library: use c string literals

library/std/src/lib.rs

@@ -308,6 +308,7 @@
 //
 // Library features (core):
 // tidy-alphabetical-start
+#![cfg_attr(bootstrap, feature(c_str_literals))]
 #![feature(char_internals)]
 #![feature(core_intrinsics)]
 #![feature(core_io_borrowed_buf)]