Interfaces can be embedded in records with optional<>

[msjarrett]

inty = interface +c { ... }

recy = record {
  field: optional<inty>;
}

This compiles. But remove the optional<...> and you get:
Interface reference cannot live in a record

[artwyman]

Seems like a bug, though the fix is to update the error-checking to disallow this case.
As separate goal might be to support interfaces in records, though that would be a larger feature-request, and violate the "records are pure data" separation.