C#: Improve upgrade script.

csharp/ql/lib/upgrades/a2bda57dbc6eea94c50128522aae536e8edd5a3c/string_interpol_insert.ql

@@ -0,0 +1,57 @@
+class Expr extends @expr {
+  string toString() { none() }
+}
+
+class TypeOrRef extends @type_or_ref {
+  string toString() { none() }
+}
+
+class StringLiteral extends Expr, @string_literal_expr { }
+
+class InterpolatedStringExpr extends Expr, @interpolated_string_expr { }
+
+class StringInterpolationInsert extends Expr, @element {
+  StringInterpolationInsert() {
+    expressions(this, _, _) and
+    expr_parent(this, _, any(InterpolatedStringExpr x)) and
+    not this instanceof StringLiteral
+  }
+}
+
+newtype TAddedElement = TInsert(StringInterpolationInsert e)
+
+module Fresh = QlBuiltins::NewEntity<TAddedElement>;
+
+class TNewExpr = @expr or Fresh::EntityId;
+
+class NewExpr extends TNewExpr {
+  string toString() { none() }
+}
+
+query predicate new_expressions(NewExpr id, int kind, TypeOrRef t) {
+  expressions(id, kind, t)
+  or
+  exists(StringInterpolationInsert e |
+    // The type of `e` is just copied even though a null type would be preferred.
+    expressions(e, _, t) and
+    Fresh::map(TInsert(e)) = id and
+    kind = 138
+  )
+}
+
+query predicate new_expr_parent(NewExpr id, int child, NewExpr parent) {
+  // Keep all parent child relationships except for string interpolation inserts
+  expr_parent(id, child, parent) and not id instanceof StringInterpolationInsert
+  or
+  exists(StringInterpolationInsert e, int child0, NewExpr p0, NewExpr new_id |
+    expr_parent(e, child0, p0) and new_id = Fresh::map(TInsert(e))
+  |
+    id = new_id and
+    child = child0 and
+    parent = p0
+    or
+    id = e and
+    child = 0 and
+    parent = new_id
+  )
+}

csharp/ql/lib/upgrades/a2bda57dbc6eea94c50128522aae536e8edd5a3c/upgrade.properties

@@ -1,2 +1,4 @@
 description: Add `interpolated_string_insert_expr` kind.
-compatibility: partial
+compatibility: backwards
+expressions.rel: run string_interpol_insert.qlo new_expressions
+expr_parent.rel: run string_interpol_insert.qlo new_expr_parent