[rust] Allow for running previous days

rust/src/main.rs

@@ -1,3 +1,4 @@
+use std::env;
 use std::io::prelude::*;
 use std::io;
 
@@ -5,7 +6,22 @@ extern crate advent_of_code;
 use advent_of_code::*;
 
 fn main() {
-    let mut input = String::new();
-    io::stdin().read_to_string(&mut input).ok();
-    println!("{}", day_10::solve(&input));
+  let mut input = String::new();
+  io::stdin().read_to_string(&mut input).ok();
+
+  let solution: String =
+    match env::args().nth(1).as_ref().map(String::as_ref) {
+      Some("1") => day_01::solve(&input).to_string(),
+      Some("2") => day_02::solve(&input).to_string(),
+      Some("3") => day_03::solve(&input).to_string(),
+      Some("4") => day_04::solve(&input).to_string(),
+      Some("5") => day_05::solve(&input).to_string(),
+      Some("6") => day_06::solve(&input).to_string(),
+      Some("7") => day_07::solve(&input, "a").to_string(),
+      Some("8") => day_08::solve(&input).to_string(),
+      Some("9") => day_09::solve(&input).to_string(),
+      _ => day_10::solve(&input),
+    };
+
+  println!("{}", solution);
 }