Difficulty creating `&mut Trait`

[alexcrichton]

This test case:

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

fn foo(_: &mut A) {
}

fn main() {
    let mut b = B;
    foo(&mut b as &mut A);
}

fails with

foo.rs:10:8: 10:14 error: borrowed value does not live long enough
foo.rs:10     foo(&mut b as &mut A);
                  ^~~~~~
note: borrowed pointer must be valid for the static lifetime...
foo.rs:8:10: 11:1 note: ...but borrowed value is only valid for the block at 8:10
foo.rs:8 fn main() {
foo.rs:9     let mut b = B;
foo.rs:10     foo(&mut b as &mut A);
foo.rs:11 }
error: aborting due to previous error

cc @msullivan

[nikomatsakis]

cc @nikomatsakis

[alexcrichton]

Yay this works now!

Flagging as needstest

btw, @nikomatsakis you are awesome.