Reference recommends union pattern matching pattern that is not actually guaranteed to work

[RalfJung]

The reference recommends code like this

#[repr(u32)]
enum Tag { I, F }

#[repr(C)]
union U {
    i: i32,
    f: f32,
}

#[repr(C)]
struct Value {
    tag: Tag,
    u: U,
}

fn is_zero(v: Value) -> bool {
    unsafe {
        match v {
            Value { tag: Tag::I, u: U { i: 0 } } => true,
            Value { tag: Tag::F, u: U { f: num } } if num == 0.0 => true,
            _ => false,
        }
    }
}

However, we don't actually guarantee the order in which patterns are evaluated, and in fact some variants of this pattern are UB today.

It's unclear how this example ended up there, it exists at least since ae361e1 which is 8 years old (Github is too broken right now to easily track this back further). None of the folks involved in opsem or pattern matching (or at least, neither @Nadrieril nor me) were aware of this.

It's probably too late to take this pattern out of the reference, it will already have been copied many times...