Implement Error for a few error types (#511)

tokio-executor/src/enter.rs

@@ -1,5 +1,6 @@
 use std::prelude::v1::*;
 use std::cell::Cell;
+use std::error::Error;
 use std::fmt;
 
 #[cfg(feature = "unstable-futures")]
@@ -27,11 +28,23 @@ pub struct EnterError {
 impl fmt::Debug for EnterError {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         f.debug_struct("EnterError")
-            .field("reason", &"attempted to run an executor while another executor is already running")
+            .field("reason", &self.description())
             .finish()
     }
 }
 
+impl fmt::Display for EnterError {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        write!(fmt, "{}", self.description())
+    }
+}
+
+impl Error for EnterError {
+    fn description(&self) -> &str {
+        "attempted to run an executor while another executor is already running"
+    }
+}
+
 /// Marks the current thread as being within the dynamic extent of an
 /// executor.
 ///

tokio-executor/src/lib.rs

@@ -52,6 +52,9 @@ pub use global::spawn2;
 
 use futures::Future;
 
+use std::error::Error;
+use std::fmt;
+
 /// A value that executes futures.
 ///
 /// The [`spawn`] function is used to submit a future to an executor. Once
@@ -238,3 +241,15 @@ impl SpawnError {
         !self.is_shutdown
     }
 }
+
+impl fmt::Display for SpawnError {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        write!(fmt, "{}", self.description())
+    }
+}
+
+impl Error for SpawnError {
+    fn description(&self) -> &str {
+        "attempted to spawn task while the executor is at capacity or shut down"
+    }
+}