Collatz Conjeture in Rust

As a test of Rust language I've done a simple Rust checker from 1 to 10.000.000 it takes 18 seconds in my MacBook.

Sample output (after 18 seconds):
10000000 numbers checked in 18 seconds (555555 numbers per second)

use std::process::exit;
use std::time::Instant;

fn main() {
    let mut i=1;
    let start = Instant::now();

    while i<=10000000
    {
        let res=collatz(i);
        if res
        {
            i=i+1;
        }
        else
        {
            println!("{} NO!!!", i);
            exit(1);
        }
    }
    let elapsed=start.elapsed().as_secs();
    println!("{} numbers checked in {} seconds ({} numbers per second)", i-1, elapsed, (i-1)/elapsed);
}

fn collatz(i: u64) -> bool
{
    let mut res;
    res=false;

    if i==1
    {
        res=true
    }
    else
    {
        if i%2==0
        {// par
            res=collatz(i/2);
        }
        else
        {// impar
            res=collatz((i*3)+1);
        }
    }
    res
}