Just take the code as-is and comment out the `#[dead_code]`:

```
/home/clark/development/rust/rust/src/libcollections/hashmap.rs:337:9: 339:10 warning: code is never used: `can_alias_safehash_as_u64`, #[warn(dead_code)] on by default
/home/clark/development/rust/rust/src/libcollections/hashmap.rs:337         fn can_alias_safehash_as_u64() {
/home/clark/development/rust/rust/src/libcollections/hashmap.rs:338             unsafe { assert_eq!(size_of::<SafeHash>(), size_of::<u64>()) };
/home/clark/development/rust/rust/src/libcollections/hashmap.rs:339         }
<std macros>:21:9: 23:10 warning: code is never used: `run_fmt`, #[warn(dead_code)] on by default
<std macros>:21         fn run_fmt(fmt: &::std::fmt::Arguments) -> ! {
<std macros>:22             ::std::rt::begin_unwind_fmt(fmt, file!(), line!())
<std macros>:23         }
<std macros>:1:1: 26:2 note: in expansion of fail!
<std macros>:8:21: 10:18 note: expansion site
<std macros>:1:1: 14:2 note: in expansion of assert_eq!
/home/clark/development/rust/rust/src/libcollections/hashmap.rs:338:22: 338:75 note: expansion site
```