task: add manual Sync impl for VerificationCache to avoid overflowing…

… evaluation requirements

crates/matrix-sdk-base/Cargo.toml

@@ -30,7 +30,7 @@ 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 = []
+test-send-sync = ["matrix-sdk-crypto?/test-send-sync"]
 
 # "message-ids" feature doesn't do anything and is deprecated.
 message-ids = []

crates/matrix-sdk-crypto/Cargo.toml

@@ -23,6 +23,11 @@ experimental-algorithms = []
 uniffi = ["dep:uniffi"]
 _disable-minimum-rotation-period-ms = []
 
+# Private feature, see
+# https://github.com/matrix-org/matrix-rust-sdk/pull/3749#issuecomment-2312939823 for the gory
+# details.
+test-send-sync = []
+
 # "message-ids" feature doesn't do anything and is deprecated.
 message-ids = []
 

crates/matrix-sdk-crypto/src/verification/cache.rs

@@ -33,6 +33,19 @@ pub struct VerificationCache {
     inner: Arc<VerificationCacheInner>,
 }
 
+// See https://github.com/matrix-org/matrix-rust-sdk/pull/3749#issuecomment-2312939823.
+#[cfg(not(feature = "test-send-sync"))]
+unsafe impl Sync for VerificationCache {}
+
+#[cfg(feature = "test-send-sync")]
+#[test]
+// See https://github.com/matrix-org/matrix-rust-sdk/pull/3749#issuecomment-2312939823.
+fn test_send_sync_for_room() {
+    fn assert_send_sync<T: Send + Sync>() {}
+
+    assert_send_sync::<VerificationCache>();
+}
+
 #[derive(Debug, Default)]
 struct VerificationCacheInner {
     verification: StdRwLock<BTreeMap<OwnedUserId, BTreeMap<String, Verification>>>,