refactor(error): improve error message when tokio::spawn fails

Closes #1635

src/common/exec.rs

@@ -1,3 +1,4 @@
+use std::error::Error as StdError;
 use std::fmt;
 use std::sync::Arc;
 
@@ -40,7 +41,7 @@ impl Exec {
                         .spawn(Box::new(fut))
                         .map_err(|err| {
                             warn!("executor error: {:?}", err);
-                            ::Error::new_execute()
+                            ::Error::new_execute(TokioSpawnError)
                         })
                 }
                 #[cfg(not(feature = "runtime"))]
@@ -53,7 +54,7 @@ impl Exec {
                 e.execute(Box::new(fut))
                     .map_err(|err| {
                         warn!("executor error: {:?}", err.kind());
-                        ::Error::new_execute()
+                        ::Error::new_execute("custom executor failed")
                     })
             },
         }
@@ -101,7 +102,7 @@ where
         self.execute(fut)
             .map_err(|err| {
                 warn!("executor error: {:?}", err.kind());
-                ::Error::new_execute()
+                ::Error::new_execute("custom executor failed")
             })
     }
 }
@@ -117,7 +118,30 @@ where
         self.execute(fut)
             .map_err(|err| {
                 warn!("executor error: {:?}", err.kind());
-                ::Error::new_execute()
+                ::Error::new_execute("custom executor failed")
             })
     }
 }
+
+// ===== StdError impls =====
+
+struct TokioSpawnError;
+
+impl fmt::Debug for TokioSpawnError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::Debug::fmt("tokio::spawn failed (is a tokio runtime running this future?)", f)
+    }
+}
+
+impl fmt::Display for TokioSpawnError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::Display::fmt("tokio::spawn failed (is a tokio runtime running this future?)", f)
+    }
+}
+
+impl StdError for TokioSpawnError {
+    fn description(&self) -> &str {
+        "tokio::spawn failed"
+    }
+}
+

src/error.rs

@@ -258,8 +258,8 @@ impl Error {
         Error::new(Kind::Shutdown, Some(Box::new(cause)))
     }
 
-    pub(crate) fn new_execute() -> Error {
-        Error::new(Kind::Execute, None)
+    pub(crate) fn new_execute<E: Into<Cause>>(cause: E) -> Error {
+        Error::new(Kind::Execute, Some(cause.into()))
     }
 
     pub(crate) fn new_h2(cause: ::h2::Error) -> Error {
@@ -269,10 +269,12 @@ impl Error {
 
 impl fmt::Debug for Error {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        f.debug_struct("Error")
-            .field("kind", &self.inner.kind)
-            .field("cause", &self.inner.cause)
-            .finish()
+        let mut f = f.debug_struct("Error");
+        f.field("kind", &self.inner.kind);
+        if let Some(ref cause) = self.inner.cause {
+            f.field("cause", cause);
+        }
+        f.finish()
     }
 }