Add rust_2018_preview feature and fix rustfmt annotation

Author: mati865

clippy_lints/src/lib.rs

@@ -11,6 +11,7 @@
 #![allow(stable_features)]
 #![feature(iterator_find_map)]
 #![feature(macro_at_most_once_rep)]
+#![feature(rust_2018_preview)]
 
 extern crate cargo_metadata;
 #[macro_use]

clippy_lints/src/new_without_default.rs

@@ -153,7 +153,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NewWithoutDefault {
 }
 
 fn create_new_without_default_suggest_msg(ty: Ty) -> String {
-    #[rustfmt_skip]
+    #[cfg_attr(rustfmt, rustfmt_skip)]
     format!(
 "impl Default for {} {{
     fn default() -> Self {{

src/lib.rs

@@ -1,5 +1,6 @@
 // error-pattern:cargo-clippy
 #![feature(plugin_registrar)]
+#![feature(rust_2018_preview)]
 #![feature(rustc_private)]
 #![feature(macro_vis_matcher)]
 #![allow(unknown_lints)]