Inference rules changed on 1.10 nightly

``` Rust
struct Foo;
struct Bar;

fn foo_ref_to_bar_ptr(foo: &Foo) -> *const Bar {
    foo as *const _ as _
}

```

This builds on 1.10 nightly, but inference fails on beta and stable.

If this is a desired change, it should probably be documented, and regression tests added.
If not, it should be made into an error.
