rust-postgres | Native PostgreSQL driver for the Rust programming language

The reason for this was that the trait implementation Send for Arc was defined as

You need to enumerate the rows and columns, doing so you can get the column reference while enumerating, and from that get the postgresql-type. With the type information it's possible to have conditional logic to choose different sub-functions to both: i) get the strongly typed variable; and, ii) convert to a JSON value.

Here is main.rs:

Postgresql supports client connections via local Unix sockets or via TCP/IP connections.

In the default configuration, though, it will not listen for TCP/IP connections. It will only listen for connections to a local Unix socket. The location of this socket is defined by the postgresql configuration variable unix_socket_directories.

In your tests, you have found that when running the psql command line tool with no arguments, it manages to connect to the database. This works because psql uses the postgresql supplied client library (libpq) and this client library has the default behavior of connecting to the local Unix socket if no hostname is supplied.

However, when using rust-postgres, you are supplying a connection string including the text "host=localhost". This is instructing rust-postgres to make a connection to IP address localhost. It fails because the postgresql server is not configured to listen on any IP interfaces, and only listen on a Unix socket.

You could change the connection string to specify the unix socket to connect to, for example: