Putting `&mut Trait` in a struct causes an ICE

[alexcrichton]

This code

trait A {}
struct B;
impl A for B {}

struct C<'self> {
    foo: &'self mut A,
}

fn foo(a: &mut A) {
    C{ foo: a };
}

fn main() {
}

fails with this message:

foo.rs:10:12: 10:13 error: internal compiler error: Cannot relate bound region as subregion: br_self                                                
foo.rs:10     C{ foo: a };                                                                                                                          
                      ^                                                                                                                             

cc @msullivan

[nikomatsakis]

cc @nikomatsakis

[alexcrichton]

Yay, this works now! Removing ICE tag and adding needstest tag