For more see: https://blog.regehr.org/archives/213 and https://raphlinus.github.io/programming/rust/2018/08/17/unde... .
> In absence of a standard effectively every language construct is 'implementation defined behaviour'.
But the parent said:
> In absence of a standard effectively every language construct is 'undefined behavior'.
Undefined behaviour != implementation defined behaviour. Both of these things exist in C and are separate. You can rely on implementation defined behaviour giving some consistent result on a given, compiler and hardware. You cannot rely on the behaviour of your program if you hit undefined behaviour.
That's clear enough for programmers to rely on. It is nonsense to argue that this is equivalent to C's "the compiler may do anything it wants" just because the "Rust Book" is not called the "Rust Standard" or not "formal" enough.