Add private arg to fmt::UnsafeArg As discussed [here](https://github.com/rust-lang/rust/pull/89139#discussion_r719467357) r? `@Mark-Simulacrum`
Add private arg to fmt::UnsafeArg As discussed [here](https://github.com/rust-lang/rust/pull/89139#discussion_r719467357) r? `@Mark-Simulacrum`