macros: silence unreachable_code warning in select! (#2678)

Solves #2665 by adding #[allow(unreachable_code)] inside a branch
matching arm.

Co-authored-by: Alice Ryhl <alice@ryhl.io>

tokio/src/macros/select.rs

@@ -366,6 +366,7 @@ macro_rules! select {
                     }
                     match branch {
                         $(
+                            #[allow(unreachable_code)]
                             $crate::count!( $($skip)* ) => {
                                 // First, if the future has previously been
                                 // disabled, do not poll it again. This is done

tokio/tests/macros_select.rs

@@ -440,9 +440,25 @@ async fn many_branches() {
     assert_eq!(1, num);
 }
 
+#[tokio::test]
+async fn never_branch_no_warnings() {
+    let t = tokio::select! {
+        _ = async_never() => 0,
+        one_async_ready = one() => one_async_ready,
+    };
+    assert_eq!(t, 1);
+}
+
 async fn one() -> usize {
     1
 }
 
 async fn require_mutable(_: &mut i32) {}
 async fn async_noop() {}
+
+async fn async_never() -> ! {
+    use tokio::time::Duration;
+    loop {
+        tokio::time::delay_for(Duration::from_millis(10)).await;
+    }
+}