Add feature so doctest works.

Author: finnbear

library/alloc/src/lib.rs

@@ -146,6 +146,7 @@
 #![feature(slice_ptr_len)]
 #![feature(slice_range)]
 #![feature(str_internals)]
+#![feature(string_leak)]
 #![feature(strict_provenance)]
 #![feature(trusted_len)]
 #![feature(trusted_random_access)]