Split up setting up Rocket into a separate function

src/main.rs

@@ -3,6 +3,8 @@
 
 extern crate rocket;
 
+use rocket::Rocket;
+
 mod static_files;
 
 #[get("/")]
@@ -10,8 +12,11 @@ fn index() -> &'static str {
     "Hello, world!"
 }
 
-fn main() {
+fn rocket() -> Rocket {
     rocket::ignite()
         .mount("/", routes![index, static_files::all])
-        .launch();
+}
+
+fn main() {
+    rocket().launch();
 }