allow box_syntax and unknown features in the rustc_driver crate.

src/librustc_driver/lib.rs

@@ -22,8 +22,10 @@
       html_favicon_url = "http://www.rust-lang.org/favicon.ico",
       html_root_url = "http://doc.rust-lang.org/nightly/")]
 
+#![allow(unknown_features)]
 #![feature(quote)]
 #![feature(slicing_syntax, unsafe_destructor)]
+#![feature(box_syntax)]
 #![feature(rustc_diagnostic_macros)]
 
 extern crate arena;