events: Add method to add mentions instead of replacing them

crates/ruma-common/src/events/room/message.rs

@@ -350,6 +350,10 @@ impl RoomMessageEventContent {
     /// `m.new_content` so only new mentions will trigger a notification. As such, this needs to be
     /// called after [`Self::make_replacement()`].
     ///
+    /// It is not recommended to call this method after one that sets mentions automatically, like
+    /// [`Self::make_reply_to()`] as these will be overwritten. [`Self::add_mentions()`] should be
+    /// used instead.
+    ///
     /// [mentions]: https://spec.matrix.org/latest/client-server-api/#user-and-room-mentions
     pub fn set_mentions(mut self, mentions: Mentions) -> Self {
         if let Some(Relation::Replacement(replacement)) = &mut self.relates_to {
@@ -381,6 +385,28 @@ impl RoomMessageEventContent {
         self
     }
 
+    /// Add the given [mentions] to this event.
+    ///
+    /// If no [`Mentions`] was set on this events, this sets it. Otherwise, this updates the current
+    /// mentions by extending the previous `user_ids` with the new ones, and applies a logical OR to
+    /// the values of `room`.
+    ///
+    /// This is recommended over [`Self::set_mentions()`] to avoid to overwrite any mentions set
+    /// automatically by another method, like [`Self::make_reply_to()`]. However, this method has no
+    /// special support for replacements.
+    ///
+    /// [mentions]: https://spec.matrix.org/latest/client-server-api/#user-and-room-mentions
+    pub fn add_mentions(mut self, mentions: Mentions) -> Self {
+        if let Some(m) = &mut self.mentions {
+            m.user_ids.extend(mentions.user_ids);
+            m.room |= mentions.room;
+        } else {
+            self.mentions = Some(mentions);
+        }
+
+        self
+    }
+
     /// Returns a reference to the `msgtype` string.
     ///
     /// If you want to access the message type-specific data rather than the message type itself,

crates/ruma-common/tests/events/room_message.rs

@@ -271,6 +271,7 @@ fn escape_tags_in_plain_reply_body() {
         ForwardThread::Yes,
         AddMentions::No,
     );
+    assert_matches!(second_message.mentions, None);
 
     assert_matches!(
         first_message.content.msgtype,
@@ -406,6 +407,43 @@ fn reply_sanitize() {
     );
 }
 
+#[test]
+fn reply_add_mentions() {
+    let user = owned_user_id!("@user:example.org");
+    let friend = owned_user_id!("@friend:example.org");
+    let other_friend = owned_user_id!("@other_friend:example.org");
+
+    let mut first_message_content = RoomMessageEventContent::text_plain("My friend!");
+    first_message_content.mentions = Some(Mentions::with_user_ids([friend.clone()]));
+    let first_message = OriginalRoomMessageEvent {
+        content: first_message_content,
+        event_id: owned_event_id!("$143273582443PhrSn"),
+        origin_server_ts: MilliSecondsSinceUnixEpoch(uint!(10_000)),
+        room_id: owned_room_id!("!testroomid:example.org"),
+        sender: user.clone(),
+        unsigned: MessageLikeUnsigned::default(),
+    };
+    let mut second_message = RoomMessageEventContent::text_plain("User! Other friend!")
+        .make_reply_to(&first_message, ForwardThread::Yes, AddMentions::Yes);
+
+    let mentions = second_message.mentions.clone().unwrap();
+    assert_eq!(mentions.user_ids, [friend.clone(), user.clone()].into());
+    assert!(!mentions.room);
+
+    second_message =
+        second_message.add_mentions(Mentions::with_user_ids([user.clone(), other_friend.clone()]));
+
+    let mentions = second_message.mentions.clone().unwrap();
+    assert_eq!(mentions.user_ids, [friend.clone(), other_friend.clone(), user.clone()].into());
+    assert!(!mentions.room);
+
+    second_message = second_message.add_mentions(Mentions::with_room_mention());
+
+    let mentions = second_message.mentions.unwrap();
+    assert_eq!(mentions.user_ids, [friend, other_friend, user].into());
+    assert!(mentions.room);
+}
+
 #[test]
 fn make_replacement_no_reply() {
     let content = RoomMessageEventContent::text_html(