rt: worker_threads must be non-zero (#2947)

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

benches/signal.rs

@@ -45,9 +45,8 @@ fn many_signals(bench: &mut Bencher) {
     let num_signals = 10;
     let (tx, mut rx) = mpsc::channel(num_signals);
 
-    let rt = runtime::Builder::new_multi_thread()
-        // Intentionally single threaded to measure delays in propagating wakes
-        .worker_threads(0)
+    // Intentionally single threaded to measure delays in propagating wakes
+    let rt = runtime::Builder::new_current_thread()
         .enable_all()
         .build()
         .unwrap();

benches/sync_semaphore.rs

@@ -49,8 +49,7 @@ fn uncontended_concurrent_multi(b: &mut Bencher) {
 }
 
 fn uncontended_concurrent_single(b: &mut Bencher) {
-    let rt = tokio::runtime::Builder::new_multi_thread()
-        .worker_threads(0)
+    let rt = tokio::runtime::Builder::new_current_thread()
         .build()
         .unwrap();
 

tokio/src/runtime/builder.rs

@@ -192,7 +192,12 @@ impl Builder {
     /// // This will run the runtime and future on the current thread
     /// rt.block_on(async move {});
     /// ```
+    ///
+    /// # Panic
+    ///
+    /// This will panic if `val` is not larger than `0`.
     pub fn worker_threads(&mut self, val: usize) -> &mut Self {
+        assert!(val > 0, "Worker threads cannot be set to 0");
         self.worker_threads = Some(val);
         self
     }