fix: correct extend_to logic in parser

Previously we swapped to events in the buffer, but that might be wrong
if there aer `forward_parent` links pointing to the swapped-out node.

Let's do the same via parent links instead, keeping the nodes in place

Author: matklad

crates/parser/src/event.rs

@@ -93,8 +93,6 @@ pub(super) fn process(sink: &mut dyn TreeSink, mut events: Vec<Event>) {
 
     for i in 0..events.len() {
         match mem::replace(&mut events[i], Event::tombstone()) {
-            Event::Start { kind: TOMBSTONE, .. } => (),
-
             Event::Start { kind, forward_parent } => {
                 // For events[A, B, C], B is A's forward_parent, C is B's forward_parent,
                 // in the normal control flow, the parent-child relation: `A -> B -> C`,
@@ -109,9 +107,7 @@ pub(super) fn process(sink: &mut dyn TreeSink, mut events: Vec<Event>) {
                     // append `A`'s forward_parent `B`
                     fp = match mem::replace(&mut events[idx], Event::tombstone()) {
                         Event::Start { kind, forward_parent } => {
-                            if kind != TOMBSTONE {
-                                forward_parents.push(kind);
-                            }
+                            forward_parents.push(kind);
                             forward_parent
                         }
                         _ => unreachable!(),
@@ -120,7 +116,9 @@ pub(super) fn process(sink: &mut dyn TreeSink, mut events: Vec<Event>) {
                 }
 
                 for kind in forward_parents.drain(..).rev() {
-                    sink.start_node(kind);
+                    if kind != TOMBSTONE {
+                        sink.start_node(kind);
+                    }
                 }
             }
             Event::Finish => sink.finish_node(),

crates/parser/src/parser.rs

@@ -340,10 +340,14 @@ impl CompletedMarker {
 
     /// Extends this completed marker *to the left* up to `m`.
     pub(crate) fn extend_to(self, p: &mut Parser, mut m: Marker) {
-        assert!(m.pos <= self.pos);
         m.bomb.defuse();
-
-        p.events.swap(self.pos as usize, m.pos as usize);
+        let idx = m.pos as usize;
+        match &mut p.events[idx] {
+            Event::Start { forward_parent, .. } => {
+                *forward_parent = Some(self.pos - m.pos);
+            }
+            _ => unreachable!(),
+        }
     }
 
     pub(crate) fn kind(&self) -> SyntaxKind {