Remove optional failure dependency and impl std::error::Error.

Cargo.toml

@@ -51,7 +51,6 @@ clear_on_drop = "=0.2.3"
 subtle = { version = "2", default-features = false }
 serde = { version = "1.0", default-features = false, optional = true }
 packed_simd = { version = "0.3.0", features = ["into_bits"], optional = true }
-failure = { version = "0.1", optional = true }
 
 [build-dependencies]
 rand_core = { version = "0.3.0", default-features = false }
@@ -61,7 +60,6 @@ clear_on_drop = "=0.2.3"
 subtle = { version = "2", default-features = false }
 serde = { version = "1.0", default-features = false, optional = true }
 packed_simd = { version = "0.3.0", features = ["into_bits"], optional = true }
-failure = { version = "0.1", optional = true }
 
 [features]
 nightly = ["subtle/nightly", "clear_on_drop/nightly"]

build.rs

@@ -17,9 +17,6 @@ extern crate subtle;
 #[cfg(all(feature = "nightly", feature = "packed_simd"))]
 extern crate packed_simd;
 
-#[cfg(feature = "failure")]
-extern crate failure;
-
 use std::env;
 use std::fs::File;
 use std::io::Write;

src/errors.rs

@@ -11,12 +11,15 @@
 //!
 //! Currently, these are only used in the implementations of `TryFrom`.
 //!
-//! This module optionally implements support for the types in the `failure`
-//! crate.  This can be enabled by building with `--features failure`.
+//! If used with `std` support, this public types in this module implement the
+//! `std::error::Error` trait.
 
 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)]
@@ -41,8 +44,8 @@ impl Display for InternalError {
     }
 }
 
-#[cfg(feature = "failure")]
-impl ::failure::Fail for InternalError {}
+#[cfg(feature = "std")]
+impl Error for InternalError { }
 
 /// Errors which may occur.
 ///
@@ -58,9 +61,9 @@ impl Display for CurveError {
     }
 }
 
-#[cfg(feature = "failure")]
-impl ::failure::Fail for CurveError {
-    fn cause(&self) -> Option<&dyn (::failure::Fail)> {
+#[cfg(feature = "std")]
+impl Error for CurveError {
+    fn source(&self) -> Option<&(dyn Error + 'static)> {
         Some(&self.0)
     }
 }

src/lib.rs

@@ -44,8 +44,6 @@ extern crate packed_simd;
 extern crate byteorder;
 extern crate clear_on_drop;
 pub extern crate digest;
-#[cfg(feature = "failure")]
-extern crate failure;
 extern crate rand_core;
 #[cfg(all(test, feature = "stage2_build"))]
 extern crate rand_os;