Wrong code with assignment in match body

[eefriedman]

fn main() {
    let mut c = (1, "".to_owned());
    match c {
        c2 => { c.0 = 2; assert_eq!(c2.0, 1); }
    }
}

thread '<main>' panicked at 'assertion failed: `(left == right)` (left: `2`, right: `1`)', <anon>:4
playpen: application terminated with error code 101

[Thiez]

Actually what happens here is the following:

The tuple you create in c is moved, because it is not Copy. When you perform the match you move out of c, into c2, leaving c uninitialized. Your assignment to c.0 is basically to uninitialized memory, and doesn't have an effect. When you perform the assertion you see the old value of c.0, because your assignment to c.0 had no effect.

The whole thing is confusing because Rust does not warn when you perform useless assignments to moved variables, and it probably should. The whole thing becomes obviously when you add a statement that uses c in the match arm:

fn main() {
    let mut c = (1, "".to_owned());
    match c {
        c2 => { println!("{}", c.0); c.0 = 2; assert_eq!(c2.0, 1); }
    }
}

Edit: @Aatch the labels you have assigned to this issue are incorrect.

[mitaa]

Your assignment to c.0 is basically to uninitialized memory, and doesn't have an effect. When you perform the assertion you see the old value of c.0, because your assignment to c.0 had no effect.

Don't we see the new value c.0 = 2;, even though we're accessing c2.0, when performing the assertion, which causes it to fail?

Doing this without a match statement passes the same assertion sucessfully:

fn main() {
    let mut c = (1, "".to_owned());
    let c2 = c;
    c.0 = 2;
    assert_eq!(c2.0, 1);
}

[Thiez]

Oops, you are correct, I am mistaken :(

On Mon, Jul 13, 2015 at 10:52 AM, mitaa [email protected] wrote:

Your assignment to c.0 is basically to uninitialized memory, and doesn't
have an effect. When you perform the assertion you see the old value of
c.0, because your assignment to c.0 had no effect.

Don't we see the new value c.0 = 2;, even though we're accessing c2.0,
when performing the assertion, which causes it to fail?

Doing this without a match statement passes the same assertion sucessfully:

fn main() {
let mut c = (1, "".to_owned());
let c2 = c;
c.0 = 2;
assert_eq!(c2.0, 1);
}

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[Aatch]

@Thiez heh, I made the same mistake. Typed out most of a response and then realised that I'd mis-read the code.