Implement ExactSizeIterator for DocumentIter.

src/iter/ast_node_iter.rs

@@ -202,14 +202,35 @@ impl<'r, 's> AstNodeIter<'r, 's> {
     }
 }
 
-multi_field_iter!(
-    Document<'s>,
-    DocumentIter,
-    zeroth_section,
-    std::option::Iter<'r, Section<'s>>,
-    children,
-    std::slice::Iter<'r, Heading<'s>>
-);
+pub struct DocumentIter<'r, 's> {
+    zeroth_section: std::option::Iter<'r, Section<'s>>,
+    children: std::slice::Iter<'r, Heading<'s>>,
+}
+impl<'r, 's> Iterator for DocumentIter<'r, 's> {
+    type Item = AstNode<'r, 's>;
+    fn next(&mut self) -> Option<Self::Item> {
+        self.zeroth_section
+            .next()
+            .map(Into::<AstNode>::into)
+            .or_else(|| self.children.next().map(Into::<AstNode>::into))
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let size = self.zeroth_section.len() + self.children.len();
+        (size, Some(size))
+    }
+}
+impl<'r, 's> ExactSizeIterator for DocumentIter<'r, 's> {}
+impl<'r, 's> IntoIterator for &'r Document<'s> {
+    type Item = AstNode<'r, 's>;
+    type IntoIter = DocumentIter<'r, 's>;
+    fn into_iter(self) -> Self::IntoIter {
+        DocumentIter {
+            zeroth_section: self.zeroth_section.iter(),
+            children: self.children.iter(),
+        }
+    }
+}
 multi_field_iter!(
     Heading<'s>,
     HeadingIter,