Implement Error::source for h2::Error.

This makes it easier to detect an IO error at the bottom of an error
cause chain (e.g. behind a `hyper::Error`).

Also, don't implement `std::error::Error` for the private error types,
to avoid confusion.

src/codec/error.rs

@@ -1,6 +1,6 @@
 use crate::frame::{Reason, StreamId};
 
-use std::{error, fmt, io};
+use std::{fmt, io};
 
 /// Errors that are received
 #[derive(Debug)]
@@ -73,8 +73,6 @@ impl From<io::Error> for RecvError {
     }
 }
 
-impl error::Error for RecvError {}
-
 impl fmt::Display for RecvError {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         use self::RecvError::*;
@@ -89,8 +87,6 @@ impl fmt::Display for RecvError {
 
 // ===== impl SendError =====
 
-impl error::Error for SendError {}
-
 impl fmt::Display for SendError {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         use self::SendError::*;
@@ -117,8 +113,6 @@ impl From<UserError> for SendError {
 
 // ===== impl UserError =====
 
-impl error::Error for UserError {}
-
 impl fmt::Display for UserError {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         use self::UserError::*;

src/error.rs

@@ -132,4 +132,11 @@ impl fmt::Display for Error {
     }
 }
 
-impl error::Error for Error {}
+impl error::Error for Error {
+    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
+        match self.kind {
+            Kind::Io(ref e) => Some(e),
+            _ => None,
+        }
+    }
+}