Use a vector to store known primes.

src/main.rs

@@ -1,34 +1,39 @@
 use std::env;
+use std::collections::VecDeque;
 
 fn main() {
     let args:Vec<String> = env::args().collect();
 
     // get the first `n` primes
-    let n:u64 = args[1].parse().unwrap();
+    let n:usize = args[1].parse().unwrap();
+    let mut primes:VecDeque<u64> = VecDeque::with_capacity(n);
     // first prime
     println!("{}", 2);
-    // already have first prime
-    let mut prime_count:u64 = 1;
+    primes.push_back(2);
+
     let mut candidate:u64 = 3;
 
-    while prime_count < n
+    while primes.len() < n
     {
-        let mut aux:u64 = 3;
+        let mut i = 0;
+        let mut test_prime = primes.get(i).unwrap();
         let mut is_prime = true;
-        while aux < (candidate as f64).sqrt() as u64
+        while *test_prime < (candidate as f64).sqrt() as u64
         {
-            if candidate % aux == 0
+            if candidate % test_prime == 0
             {
                 is_prime = false;
                 break;
             }
-            aux += 2;
+
+            i += 1;
+            test_prime = primes.get(i).unwrap();
         }
 
         if is_prime
         {
             println!("{}", candidate);
-            prime_count += 1;
+            primes.push_back(candidate);
         }
         candidate += 2;
     }