Remove usage of deprecated std::error::Error methods (#1206) (#1245)

tokio/src/codec/length_delimited.rs

@@ -940,12 +940,8 @@ impl fmt::Debug for FrameTooBig {
 
 impl fmt::Display for FrameTooBig {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.write_str(self.description())
+        f.write_str("frame size too big")
     }
 }
 
-impl StdError for FrameTooBig {
-    fn description(&self) -> &str {
-        "frame size too big"
-    }
-}
+impl StdError for FrameTooBig {}

tokio/src/runtime/current_thread/runtime.rs

@@ -80,15 +80,8 @@ impl fmt::Display for RunError {
 }
 
 impl Error for RunError {
-    fn description(&self) -> &str {
-        self.inner.description()
-    }
-
-    // FIXME(taiki-e): When the minimum support version of tokio reaches Rust 1.30,
-    // replace this with Error::source.
-    #[allow(deprecated)]
-    fn cause(&self) -> Option<&dyn Error> {
-        self.inner.cause()
+    fn source(&self) -> Option<&(dyn Error + 'static)> {
+        self.inner.source()
     }
 }