Introduce a macro for empty iterators.

src/iter/macros.rs

@@ -1,5 +1,5 @@
 /// Create iterators for ast nodes where it only has to iterate over children
-macro_rules! simple_iter {
+macro_rules! children_iter {
     ($astnodetype:ty, $itertype:ident, $innertype:ty) => {
         pub struct $itertype<'r, 's> {
             next: $innertype,
@@ -27,4 +27,35 @@ macro_rules! simple_iter {
     };
 }
 
-pub(crate) use simple_iter;
+pub(crate) use children_iter;
+
+/// Create iterators for ast nodes that do not contain any ast node children.
+macro_rules! empty_iter {
+    ($astnodetype:ty, $itertype:ident) => {
+        pub struct $itertype<'r, 's> {
+            phantom: PhantomData<&'r $astnodetype>,
+        }
+
+        impl<'r, 's> Iterator for $itertype<'r, 's> {
+            type Item = AstNode<'r, 's>;
+
+            fn next(&mut self) -> Option<Self::Item> {
+                None
+            }
+        }
+
+        impl<'r, 's> IntoIterator for &'r $astnodetype {
+            type Item = AstNode<'r, 's>;
+
+            type IntoIter = $itertype<'r, 's>;
+
+            fn into_iter(self) -> Self::IntoIter {
+                $itertype {
+                    phantom: PhantomData,
+                }
+            }
+        }
+    };
+}
+
+pub(crate) use empty_iter;