According tohttps://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-PARAMKEYWORDS

The default behavior whenhost is not specified, or is empty, is to connect to a Unix-domain socket in /tmp (or whatever socket directory was specified when PostgreSQL was built). On Windows, the default is to connect to localhost.

However tokio-posgresconnect preventshost &hostaddr being omitted:

https://github.com/sfackler/rust-postgres/blob/02463b1/tokio-postgres/src/connect.rs#L20-L22