rt: add a method to determine the flavor of the runtime.

tokio/src/runtime/handle.rs

@@ -1,4 +1,4 @@
-use crate::runtime::scheduler;
+use crate::runtime::{scheduler, RuntimeFlavor};
 
 /// Handle to the runtime.
 ///
@@ -277,6 +277,34 @@ impl Handle {
         let future = crate::util::trace::task(future, "task", _name, id.as_u64());
         self.inner.spawn(future, id)
     }
+
+    /// Returns the flavor of the current `Runtime`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::runtime::{Handle, RuntimeFlavor};
+    ///
+    /// #[tokio::main(flavor = "current_thread")]
+    /// async fn main() {
+    ///   assert_eq!(RuntimeFlavor::CurrentThread, Handle::current().runtime_flavor());
+    /// }
+    /// ```
+    ///
+    /// ```
+    /// use tokio::runtime::{Handle, RuntimeFlavor};
+    ///
+    /// #[tokio::main(flavor = "multi_thread", worker_threads = 4)]
+    /// async fn main() {
+    ///   assert_eq!(RuntimeFlavor::MultiThread, Handle::current().runtime_flavor());
+    /// }
+    /// ```
+    pub fn runtime_flavor(&self) -> RuntimeFlavor {
+        match self.inner {
+            scheduler::Handle::CurrentThread(_) => RuntimeFlavor::CurrentThread,
+            scheduler::Handle::MultiThread(_) => RuntimeFlavor::MultiThread,
+        }
+    }
 }
 
 cfg_metrics! {

tokio/src/runtime/mod.rs

@@ -312,6 +312,19 @@ cfg_rt! {
         blocking_pool: BlockingPool,
     }
 
+    /// The flavor of a `Runtime`.
+    ///
+    /// This is the return type for [`Handle::runtime_flavor`](crate::runtime::Handle::runtime_flavor()).
+    #[derive(Debug, PartialEq, Eq)]
+    #[non_exhaustive]
+    pub enum RuntimeFlavor {
+        /// The flavor that executes all tasks on the current thread.
+        CurrentThread,
+        /// The flavor that executes tasks across multiple threads.
+        #[cfg(all(feature = "rt-multi-thread", not(tokio_wasi)))]
+        MultiThread,
+    }
+
     /// The runtime scheduler is either a multi-thread or a current-thread executor.
     #[derive(Debug)]
     enum Scheduler {