Can't bound types with paths from macros

This code:

``` rust
macro_rules! foo(($t:path) => {
    impl<T:$t> Foo for T {}
})

foo!(A)

fn main() {}
```

fails with

```
foo.rs:2:12: 2:13 error: expected `,` but found `an interpolated path`
foo.rs:2     impl<T:$t> Foo for T {}
                     ^
```

I may be doing something wrong, but this should be allowed, right?
