foo.rs

use std::time::{Duration, Instant};

struct Limiter {
    instant: Option<Instant>,
    duration: Duration,
}

impl Limiter {
    pub fn new(secs: u64) -> Limiter {
        Limiter {
            instant: None,
            duration: Duration::from_secs(secs),
        }
    }

    pub fn call<F, R>(&mut self, cb: F) -> Option<R>
    where
        F: Fn() -> R,
    {
        if self.instant.is_none() || self.instant.unwrap().elapsed() > self.duration {
            self.instant = Some(Instant::now());
            Some(cb())
        } else {
            None
        }
    }
}

fn main() {
    let mut limiter = Limiter::new(3);
    for i in { 0..10 } {
        std::thread::sleep(Duration::from_secs(1));
        println!("i {}", i);
        limiter.call(&|| 1);
        limiter.call(&|| println!("Hello"));
    }
}