Require a main function in all exercises

exercises/18_iterators/iterators2.rs

@@ -33,6 +33,10 @@ pub fn capitalize_words_string(words: &[&str]) -> String {
     String::new()
 }
 
+fn main() {
+    // You can optionally experiment here.
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

exercises/18_iterators/iterators3.rs

@@ -43,6 +43,10 @@ fn list_of_results() -> () {
     let division_results = numbers.into_iter().map(|n| divide(n, 27));
 }
 
+fn main() {
+    // You can optionally experiment here.
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

exercises/18_iterators/iterators4.rs

@@ -15,6 +15,10 @@ pub fn factorial(num: u64) -> u64 {
     // Execute `rustlings hint iterators4` for hints.
 }
 
+fn main() {
+    // You can optionally experiment here.
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

exercises/18_iterators/iterators5.rs

@@ -55,6 +55,10 @@ fn count_collection_iterator(collection: &[HashMap<String, Progress>], value: Pr
     todo!();
 }
 
+fn main() {
+    // You can optionally experiment here.
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;