Add compiler error for DISTINCT ON that doesn't match initial ORDER BY expressions

[connec]

For example:

table! {
    entries (id) {
        id -> Int4,
        name -> Text,
        date -> Timestamp,
    }
}

// Attempt to get the latest entries for each `name`
entries
    .order_by(date.desc())
    .distinct_on(name)
    .load(&conn)

This compiles fine, but inevitably causes "SELECT DISTINCT ON expressions must match initial ORDER BY expressions" when executed.

The correct version, of course, is:

entries
    .order_by((name, date.desc()))
    .distinct_on(name)
    .load(&conn)

According to the PostgreSQL docs DISTINCT ON is valid with no ordering, or with a matching prefix. I don't know enough about Rust to have any idea how feasible it is to encode that in the types, but it would be a neat sanity check!

Checklist

• I have already looked over the issue tracker for similar issues.

[sgrif]

I'm not sure that it's possible for us to encode this (without a breaking change), but we definitely should be