Fix conflict between no_std and no_core when building rustc with custom libc
// Enable extra lints:
#![cfg_attr(feature = "extra_traits", deny(missing_debug_implementations))]
#![deny(missing_copy_implementations, safe_packed_borrows)]
-#![no_std]
+#![cfg_attr(not(feature = "rustc-dep-of-std"), no_std)]
#![cfg_attr(feature = "rustc-dep-of-std", no_core)]
#![cfg_attr(
any(feature = "rustc-dep-of-std", target_os = "redox"),
