Avoid .take().unwrap() with FnOnce closures

src/libstd/lib.rs

@@ -106,7 +106,7 @@
 #![allow(unknown_features)]
 #![feature(macro_rules, globs, linkage, thread_local, asm)]
 #![feature(default_type_params, phase, lang_items, unsafe_destructor)]
-#![feature(import_shadowing, slicing_syntax)]
+#![feature(import_shadowing, slicing_syntax, tuple_indexing)]
 #![feature(unboxed_closures)]
 
 // Don't link to std. We are std.