Support stable identifiers for MSC4115

[richvdh]

matrix-org/matrix-spec-proposals#4115 has now completed FCP; servers will soon start exposing the information using stable identifiers (element-hq/synapse#17264) and we should add support for them.

[richvdh]

I think this might actually be a no-op?

The code in question has:

            if let Ok(Some(unsigned)) = raw_event.get_field::<UnsignedWithMembership>("unsigned") {
                if let Membership::Leave = unsigned.membership {
                    // We were not a member - this is the cause of the UTD
                    return UtdCause::Membership;
                }
            }

where UnsignedWithMembership is defined as:

/// MSC4115 membership info in the unsigned area.
#[derive(Deserialize)]
struct UnsignedWithMembership {
    #[serde(alias = "io.element.msc4115.membership")]
    membership: Membership,
}

Given that UnsignedWithMembership.membership is defined with #[serde(alias)] (rather than #[serde(rename)], it should accept either the unstable name or just membership.

Would be good to test this works once element-hq/synapse#17282 lands.

[richvdh]

(by the way, @andybalaam: accepting the proposed stable name before the MSC lands is considered bad karma: there's no guarantee that membership actually ends up meaning what you expect it to mean. In this case, we got away with it :) )

[richvdh]

In fact, #3337 even included tests for the stable name, so I think we can consider this done.

[andybalaam]

(by the way, @andybalaam: accepting the proposed stable name before the MSC lands is considered bad karma: there's no guarantee that membership actually ends up meaning what you expect it to mean. In this case, we got away with it :) )

Ouch, sorry/thanks.