generalize get_room_version

There are other fields of `m.room.create` events that are useful to
individually extract without caring about the values of other fields.

src/service/rooms/state.rs

@@ -7,14 +7,14 @@ use std::{
 use ruma::{
     api::client::error::ErrorKind,
     events::{
-        room::{create::RoomCreateEventContent, member::MembershipState},
-        AnyStrippedStateEvent, StateEventType, TimelineEventType,
+        room::member::MembershipState, AnyStrippedStateEvent, StateEventType,
+        TimelineEventType,
     },
     serde::Raw,
     state_res::{self, StateMap},
     EventId, OwnedEventId, OwnedRoomId, RoomId, RoomVersionId, UserId,
 };
-use serde::Deserialize;
+use serde::{de::DeserializeOwned, Deserialize};
 use tracing::warn;
 
 use super::{short::ShortStateHash, state_compressor::CompressedStateEvent};
@@ -31,6 +31,26 @@ mod data;
 
 pub(crate) use data::Data;
 
+pub(crate) trait ExtractCreateContent: DeserializeOwned {
+    type Extract;
+
+    fn extract(self) -> Self::Extract;
+}
+
+/// Extract the `room_version` from an `m.room.create` event
+#[derive(Deserialize)]
+pub(crate) struct ExtractVersion {
+    room_version: RoomVersionId,
+}
+
+impl ExtractCreateContent for ExtractVersion {
+    type Extract = RoomVersionId;
+
+    fn extract(self) -> Self::Extract {
+        self.room_version
+    }
+}
+
 pub(crate) struct Service {
     pub(crate) db: &'static dyn Data,
 }
@@ -315,22 +335,22 @@ impl Service {
         self.db.set_room_state(room_id, shortstatehash)
     }
 
-    /// Returns the room's version.
+    /// Returns the value of a field of an `m.room.create` event's `content`.
     #[tracing::instrument(skip(self))]
-    pub(crate) fn get_room_version(
+    pub(crate) fn get_create_content<T: ExtractCreateContent>(
         &self,
         room_id: &RoomId,
-    ) -> Result<RoomVersionId> {
+    ) -> Result<T::Extract> {
         let create_event = services().rooms.state_accessor.room_state_get(
             room_id,
             &StateEventType::RoomCreate,
             "",
         )?;
 
-        let create_event_content: RoomCreateEventContent = create_event
+        let content_field = create_event
             .as_ref()
             .map(|create_event| {
-                serde_json::from_str(create_event.content.get()).map_err(
+                serde_json::from_str::<T>(create_event.content.get()).map_err(
                     |error| {
                         warn!(%error, "Invalid create event");
                         Error::BadDatabase("Invalid create event in db.")
@@ -345,7 +365,7 @@ impl Service {
                 )
             })?;
 
-        Ok(create_event_content.room_version)
+        Ok(content_field.extract())
     }
 
     #[tracing::instrument(skip(self))]