For implementers of third-party compilers, researchers of the Rust programming language, and programmers who write unsafe code, this is indeed a problem. It's bad.
For the designers of Rust, "no formal specification" allows them to make changes as long as it is not breaking. It's good.
Medical or Miltary often require the software stack/tooling to be certified following certain rules. I know most of this certifications are bogus, but how is that handled with Rust?
Ferrocene provides a certified compiler (based on a spec they've written documenting how it behaves) which is usable for many uses cases, but it obviously depends what exactly your specific domain needs.
For implementers of third-party compilers, researchers of the Rust programming language, and programmers who write unsafe code, this is indeed a problem. It's bad.
For the designers of Rust, "no formal specification" allows them to make changes as long as it is not breaking. It's good.