Use ptr::metadata in <[T]>::len implementation This avoids duplication of ptr::metadata code. I believe this is acceptable as the previous approach essentially duplicated `ptr::metadata` because back then `rustc_allow_const_fn_unstable` annotation did not exist. I would like somebody to ping `@rust-lang/wg-const-eval` as the documentation says: > Always ping `@rust-lang/wg-const-eval` if you are adding more rustc_allow_const_fn_unstable attributes to any const fn.