Let's say we have code like:

```
let badptr = 0 as *const u8;
if odd_perfect_number_exists() {
*badptr;
}
```

Even if the compiler can see and "understand" 100% of that code, it would have to crack an unsolved problem in mathematics to say whether this code is unsafe or not.

If the condition is `false`

, then the code is safe. If you want to be able to analyze arbitrary code, you can't forbid or ignore such cases. For example, you need to analyze dynamic conditions to prove that Rust's `Cow`

runs destructor only when that is safe to do.

Halting Problem is a name derived from the proof that arbitrary code can't be analyzed. It's not about halting, but lots of other analysis problems can be reduced to that problem by making them depend on whether part of a program will halt (such as whether a loop that stops on the first odd perfect number will stop or not).

There is analyzable subset of code, e.g. `while true {}`

obviously never halts, and `while false {}`

obviously always does. But that is not all code, and there's a proof that you will never be able to analyze **all** code. Rust can obviously analyze safe subset of Rust. It is possible to analyze more than that, but this usually gets exponentially more difficult (e.g. if you try to symbolically execute all possible paths through a function with all possible values), so there has to be a line somewhere between what is analyzed, and where analysis has to give up.