the feature rust_2018_preview is already implied by usage of 2018 edition.

clippy_lints/src/lib.rs

@@ -12,7 +12,6 @@
 #![feature(iterator_find_map)]
 #![feature(macro_at_most_once_rep)]
 #![feature(tool_attributes)]
-#![feature(rust_2018_preview)]
 #![warn(rust_2018_idioms)]
 
 use toml;

mini-macro/src/lib.rs

@@ -1,4 +1,4 @@
-#![feature(use_extern_macros, proc_macro_quote, proc_macro_non_items)]
+#![feature(proc_macro_quote, proc_macro_non_items)]
 extern crate proc_macro;
 
 use proc_macro::{TokenStream, quote};
@@ -8,4 +8,4 @@ pub fn mini_macro(_: TokenStream) -> TokenStream {
     quote!(
         #[allow(unused)] fn needless_take_by_value(s: String) { println!("{}", s.len()); }
     )
-}
+}

src/lib.rs

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