Can not borrow a `&Trait` from a `& &mut Trait`

[Kimundi]

Maybe I'm missing something, but afaik the only way to do this is with automatic coercion at the moment, which does not work here:

trait Bar{}
impl Bar for uint {};

let x = &mut 7u as &mut Bar;
let y = &x; 
let z: &Bar = *y;

<anon>:9:97: 9:99 error: cannot borrow an `&mut` in a `&` pointer; try an `&mut` instead
<anon>:9         trait Bar{} impl Bar for uint {}; let x = &mut 7u as &mut Bar; let y = &x; let z: &Bar = *y;
                                                                                                          ^~

[Kimundi]

See also #9629

[nikomatsakis]

This is pretty much a dup of #9629.