Cargo.toml

[package]
name = "nth-prime"
version = "0.1.0"
edition = "2018"

[dependencies]
futures-preview = "0.3.0-alpha.16"

nth_prime.rs

#![feature(async_await, await_macro)]

extern crate futures;

use core::future::Future;
use core::pin::Pin;
use core::task::{Context, Poll};

fn is_prime(x: u64) -> bool {
    for i in 2..=((x as f64).sqrt().floor() as u64) {
        if x % i == 0 {
            return false;
        }
    }
    true
}

struct NthPrime {
    count: usize,
    value: u64,
}

impl NthPrime {
    pub fn new(count: usize) -> Self {
        NthPrime { count, value: 2 }
    }
}

impl Future for NthPrime {
    type Output = u64;
    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        let mut data = self.get_mut();
        if data.count == 0 {
            Poll::Ready(data.value)
        } else {
            if is_prime(data.value + 1) {
                data.count -= 1;
            }
            data.value += 1;
            cx.waker().clone().wake();
            Poll::Pending
        }
    }
}

async fn nth_prime(count: usize) -> u64 {
    await!(NthPrime::new(count))
}

fn main() {
    let nth = 100000;
    let future = nth_prime(nth);
    let prime = futures::executor::block_on(future);
    println!("nth_prime({}) = {}", nth, prime);
}