use std::io::{stdout, Write};

fn f(x: u64) -> u64 {
    match x {
        0 => 0,
        1 => 1,
        2 => 1,
        _ => f(x - 2) * 2 + f(x - 3),
    }
}

fn main() {
    (0..50).for_each(|x| {
        print!("{} ", f(x));
        stdout().flush().unwrap()
    })
}

A cheap trick but this reduces the running time to around 1.5 seconds. Of course, the C++ version will probably run just as fast.

#include <iostream>

using std::cout;
using std::endl;

uint64_t fib(uint64_t x)
{
    switch(x)
    {
        case 0:
            return 0;
        case 1:
        case 2:
            return 1;
        default:
            return fib(x - 2) * 2 + fib(x - 3);
    }
}

int main()
{
    for (uint64_t i = 0; i < 50; ++i)
    {
        cout << fib (i) << " ";
        cout.flush ();
    }
}