macros: allow select with only else branch (#6339)

tokio/src/macros/select.rs

@@ -608,6 +608,10 @@ macro_rules! select {
 
     // ===== Entry point =====
 
+    ($(biased;)? else => $else:expr $(,)? ) => {{
+        $else
+    }};
+
     (biased; $p:pat = $($t:tt)* ) => {
         $crate::select!(@{ start=0; () } $p = $($t)*)
     };
@@ -617,6 +621,7 @@ macro_rules! select {
         // fair and avoids always polling the first future.
         $crate::select!(@{ start={ $crate::macros::support::thread_rng_n(BRANCHES) }; () } $p = $($t)*)
     };
+
     () => {
         compile_error!("select! requires at least one branch.")
     };

tokio/tests/macros_select.rs

@@ -22,6 +22,25 @@ async fn sync_one_lit_expr_comma() {
     assert_eq!(foo, 1);
 }
 
+#[maybe_tokio_test]
+async fn no_branch_else_only() {
+    let foo = tokio::select! {
+        else => 1,
+    };
+
+    assert_eq!(foo, 1);
+}
+
+#[maybe_tokio_test]
+async fn no_branch_else_only_biased() {
+    let foo = tokio::select! {
+        biased;
+        else => 1,
+    };
+
+    assert_eq!(foo, 1);
+}
+
 #[maybe_tokio_test]
 async fn nested_one() {
     let foo = tokio::select! {