Use the alloc_layout_extra feature for Layout::array::<T>

src/vec-final.md

@@ -3,6 +3,7 @@
 ```rust
 #![feature(ptr_internals)]
 #![feature(allocator_api)]
+#![feature(alloc_layout_extra)]
 
 use std::ptr::{Unique, NonNull, self};
 use std::mem;