feat: panic trying to mutate stack initial state

src/parsers/stack.rs

@@ -78,12 +78,12 @@ impl Stack {
     }
 
     pub fn pop(&mut self) {
-        // Panic if the top element is the initial state.
+        self.guard_immutable_initial_state();
         self.items.pop();
     }
 
     pub fn swap_top(&mut self, new_item: State) {
-        // Panic if the top element is the initial state.
+        self.guard_immutable_initial_state();
         self.items.pop();
         self.push(new_item);
     }
@@ -93,14 +93,24 @@ impl Stack {
     /// # Panics
     ///
     /// Will panic is the stack is empty. The stack is never empty because it's
-    /// not allowed to pop the initial state.
+    /// not allowed to pop or change the initial state.
     #[must_use]
     pub fn peek(&self) -> State {
         match self.items.last() {
             Some(top) => top.clone(),
             None => panic!("empty stack!"),
         }
     }
+
+    fn guard_immutable_initial_state(&self) {
+        if let Some(top) = self.items.last() {
+            if *top != State::Initial {
+                return;
+            }
+        };
+
+        panic!("trying to mutate immutable initial state. It can't be popped or swapped!")
+    }
 }
 
 #[cfg(test)]
@@ -114,6 +124,18 @@ mod tests {
             assert_eq!(Stack::default().peek(), State::Initial);
         }
 
+        #[test]
+        #[should_panic(expected = "trying to mutate")]
+        fn not_allow_to_pop_the_initial_state() {
+            Stack::default().pop();
+        }
+
+        #[test]
+        #[should_panic(expected = "trying to mutate")]
+        fn not_allow_to_swap_the_initial_state() {
+            Stack::default().swap_top(State::Initial);
+        }
+
         // todo: the rest of operations on the stack.
     }
 }