Type inference failure when cloning

``` rust
fn foo() -> String {
    let x = std::default::Default::default();
    drop(x.clone());
    x
}

fn main() {}
```

```
test.rs:3:10: 3:19 error: the type of this value must be known in this context
test.rs:3     drop(x.clone());
                   ^~~~~~~~~
error: aborting due to previous error
```

cc @nikomatsakis 
