Mutable trait object cast counts as move, not as temporary borrow

[Kimundi]

struct MovingType { data: ~() }
trait Trait {}
impl Trait for MovingType {}

fn main() {
    let mut x: MovingType = MovingType { data: ~() };
    a(&mut x);
}

fn a(x: &mut MovingType) {
    b(x);
    {
        c(x as &mut Trait);
        //c_cast_wrapper(x);
    }
    b(x);
}

fn b(_: &mut MovingType) {}
fn c(_: &mut Trait) {}

fn c_cast_wrapper(x: &mut MovingType) {
    c(x as &mut Trait);
}

The compiler currently rejects the code above, because the as &mut Trait cast counts as moving the &mut MovingType reference:

moved_mutable_ref_trait_cast.rs:16:6: 16:7 error: use of moved value: `x`
moved_mutable_ref_trait_cast.rs:16     b(x);
                                         ^
moved_mutable_ref_trait_cast.rs:13:10: 13:11 note: `x` moved here because it has type `&mut MovingType`, which is non-copyable (perhaps you meant to use clone()?)
moved_mutable_ref_trait_cast.rs:13         c(x as &mut Trait);
                                             ^
error: aborting due to previous error
task 'rustc' failed at 'explicit failure', /build/rust-git/src/rust/src/libsyntax/diagnostic.rs:102
task '<main>' failed at 'explicit failure', /build/rust-git/src/rust/src/librustc/lib.rs:440

However, if you introduce indirection by having the cast be performed by a wrapper that takes &mut MovingType, then the program compiles.

This can be demonstrated by swapping the the commented and uncommented lines in the code block.

As far as I can see, this is a bug, as either the compiler should allow both or forbid both.

[Kimundi]

cc @nikomatsakis

[alexcrichton]

Possible dupe of #8813, but I don't know enough about this to close one way or the other.

[nikomatsakis]

Dup of #8813