Rust: scrub deleted-node references in downgrade script

A full new->old->new dataset upgrade/downgrade round-trip surfaced dangling
references: deleting the new node kinds left `macro_call_macro_call_expansions`
rows (and potentially `comments` rows) pointing at now-undefined `@ast_node`s.
These are the only two relations with a generic `@ast_node`-typed value column,
so the downgrade now drops rows in both that reference a deleted node, alongside
the existing `locatable_locations` scrub. `codeql dataset check` is now clean at
every stage of the round-trip.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 890cb55d-0437-4bd7-9268-87b8897ada01

Author: redsun82

rust/downgrades/852696f20117c338fbb4e6a4f5ed69df79682c37/downgrade.ql

@@ -7,9 +7,10 @@ class Location extends @location_default {
 }
 
 // Genuinely-new node kinds with no representation in the old schema. Their own child relations are
-// dropped via `delete` in upgrade.properties; here we additionally drop their locations so no
-// dangling `locatable_locations` rows remain.
-private predicate deletedElement(Element id) {
+// dropped via `delete` in upgrade.properties; the references to them from `@ast_node`-typed columns
+// (`macro_call_macro_call_expansions`, `comments`) and their `locatable_locations` are scrubbed
+// below.
+private predicate deletedNode(Element id) {
   deref_pats(id) or
   not_nulls(id) or
   include_bytes_exprs(id) or
@@ -19,6 +20,14 @@ private predicate deletedElement(Element id) {
   visibility_inners(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))
+}
+
 // A `@name` used as a format argument's name. The old schema represents these as dedicated text-less
 // `@format_args_arg_name` placeholders, so we repurpose these ids into that entity table and drop
 // them (and their text) from `names`/`name_texts`.
@@ -48,3 +57,14 @@ query predicate new_name_texts(Element id, string text) {
 query predicate new_locatable_locations(Element id, Location location) {
   locatable_locations(id, location) and not deletedElement(id)
 }
+
+// `macro_call_macro_call_expansions` and `comments` are the only two relations with a generic
+// `@ast_node`-typed value column, so a deleted node reachable through a macro expansion or as a
+// comment's parent would otherwise dangle here.
+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/852696f20117c338fbb4e6a4f5ed69df79682c37/upgrade.properties

@@ -12,6 +12,8 @@ names.rel: run downgrade.ql new_names
 name_texts.rel: run downgrade.ql new_name_texts
 
 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
 
 deref_pats.rel: delete
 deref_pat_pats.rel: delete