Fix server arbiter support (#417)

actix-utils/Cargo.toml

@@ -23,3 +23,4 @@ local-waker = "0.1"
 [dev-dependencies]
 actix-rt = "2.0.0"
 futures-util = { version = "0.3.7", default-features = false }
+static_assertions = "1.1"

actix-utils/src/future/ready.rs

@@ -103,10 +103,16 @@ pub fn err<T, E>(err: E) -> Ready<Result<T, E>> {
 
 #[cfg(test)]
 mod tests {
+    use std::rc::Rc;
+
     use futures_util::task::noop_waker;
+    use static_assertions::{assert_impl_all, assert_not_impl_all};
 
     use super::*;
 
+    assert_impl_all!(Ready<()>: Send, Sync, Clone);
+    assert_not_impl_all!(Ready<Rc<()>>: Send, Sync);
+
     #[test]
     #[should_panic]
     fn multiple_poll_panics() {