I thought Rust didn't have a spec so everything in Rust was essentially undefined behaviour.
Has this changed or is the "defined" part still the compiler source code? In that case taking the source code of any C compiler as the _blessed_ one should get rid of any undefined behaviour problems as well.
You don't need a spec for the concepts of undefined vs defined behaviour. LLVM IR lacks a spec as well and is still built upon these concepts (LLVM IR does have documentation but so does Rust. There is no comprehensive document like the C specification for either).
Indeed the notion of what behaviour is considered undefined changes with compiler versions, and it is not fixed yet. E.g. mem::unused() for example is now basically always undefined and you are supposed to use MaybeUninit. But you get a warning if you try to use the old API.
This is for unsafe Rust however. With safe Rust, even though there is no spec, the guarantee is that, unless you hit one of the soundness holes in the language, or a piece of user code that uses unsafe internally, you are safe.
Has this changed or is the "defined" part still the compiler source code? In that case taking the source code of any C compiler as the _blessed_ one should get rid of any undefined behaviour problems as well.