Put slicing syntax behind a feature gate.

[breaking-change]

If you are using slicing syntax you will need to add #![feature(slicing_syntax)] to your crate.

src/libcore/lib.rs

@@ -57,8 +57,9 @@
        html_playground_url = "http://play.rust-lang.org/")]
 
 #![no_std]
+#![allow(unknown_features)]
 #![feature(globs, intrinsics, lang_items, macro_rules, managed_boxes, phase)]
-#![feature(simd, unsafe_destructor)]
+#![feature(simd, unsafe_destructor, slicing_syntax)]
 #![deny(missing_doc)]
 
 mod macros;