Replace failure dependency with impls of std::error::Error.

Cargo.toml

@@ -19,7 +19,6 @@ travis-ci = { repository = "dalek-cryptography/ed25519-dalek", branch = "master"
 [dependencies]
 clear_on_drop = { version = "0.2" }
 curve25519-dalek = { version = "1", default-features = false }
-failure = { version = "0.1", default-features = false }
 rand = { version = "0.7", default-features = false, optional = true }
 serde = { version = "1.0", optional = true }
 sha2 = { version = "0.8", default-features = false }

src/errors.rs

@@ -16,6 +16,9 @@
 use core::fmt;
 use core::fmt::Display;
 
+#[cfg(feature = "std")]
+use std::error::Error;
+
 /// Internal errors.  Most application-level developers will likely not
 /// need to pay any attention to these.
 #[derive(Clone, Copy, Debug, Eq, PartialEq, Hash)]
@@ -50,7 +53,8 @@ impl Display for InternalError {
     }
 }
 
-impl ::failure::Fail for InternalError {}
+#[cfg(feature = "std")]
+impl Error for InternalError { }
 
 /// Errors which may occur while processing signatures and keypairs.
 ///
@@ -75,8 +79,9 @@ impl Display for SignatureError {
     }
 }
 
-impl ::failure::Fail for SignatureError {
-    fn cause(&self) -> Option<&dyn (::failure::Fail)> {
+#[cfg(feature = "std")]
+impl Error for SignatureError {
+    fn source(&self) -> Option<&(dyn Error + 'static)> {
         Some(&self.0)
     }
 }

src/lib.rs

@@ -241,7 +241,6 @@ extern crate std;
 extern crate alloc;
 extern crate clear_on_drop;
 extern crate curve25519_dalek;
-extern crate failure;
 #[cfg(any(feature = "batch", feature = "std", feature = "alloc", test))]
 extern crate rand;
 #[cfg(feature = "serde")]