Only require a mutable borrow by using option's take().

src/parser/list.rs

@@ -35,8 +35,8 @@ impl<T: Debug> List<T> {
         }
     }
 
-    pub fn pop_front(self) -> (Option<T>, List<T>) {
-        match self.head {
+    pub fn pop_front(&mut self) -> (Option<T>, List<T>) {
+        match self.head.take() {
             None => (None, List::new()),
             Some(popped_node) => {
                 let extracted_node =