Rust: Remove references to deleted IDs in downgrade script

Author: paldepind

rust/downgrades/9487e73d9da6508522f07a34bc4aafa8d55c0f5f/downgrade.ql

@@ -7,7 +7,24 @@ class Location extends @location_default {
 }
 
 // `CfgPredExpr` and `CfgPredPat` are new node kinds with no representation in the old schema, so we
-// scrub their locations.
+// scrub their locations and references to them.
+private predicate deletedNode(Element id) { cfg_pred_exprs(id) or cfg_pred_pats(id) }
+
+// Comments attached to deleted nodes are also removed, along with their locations.
+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 cfg_pred_exprs(id) and not cfg_pred_pats(id)
+  locatable_locations(id, location) and not deletedElement(id)
+}
+
+query predicate new_macro_call_macro_call_expansions(Element macroCall, Element expansion) {
+  macro_call_macro_call_expansions(macroCall, expansion) and not deletedNode(expansion)
+}
+
+query predicate new_comments(Element id, Element parent, string text) {
+  comments(id, parent, text) and not deletedNode(parent)
 }

rust/downgrades/9487e73d9da6508522f07a34bc4aafa8d55c0f5f/upgrade.properties

@@ -6,3 +6,5 @@ cfg_pred_expr_cfg_predicates.rel: delete
 cfg_pred_pats.rel: delete
 cfg_pred_pat_cfg_predicates.rel: delete
 locatable_locations.rel: run downgrade.ql new_locatable_locations
+macro_call_macro_call_expansions.rel: run downgrade.ql new_macro_call_macro_call_expansions
+comments.rel: run downgrade.ql new_comments