Modifying a Option<String> via pattern matching results in inconsistent behavior

[ghost]

The following code worked as of 0.11.0 but no longer produces an expected result on master.

fn main() {
    let mut foo = Some("Kitten".to_string());
    match foo {
        None => {},
        Some(x) => {
            foo = Some(x);
        }
    };
    println!("'{:s}'", foo.unwrap());
}

on 0.11.0:

'Kitten'

on master:

''

Another example

fn main() {
    let mut foo = Some(box 5i);
    match foo {
        None => {},
        Some(x) => {
            foo = Some(x);
        }
    };
    println!("'{}'", foo.unwrap());
}

on 0.11.0:

'5'

on master:

task '<main>' failed at 'called `Option::unwrap()` on a `None` value', /build/rust-git/src/rust/src/libcore/option.rs:265

[pcwalton]

cc @luqmana, I think this is probably a bug introduced by your change