StreamExt that supports wasm platforms

The .boxed() method requires a Send trait that makes it
unusable on Wasm platforms. This re-exports the existing StreamExt,
while providing a new extension for the wasm family of targets.

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

@@ -30,6 +30,7 @@ pub mod locks;
 pub mod ring_buffer;
 pub mod sleep;
 pub mod store_locks;
+pub mod stream;
 pub mod timeout;
 pub mod tracing_timer;
 pub mod ttl_cache;

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

@@ -0,0 +1,57 @@
+// Copyright 2025 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Platform-specific stream utilities.
+//!
+//! This module provides a unified BoxStream + StreamExt class for working
+//! with boxed streams across different platforms. On native platforms,
+//! streams can be `Send`, but on Wasm they cannot. This module abstracts
+//! over that difference.
+
+#[cfg(not(target_family = "wasm"))]
+mod sys {
+    // On native platforms, just re-export everything from futures_util
+    pub use futures_util::{stream::BoxStream, StreamExt};
+}
+
+#[cfg(target_family = "wasm")]
+mod sys {
+    use futures_core::Stream;
+    // On Wasm, BoxStream is LocalBoxStream
+    pub use futures_util::stream::LocalBoxStream as BoxStream;
+
+    /// Custom StreamExt trait for Wasm that provides essential methods
+    /// like `.boxed()` and `.next()` without Send requirements.
+    pub trait StreamExt: Stream {
+        /// Box this stream using LocalBoxStream (no Send requirement).
+        fn boxed<'a>(self) -> BoxStream<'a, Self::Item>
+        where
+            Self: Sized + 'a,
+        {
+            futures_util::StreamExt::boxed_local(self)
+        }
+
+        /// Get the next item from this stream.
+        fn next(&mut self) -> futures_util::stream::Next<'_, Self>
+        where
+            Self: Unpin,
+        {
+            futures_util::StreamExt::next(self)
+        }
+    }
+
+    impl<S: Stream> StreamExt for S {}
+}
+
+pub use sys::*;