Segfault with pattern guards, copy bindings, impls, and structs

[pcwalton]

This code causes a free of garbage memory and crashes:

struct Url {
    scheme: ~str,
    port: Option<~str>
}

impl Url {
    fn normalized_url(&self) {
        match self.port {
            Some(copy port) if true => {}
            _ => {}
        }
    }
}

fn main() {
    let url: Url = Url {
        scheme: ~"http",
        port: None,
    };
    url.normalized_url();
}

Doesn't seem to be related to explicit self. The pattern guard is necessary in order to cause the crash.

[bblum]

Dup of #3291, I believe.