hashes: Implement std::error::Error

We have old Rust 1.29 error handling code still in `hashes`. Implement
`std::error::Error` for the `hex::Error` and `error::Error` types in
line with "modern" Rust 1.41.1 error handling.

hashes/src/error.rs

@@ -31,3 +31,15 @@ impl fmt::Display for Error {
         }
     }
 }
+
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::Error::*;
+
+        match self {
+            InvalidLength(_, _) => None,
+        }
+    }
+}

hashes/src/hex.rs

@@ -47,6 +47,18 @@ impl fmt::Display for Error {
     }
 }
 
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::Error::*;
+
+        match self {
+            InvalidChar(_) | OddLengthString(_) | InvalidLength(_, _) => None,
+        }
+    }
+}
+
 /// Trait for objects that can be deserialized from hex strings.
 pub trait FromHex: Sized {
     /// Produces an object from a byte iterator.

hashes/src/impls.rs

@@ -18,26 +18,12 @@
 //!
 
 #[cfg(feature = "std")]
-use std::{error, io};
+use std::io;
 
 #[cfg(not(feature = "std"))]
-use core2::{error, io};
+use core2::io;
 
-use crate::{Error, HashEngine, hex, sha1, sha256, sha512, ripemd160, siphash24, hmac};
-
-impl error::Error for Error {
-    #[cfg(feature = "std")]
-    fn cause(&self) -> Option<&error::Error> { None }
-    #[cfg(feature = "std")]
-    fn description(&self) -> &str { "`std::error::description` is deprecated" }
-}
-
-impl error::Error for hex::Error {
-    #[cfg(feature = "std")]
-    fn cause(&self) -> Option<&error::Error> { None }
-    #[cfg(feature = "std")]
-    fn description(&self) -> &str { "`std::error::description` is deprecated" }
-}
+use crate::{HashEngine, sha1, sha256, sha512, ripemd160, siphash24, hmac};
 
 impl io::Write for sha1::HashEngine {
     fn flush(&mut self) -> io::Result<()> { Ok(()) }