Remove unknown_lints warning on stable (#209)

* Allow the new unknown lint 'unused_doc_comment'

This is currently in nightly, but not stable. Hence, we need to allow
unknown lints so that stable doesn't produce warnings

* Deny build warnings in CI

Fixes #208

Author: Benjamin Gill

.travis.yml

@@ -35,6 +35,7 @@ env:
   global:
   - secure: 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
   - TRAVIS_CARGO_NIGHTLY_FEATURE=""
+  - RUSTFLAGS="-D warnings"
   matrix:
   - FEATURES=--features=backtrace
   - FEATURES=--no-default-features

src/error_chain.rs

@@ -172,7 +172,7 @@ macro_rules! error_chain_processed {
                 self.0.description()
             }
 
-            #[allow(unused_doc_comment)]
+            #[allow(unknown_lints, unused_doc_comment)]
             fn cause(&self) -> Option<&::std::error::Error> {
                 match self.1.next_error {
                     Some(ref c) => Some(&**c),
@@ -424,7 +424,7 @@ macro_rules! impl_extract_backtrace {
     ($error_name: ident
      $error_kind_name: ident
      $([$link_error_path: path, $(#[$meta_links: meta])*])*) => {
-        #[allow(unused_doc_comment)]
+        #[allow(unknown_lints, unused_doc_comment)]
         fn extract_backtrace(e: &(::std::error::Error + Send + 'static))
             -> Option<::std::sync::Arc<$crate::Backtrace>> {
             if let Some(e) = e.downcast_ref::<$error_name>() {

src/quick_error.rs

@@ -260,7 +260,7 @@ macro_rules! quick_error {
             $item:ident: $imode:tt [$(#[$imeta:meta])*] [$( $var:ident: $typ:ty ),*] {$( $funcs:tt )*}
         )*}
     ) => {
-        #[allow(unused, unused_doc_comment)]
+        #[allow(unknown_lints, unused, unused_doc_comment)]
         impl ::std::fmt::Display for $name {
             fn fmt(&self, fmt: &mut ::std::fmt::Formatter)
                 -> ::std::fmt::Result
@@ -310,7 +310,7 @@ macro_rules! quick_error {
                 }
             }
         }*/
-        #[allow(unused, unused_doc_comment)]
+        #[allow(unknown_lints, unused, unused_doc_comment)]
         impl $name {
             /// A string describing the error kind.
             pub fn description(&self) -> &str {