Avoid reusing the root node during incremental parsing

Fixes #712

cli/src/tests/parser_test.rs

@@ -395,6 +395,18 @@ fn test_parsing_after_editing_end_of_code() {
     assert_eq!(recorder.strings_read(), vec![" * ", "abc.d)",]);
 }
 
+#[test]
+fn test_parsing_empty_file_with_reused_tree() {
+    let mut parser = Parser::new();
+    parser.set_language(get_language("rust")).unwrap();
+
+    let tree = parser.parse("", None);
+    parser.parse("", tree.as_ref());
+
+    let tree = parser.parse("\n  ", None);
+    parser.parse("\n  ", tree.as_ref());
+}
+
 // Thread safety
 
 #[test]

lib/src/reusable_node.h

@@ -20,15 +20,6 @@ static inline void reusable_node_clear(ReusableNode *self) {
   self->last_external_token = NULL_SUBTREE;
 }
 
-static inline void reusable_node_reset(ReusableNode *self, Subtree tree) {
-  reusable_node_clear(self);
-  array_push(&self->stack, ((StackEntry) {
-    .tree = tree,
-    .child_index = 0,
-    .byte_offset = 0,
-  }));
-}
-
 static inline Subtree reusable_node_tree(ReusableNode *self) {
   return self->stack.size > 0
     ? self->stack.contents[self->stack.size - 1].tree
@@ -86,3 +77,19 @@ static inline void reusable_node_advance_past_leaf(ReusableNode *self) {
   while (reusable_node_descend(self)) {}
   reusable_node_advance(self);
 }
+
+static inline void reusable_node_reset(ReusableNode *self, Subtree tree) {
+  reusable_node_clear(self);
+  array_push(&self->stack, ((StackEntry) {
+    .tree = tree,
+    .child_index = 0,
+    .byte_offset = 0,
+  }));
+
+  // Never reuse the root node, because it has a non-standard internal structure
+  // due to transformations that are applied when it is accepted: adding the EOF
+  // child and any extra children.
+  if (!reusable_node_descend(self)) {
+    reusable_node_clear(self);
+  }
+}