Add replaces_state to state events in the /sync response

[manuroe]

In case an event has a prev_content, it would be useful to know also the id of the event where this prev_content comes from.

One use case is the redaction of a state event that was in the past like in:

• state was A, then later B, then later C
• B is redacted

The prev_content of C, that contains redacted data of B, must be pruned to avoid leaked data as described at element-hq/element-ios#443.

Knowing the event_id of the state event which C replaces, the client will be able to locally prune C without requiring any request to the homeserver.
To solve this, Riot-iOS used to make a room initial sync but is was an expensive process server side (element-hq/element-ios#1823).

[turt2live]

Related: matrix-org/matrix-spec-proposals#819

[joepie91]

This should probably be tagged with spec-omission.

[ShadowJonathan]

Shouldn't this be simply added as a MSC, or to the spec?

[turt2live]

this is the spec, and it might not need an MSC if it's a bug in the spec.