Dead code lint warns on types used in generics

[klutzy]

struct A;

fn f<T>() {}

fn main() {
    f::<A>();
    let _b = std::mem::size_of::<A>();
}

t.rs:1:0: 1:9 warning: code is never used: `A`, #[warn(dead_code)] on by default
t.rs:1 struct A;
       ^~~~~~~~~

cc @ktt3ja

[alexcrichton]

Oh whoops, I should have caught this during review! @ktt3ja, this is because walk_ty doesn't recurse further when it sees ty_path (which may have type parameters like in this case), but the good news is now that we have a generic visit_path you should be able to delete the visit_ty method entirely.