Function local_user_id 

pub(crate) fn local_user_id(
    localpart: &str,
    server_name: &ServerName,
) -> Option<OwnedUserId>

Assembles a local user id from a localpart a foreign column records.

The id is formatted into an inline buffer and parsed from that slice, which keeps a short id in inline storage; parsing against a server name instead routes through an over-allocated String and spills to the heap.