task: add methods for inspecting JoinErrors (#2051)

Adds `is_cancelled()` and `is_panic()` methods to `JoinError`, as well as
`into_panic()` and `try_into_panic()` methods which, when applicable, returns
the payload of the panic.

tokio/src/task/error.rs

@@ -39,6 +39,95 @@ impl JoinError {
             repr: Repr::Panic(Mutex::new(err)),
         }
     }
+
+    /// Returns true if the error was caused by the task being cancelled
+    pub fn is_cancelled(&self) -> bool {
+        match &self.repr {
+            Repr::Cancelled => true,
+            _ => false,
+        }
+    }
+
+    /// Returns true if the error was caused by the task panicking
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::panic;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let err = tokio::spawn(async {
+    ///         panic!("boom");
+    ///     }).await.unwrap_err();
+    ///
+    ///     assert!(err.is_panic());
+    /// }
+    /// ```
+    pub fn is_panic(&self) -> bool {
+        match &self.repr {
+            Repr::Panic(_) => true,
+            _ => false,
+        }
+    }
+
+    /// Consumes the join error, returning the object with which the task panicked.
+    ///
+    /// # Panics
+    ///
+    /// `into_panic()` panics if the `Error` does not represent the underlying
+    /// task terminating with a panic. Use `is_panic` to check the error reason
+    /// or `try_into_panic` for a variant that does not panic.
+    ///
+    /// # Examples
+    ///
+    /// ```should_panic
+    /// use std::panic;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let err = tokio::spawn(async {
+    ///         panic!("boom");
+    ///     }).await.unwrap_err();
+    ///
+    ///     if err.is_panic() {
+    ///         // Resume the panic on the main task
+    ///         panic::resume_unwind(err.into_panic());
+    ///     }
+    /// }
+    /// ```
+    pub fn into_panic(self) -> Box<dyn Any + Send + 'static> {
+        self.try_into_panic()
+            .expect("`JoinError` reason is not a panic.")
+    }
+
+    /// Consumes the join error, returning the object with which the task
+    /// panicked if the task terminated due to a panic. Otherwise, `self` is
+    /// returned.
+    ///
+    /// # Examples
+    ///
+    /// ```should_panic
+    /// use std::panic;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let err = tokio::spawn(async {
+    ///         panic!("boom");
+    ///     }).await.unwrap_err();
+    ///
+    ///     if let Ok(reason) = err.try_into_panic() {
+    ///         // Resume the panic on the main task
+    ///         panic::resume_unwind(reason);
+    ///     }
+    /// }
+    /// ```
+    pub fn try_into_panic(self) -> Result<Box<dyn Any + Send + 'static>, JoinError> {
+        match self.repr {
+            Repr::Panic(p) => Ok(p.into_inner().expect("Extracting panic from mutex")),
+            _ => Err(self),
+        }
+    }
 }
 
 impl fmt::Display for JoinError {