feat(threads): automatically subscribe a user to a thread under the semantics of MSC4306

crates/matrix-sdk-base/src/store/send_queue.rs

@@ -66,12 +66,19 @@ impl SerializableEventContent {
         self.event.deserialize_with_type(&self.event_type)
     }
 
-    /// Returns the raw event content along with its type.
+    /// Returns the raw event content along with its type, borrowed variant.
     ///
     /// Useful for callers manipulating custom events.
     pub fn raw(&self) -> (&Raw<AnyMessageLikeEventContent>, &str) {
         (&self.event, &self.event_type)
     }
+
+    /// Returns the raw event content along with its type, owned variant.
+    ///
+    /// Useful for callers manipulating custom events.
+    pub fn into_raw(self) -> (Raw<AnyMessageLikeEventContent>, String) {
+        (self.event, self.event_type)
+    }
 }
 
 /// The kind of a send queue request.

crates/matrix-sdk-common/src/serde_helpers.rs

@@ -17,7 +17,10 @@
 
 use ruma::{
     OwnedEventId,
-    events::{AnySyncMessageLikeEvent, AnySyncTimelineEvent, relation::BundledThread},
+    events::{
+        AnyMessageLikeEventContent, AnySyncMessageLikeEvent, AnySyncTimelineEvent,
+        relation::BundledThread,
+    },
     serde::Raw,
 };
 use serde::Deserialize;
@@ -45,6 +48,22 @@ struct SimplifiedContent {
     relates_to: Option<RelatesTo>,
 }
 
+/// Try to extract the thread root from an event's content, if provided.
+///
+/// The thread root is the field located at `m.relates_to`.`event_id`,
+/// if the field at `m.relates_to`.`rel_type` is `m.thread`.
+///
+/// Returns `None` if we couldn't find a thread root, or if there was an issue
+/// during deserialization.
+pub fn extract_thread_root_from_content(
+    content: Raw<AnyMessageLikeEventContent>,
+) -> Option<OwnedEventId> {
+    let relates_to = content.deserialize_as_unchecked::<SimplifiedContent>().ok()?.relates_to?;
+    match relates_to.rel_type {
+        RelationsType::Thread => relates_to.event_id,
+    }
+}
+
 /// Try to extract the thread root from a timeline event, if provided.
 ///
 /// The thread root is the field located at `content`.`m.relates_to`.`event_id`,