Rust: Add schema upgrade, downgrade and change note

Author: github-actions[bot]

rust/downgrades/9487e73d9da6508522f07a34bc4aafa8d55c0f5f/downgrade.ql

@@ -0,0 +1,23 @@
+class Element extends @element {
+  string toString() { none() }
+}
+
+class Location extends @location_default {
+  string toString() { none() }
+}
+
+// The new node kinds have no representation in the old schema. Their own relations are dropped via
+// `delete` in upgrade.properties; their `locatable_locations` rows are scrubbed below.
+private predicate deletedNode(Element id) { cfg_pred_exprs(id) or cfg_pred_pats(id) }
+
+// A deleted node, plus any comment attached to one: dropping the comment's `comments` row would
+// otherwise leave its `locatable_locations` row dangling.
+private predicate deletedElement(Element id) {
+  deletedNode(id)
+  or
+  exists(Element parent | comments(id, parent, _) and deletedNode(parent))
+}
+
+query predicate new_locatable_locations(Element id, Location location) {
+  locatable_locations(id, location) and not deletedElement(id)
+}