failure to infer move mode for a variable when the move is from `*x`

Example (play: http://is.gd/idpRaW)

``` rust
use std::mem;
struct Trace;
fn foo(b: Box<Trace>) {
    do1(|| mem::drop(*b));
}
fn do1<F:FnOnce()>(f: F) { }
fn main() { }
```

If you change `mem::drop(*b)` to `mem::drop(b)`, it compiles.
