Function managed_replica_config 

fn managed_replica_config(managed: &ClusterVariantManaged) -> ReplicaConfig

The replica config implied by a managed cluster’s config.

Every field a replica shares with its cluster is taken from the cluster. availability_zones is the cluster’s pool, which is what the provisioning paths stamp onto a managed replica.

Two things the ALTER CLUSTER path does that this does not. It normalizes logging through the plan, which discards log_logging when no interval is set, so for the odd cluster config that has debugging on and logging off the two produce different records. And it validates the size against the replica size map and the role’s allowed sizes, which cannot be done from here because the size map is not part of the durable catalog. A builtin cluster’s size only ever comes from a bootstrap flag or an ALTER that already validated it.