`PgConnectOptions.to_url_lossy()` panics due to incorrectly encoded host/socket

[InternetUnexplorer]

I have found these related issues/pull requests

I have not found any existing issues for this bug. The closest I could find was #3255, which is about a different bug in to_url_lossy().

Description

PgConnectOptions does not always set socket when a socket is used. Since PgConnectOptions.to_url_lossy only percent-encodes the socket (and not the host), this can cause it to panic:

BUG: generated un-parseable URL: EmptyHost

An easy way to reproduce this is to parse a URL with the default host, user, etc., such as postgres:///foo. No host is specified in the URL, so the default one is used, which in my case is the socket at /var/run/postgresql:

let opts: PgConnectOptions = "postgresql:///foo".parse().unwrap();
assert_eq!(opts.get_host(), "/var/run/postgresql");
assert_eq!(opts.get_socket(), None);
dbg!(opts.to_url_lossy()); // panic: BUG: generated un-parseable URL

I believe this is a bug, as passing a URL without a host seems to be supported by libpq and thus a lot of postgresql software (sqlx seemingly supports it too; connecting works fine even though PgConnectOptions.to_url_lossy panics). While this may seem like an edge-case, the ability to not specify a host in the URL is very convenient in my experience.

I'm not sure what the best way to fix this is. One option would be to percent-encode the host, while another would be to make sure that socket is always set when a socket is used, similar to the check in PgConnectOptions.parse_from_url.

Reproduction steps

The example I mentioned above should be enough to reproduce the issue, as long as one of the candidate sockets used by PgConnectOptions.default_host is available:

let opts: PgConnectOptions = "postgresql:///foo".parse().unwrap();
assert_eq!(opts.get_host(), "/var/run/postgresql");
assert_eq!(opts.get_socket(), None);
dbg!(opts.to_url_lossy()); // panic: BUG: generated un-parseable URL

SQLx version

0.8.6

Enabled SQLx features

postgres, runtime-tokio

Database server and version

PostgreSQL 17.5

Operating system

Linux 6.15.8 - NixOS 25.11.20250806.c2ae88e (Xantusia)

Rust version

rustc 1.88.0 (6b00bc388 2025-06-23)