Implement a macro for iterators with multiple fields.

src/iter/macros.rs

@@ -72,3 +72,43 @@ macro_rules! empty_iter {
 }
 
 pub(crate) use empty_iter;
+
+/// Create iterators for ast nodes where it has to iterate over multiple child lists.
+macro_rules! multi_field_iter {
+    ($astnodetype:ty, $itertype:ident, $firstfieldname: ident, $firstinnertype:ty, $($fieldname: ident, $innertype:ty),*) => {
+        pub struct $itertype<'r, 's> {
+            $firstfieldname: $firstinnertype,
+            $(
+$fieldname: $innertype,
+),*
+        }
+
+        impl<'r, 's> Iterator for $itertype<'r, 's> {
+            type Item = AstNode<'r, 's>;
+
+            fn next(&mut self) -> Option<Self::Item> {
+                self.$firstfieldname.next().map(Into::<AstNode>::into)
+                $(
+.or_else(|| self.$fieldname.next().map(Into::<AstNode>::into))
+),*
+            }
+        }
+
+        impl<'r, 's> IntoIterator for &'r $astnodetype {
+            type Item = AstNode<'r, 's>;
+
+            type IntoIter = $itertype<'r, 's>;
+
+            fn into_iter(self) -> Self::IntoIter {
+                $itertype {
+            $firstfieldname: self.$firstfieldname.iter(),
+            $(
+$fieldname: self.$fieldname.iter(),
+),*
+                }
+            }
+        }
+    };
+}
+
+pub(crate) use multi_field_iter;