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/librustdoc/lib.rs

@@ -18,6 +18,7 @@
        html_root_url = "http://doc.rust-lang.org/nightly/",
        html_playground_url = "http://play.rust-lang.org/")]
 #![feature(slicing_syntax)]
+#![feature(box_syntax)]
 
 extern crate arena;
 extern crate getopts;