feat(sdk): LatestEventValue implements Serialize and Deserialize

[Hywan]

The problem is: LatestEventContent cannot be serialized. It's annoying because it means we can't store a LatestEventValue (that wraps a LatestEventContent) in the database.

This patch revisits LatestEventValue. Before we got:

pub enum LatestEventValue {
     None,
     Remote(LatestEventContent),
     LocalIsSending(LatestEventContent),
     LocalCannotBeSent(LatestEventContent),
}

pub enum LatestEventContent {
    RoomMessage(RoomMessageEventContent),
    Sticker(StickerEventContent),
    Poll(UnstablePollStartEventContent),
    CallInvite(CallInviteEventContent),
    CallNotify(CallNotifyEventContent),
    KnockedStateEvent(RoomMemberEventContent),
    Redacted(AnySyncMessageLikeEvent),
}

LatestEventContent::Redacted contains an AnySyncMessageLikeEvent. That's the part that is not serializable.

It appears that LatestEventContent isn't necessary! The only thing we need is to filter the events by their type, no need to find and map. The LatestEventValue can contain the entry event directly (e.g. a TimelineEvent for the event cache). Okay, let's do that.

pub enum LatestEventValue {
    None,
    Remote(RemoteLatestEventValue),
    LocalIsSending(???),
    LocalCannotBeSent(???),
}

type RemoteLatestEventValue = TimelineEvent;

What about the Local* variants? We can't use a TimelineEvent. We need a new type for that:

pub enum LatestEventValue {
    None,
    Remote(RemoteLatestEventValue),
    LocalIsSending(LocalLatestEventValue),
    LocalCannotBeSent(LocalLatestEventValue),
}

pub struct LocalLatestEventValue {
    pub timestamp: MilliSecondsSinceUnixEpoch,
    pub content: Raw<AnyMessageLikeEventContent>,
    pub event_type: String,
}

We don't need the event ID nor the transaction ID in LocalLatestEventValue.

That's the only change. All the other changes are about the tests.

---

• Address [meta] Latest Event API, the missing one #4112

[codspeed-hq]

CodSpeed Performance Report

Merging #5561 will not alter performance

_{Comparing Hywan:fat-sdk-latest-events-content (edd32f9) with main (7adaf7b)}

Summary

✅ 27 untouched benchmarks
🆕 4 new benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
🆕	Restore session [SQLite][clear]	N/A	703.5 ms	N/A
🆕	Restore session [SQLite][encrypted]	N/A	1.2 s	N/A
🆕	Restore session [memory store]	N/A	181 ms	N/A
🆕	Create a timeline with initial events[10000 events]	N/A	746.7 ms	N/A

[codecov]

Codecov Report

❌ Patch coverage is 82.06278% with 40 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.64%. Comparing base (feeeb53) to head (edd32f9).
⚠️ Report is 3 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...rates/matrix-sdk/src/latest_events/latest_event.rs	81.64%	8 Missing and 30 partials ⚠️
crates/matrix-sdk/src/latest_events/mod.rs	87.50%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5561      +/-   ##
==========================================
- Coverage   88.65%   88.64%   -0.02%     
==========================================
  Files         340      340              
  Lines       95083    95029      -54     
  Branches    95083    95029      -54     
==========================================
- Hits        84292    84234      -58     
+ Misses       6609     6608       -1     
- Partials     4182     4187       +5     

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[bnjbvr]

I think that instead of:

    pub content: Raw<AnyMessageLikeEventContent>,
    pub event_type: String,

Could you use SerializableEventContent, which likely encodes the semantics you're interested in?