rollup merge of #20723: pnkfelix/feature-gate-box-syntax

Conflicts:
	src/compiletest/compiletest.rs
	src/libcollections/lib.rs
	src/libserialize/lib.rs
	src/libsyntax/feature_gate.rs

src/librustc_driver/lib.rs

@@ -23,8 +23,10 @@
       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;