Pattern matching for parameters in Impl blocks does not count struct fields as used

Example Code:

``` rust
struct A;
struct B{ b: u32 } // warning: struct field is never used: `b`
impl A {
    fn do_stuff(&B{b}: &B) {
        println!("{}", b); // b field is used
    }
}

fn main() {
    let b = B{b:3};
    A::do_stuff(&b);
}
```
