Fallback with standard PG prefixed env vars

[mehcode]

• https://www.postgresql.org/docs/current/libpq-envars.html
• https://github.com/launchbadge/sqlx/blob/master/sqlx-core/src/postgres/connection.rs#L144

Ones that make the most sense to me:

• PGHOST
• PGPORT
• PGUSER
• PGPASSWORD
• PGPASSFILE
• PGDATABASE

[tim-harding]

Digging into the linked lines in connection.rs a bit, I see that replacing the username and password being printed as part of the startup message would be pretty easy. However, the host, port, user, password, and database are all part of Url and accessed though MaybeTlsStream. These are outside the Postgres module, so it wouldn't make sense to add Postgres-specific defaults there. On the other hand, I don't see any way to modify the contents of Url in PgConnection::establish before it's passed to MaybeTlsStream::connect, short of recreating the url as a string and converting it back with .into(). Any suggestions on a better approach?

[abonander]

I feel like this issue should probably be merged with #39.

[tim-harding]

Continue the discussion there or here? I'd say the other thread definitely gives a clearer idea of where to start with the problem. Seems like a fun problem to take on.

[mehcode]

Marking this as blocked by figuring out #174

[mehcode]

#174 has been figured out on develop and this particularly has landed on develop