fix: don't project records during broadcasting; push index down (#2524)

src/awkward/_broadcasting.py

@@ -934,7 +934,13 @@ def broadcast_any_union():
         )
 
     def broadcast_any_indexed():
-        nextinputs = [x.project() if isinstance(x, IndexedArray) else x for x in inputs]
+        # The `apply` function may exit at the level of a `RecordArray`. We can avoid projection
+        # of the record array in such cases, in favour of a deferred carry. This can be done by
+        # "pushing" the `IndexedArray` _into_ the record (i.e., wrapping each `content`).
+        nextinputs = [
+            x._push_inside_record_or_project() if isinstance(x, IndexedArray) else x
+            for x in inputs
+        ]
         return apply_step(
             backend,
             nextinputs,

src/awkward/contents/indexedarray.py

@@ -1136,3 +1136,18 @@ def _is_equal_to(self, other, index_dtype, numpyarray):
         return self.index.is_equal_to(
             other.index, index_dtype, numpyarray
         ) and self.content.is_equal_to(other.content, index_dtype, numpyarray)
+
+    def _push_inside_record_or_project(self) -> Self | ak.contents.RecordArray:
+        if self.content.is_record:
+            return ak.contents.RecordArray(
+                contents=[
+                    ak.contents.IndexedArray.simplified(self._index, c)
+                    for c in self.content.contents
+                ],
+                fields=self.content._fields,
+                length=self.length,
+                backend=self._backend,
+                parameters=parameters_union(self.content._parameters, self._parameters),
+            )
+        else:
+            return self.project()