It's definitely a PoC. I started it two weeks ago. Overall I've been super happy with Rust because it was a great safety harness and it gave me a fast concurrent program. PgBouncer implements this in about 30k lines of C, while I managed to get 80% of the way and then some in less than 1k.

As for tests, it's covered with pgbench, python (psycopg2), ruby (pg / active record gems), psql and a couple inline tests for trivial things like config.

As for bytes and hardcoded ints...give pgbouncer's and postgres' source code a look :).