> Yes, and a core type implemented is also expected to be safe. If implemented unsafely, I expect assertions to come with it unless compiled with `--cfg ndebug`

I doubt even 1% of Rust users are ever going to pass this flag, so having it at all is close to useless. If Rust can't provide a high quality hash table without redundant checks being inlined into every usage of the code, then Rust has a serious problem.