Trait object coercion regression

This code has stopped compiling some time in the last week:

``` rust
trait Foo {}

impl Foo for i32 {}

fn test(_: &[&Foo]) {}

fn main() {
    test([&1i32]);
}
```

```
test.rs:8:10: 8:17 error: mismatched types: expected `&[&Foo]`, found `[&i32, .. 1]` (expected &-ptr, found vector)
test.rs:8     test([&1i32]);
                   ^~~~~~~
error: aborting due to previous error
```
