Miri is the closest to a UB specification for Rust that there is, coming in the form of a tool so you can run it. It's really cool but Valgrind, which is a C tool that also supports Rust, also supports Rust code that calls to C and that does I/O, both pretty common things for programs to do.

That doesn't seem responsive. The question wasn't whether Rust and C are literally the same language ("duh", as it were), it was effectively "are there meaningful safety features provided to the unsafe zlib-rs code in question in that aren't already available in C toolchains/ecosystems?"

And there really aren't. The abbreviated/limited safety environment being exploited by this non-idiomatic Rust code seems to me to be basically isomorphic to the way you'd solve the problem in C.