> Hopefully the programmer will be informed whenever the compiler decides it can treat some code as "undefined"? That's the wrong mental model. The compiler do

> Hopefully the programmer will be informed whenever the compiler decides it can treat some code as "undefined"? That's the wrong mental model. The compiler doesn't decide to treat some code as "undefined". Instead, the compiler assumes all code is not "undefined", and generates code according to that. The compiler might notice that the code is doing bad things, and warn the developer (or even return an error), but that's not guaranteed, since undefined behavior is generally things the compiler cannot easily check. For an example, Rust guarantees that no two mutable references can reference the same place at the same time, even in unsafe code. If a function receives a pair of mutable references, the compiler assumes that they do not overlap. If they in fact overlap, the code generated by the compiler might do the wrong thing.