Rust and the case of unexpected enum values

There’s some interesting quirks in Rust. Here’s perfectly valid Rust code:

match Some(x) {
    Some(_) => println!("Everything is normal"),
    None => println!("How can this happen!?"),

You would think that this could never take the second None branch, right? Well, that’s not what I’m writing this blog post for.

In some (very few) cases, it prints out

How can this happen!?

It’s a bit fun to try to figure out how the heck this can ever be the case! In fact, I’m surprised that it’s still possible with opt-level=3 (I would expect it would be easy to have a bug where the optimizer doesn’t consider all cases). Kudos to the rust devs.

Here’s the snippit that causes the wacky behavior:

let x: &i32 = unsafe { &*(0 as *const i32) };

Figuring out exactly why that happens is left to the reader :)