[GENERAL] Replication slot and pg_rewind

We are using the replication slot and pg_rewind feature of postgresql 9.6

Our cluster consists of 1 master and 1 slave node.

The replication slot feature allows the master to keep as much WAL as is required by the slave.

The pg_rewind command uses WALs to bring the slave in sync with the master.

By using replication slots there are always enough WAL in the pg_xlog.

In this case is it safe to use pg_rewind without WAL archiving?

Can there be a situation where pg_rewind fails?

Regards, Subhro