Add support for feature-gating in table!

[LucaCappelletti94]

Backend

PostgreSQL, MySQL, SQLite, Others

What do you want to add to diesel?

On occasion, different versions of a schema have different columns. At this time, the table! macro does not support feature-gating columns, and this makes it necessary to gate the entire table! macro. In some settings, like for the postgres schema, several columns are introduced at different versions, forcing the creation of a large number of nearly identical table! macro calls.

Implementation notes

It would be simpler if the following was supported:

diesel::table! {
    pg_catalog.pg_proc (oid, proname, pronamespace) {
        /// OID of the function.
        oid -> Oid,
        /// Several other columns here... 
        #[cfg(feature="postgres-14")]
        /// SQL body of the function; `NULL` if not applicable.
        prosqlbody -> Nullable<Text>
    }
}

Additional details

No response

Checklist

• I have already looked over the issue tracker and the discussion forum for similar possible closed issues.