[Bug] Match arm conditions should not be allowed to mutate what is being matched

[vineethk]

🐛 Bug

Consider the following example:

//# publish
module 0xc0ffee::m {
    enum W has copy, drop {
        W1(u64),
        W2(u64),
    }

    fun use_it(_x: u64) {}

    fun weird(w: &mut W): bool {
        *w = W::W2(10);
        false
    }

    public fun test(w: &mut W) {
        match (w) {
            W::W2(_) => {},
            W::W1(_) if (weird(w)) => {},
            W::W2(x) => { use_it(*x); },
            _ => {
                assert!(!(*w is W::W2));
            },
        }
    }

    public fun run() {
        let w = W::W1(5);
        test(&mut w);
    }
}

This code fails compilation with:

 error: unreachable pattern
   ┌─ TEMPFILE:16:13
   │
16 │             W::W2(x) => { use_it(*x); },
   │             ^^^^^^^^```

But if you comment out this line, then run run(), the assertion fails (that is, *w is indeed W::W2, so it is reachable, as opposed to what the compiler says).

Note: Rust disallows such mutations in the match arm conditions.