Being in Rust or not the code has 0 tests, write arbitrary bytes, hard code values etc ... looks like more a PoC project that something that going to replace pgbouncer any time soon.
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 :).
That said I've struggled with more than my fair share of pgbouncer misconfigurations and outright bugs. I personally would love a simpler (harder to misconfigure) and less buggy alternative.