World::insert_resource_by_id doesn't talk talk about whether type implements Send. It should either be part of safety invariants that the type implements Send, or if it implements !Send we're on the main thread. Or alternatively document a panic when called from not main thread with a !Send type (although I don't currently see a call to validate_non_send_access anywhere so I assume this is UB rn but it would be good for someone to check this)