fix(common): Use a trick to avoid hitting the recursion_limit too quickly.

This patch adds a trick around `SyncTimelineEvent` to avoid reaching the
`recursion_limit` too quickly. Read the documentation in this patch to
learn more.

crates/matrix-sdk-base/Cargo.toml

@@ -30,7 +30,10 @@ uniffi = ["dep:uniffi", "matrix-sdk-crypto?/uniffi", "matrix-sdk-common/uniffi"]
 # Private feature, see
 # https://github.com/matrix-org/matrix-rust-sdk/pull/3749#issuecomment-2312939823 for the gory
 # details.
-test-send-sync = ["matrix-sdk-crypto?/test-send-sync"]
+test-send-sync = [
+    "matrix-sdk-common/test-send-sync",
+    "matrix-sdk-crypto?/test-send-sync",
+]
 
 # "message-ids" feature doesn't do anything and is deprecated.
 message-ids = []

crates/matrix-sdk-common/Cargo.toml

@@ -18,6 +18,10 @@ targets = ["x86_64-unknown-linux-gnu", "wasm32-unknown-unknown"]
 [features]
 js = ["wasm-bindgen-futures"]
 uniffi = ["dep:uniffi"]
+# Private feature, see
+# https://github.com/matrix-org/matrix-rust-sdk/pull/3749#issuecomment-2312939823 for the gory
+# details.
+test-send-sync = []
 
 [dependencies]
 async-trait = { workspace = true }

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

@@ -308,6 +308,22 @@ pub struct EncryptionInfo {
 /// Previously, this differed from [`TimelineEvent`] by wrapping an
 /// [`AnySyncTimelineEvent`] instead of an [`AnyTimelineEvent`], but nowadays
 /// they are essentially identical, and one of them should probably be removed.
+//
+// 🚨 Note about this type, please read! 🚨
+//
+// `SyncTimelineEvent` is heavily used across the SDK crates. In some cases, we
+// are reaching a [`recursion_limit`] when the compiler is trying to figure out
+// if `SyncTimelineEvent` implements `Sync` when it's embedded in other types.
+//
+// We want to help the compiler so that one doesn't need to increase the
+// `recursion_limit`. We stop the recursive check by (un)safely implement `Sync`
+// and `Send` on `SyncTimelineEvent` directly.
+//
+// See
+// https://github.com/matrix-org/matrix-rust-sdk/pull/3749#issuecomment-2312939823
+// which has addressed this issue first
+//
+// [`recursion_limit`]: https://doc.rust-lang.org/reference/attributes/limits.html#the-recursion_limit-attribute
 #[derive(Clone, Debug, Serialize)]
 pub struct SyncTimelineEvent {
     /// The event itself, together with any information on decryption.
@@ -318,6 +334,23 @@ pub struct SyncTimelineEvent {
     pub push_actions: Vec<Action>,
 }
 
+// See https://github.com/matrix-org/matrix-rust-sdk/pull/3749#issuecomment-2312939823.
+#[cfg(not(feature = "test-send-sync"))]
+unsafe impl Send for SyncTimelineEvent {}
+
+// See https://github.com/matrix-org/matrix-rust-sdk/pull/3749#issuecomment-2312939823.
+#[cfg(not(feature = "test-send-sync"))]
+unsafe impl Sync for SyncTimelineEvent {}
+
+#[cfg(feature = "test-send-sync")]
+#[test]
+// See https://github.com/matrix-org/matrix-rust-sdk/pull/3749#issuecomment-2312939823.
+fn test_send_sync_for_sync_timeline_event() {
+    fn assert_send_sync<T: Send + Sync>() {}
+
+    assert_send_sync::<SyncTimelineEvent>();
+}
+
 impl SyncTimelineEvent {
     /// Create a new `SyncTimelineEvent` from the given raw event.
     ///

crates/matrix-sdk-ui/tests/integration/main.rs

@@ -12,8 +12,6 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#![recursion_limit = "256"]
-
 use itertools::Itertools as _;
 use matrix_sdk::deserialized_responses::TimelineEvent;
 use ruma::{events::AnyStateEvent, serde::Raw, EventId, RoomId};