Possible bug about an inconsistency between borrowing from boxeds and borrowing from &muts

gistfile1.rs

struct Thing{  a: u8,  b: u8  }
fn main(){
	let mut boxed_thing = box Thing{a:2, b:2};
	let btr = &mut boxed_thing;
	let mut unboxed_thing = Thing{a:3, b:3};
	let utr = &mut unboxed_thing;
	fn pass(a:&mut u8, b:&mut u8){}
	pass(&mut utr.a, &mut utr.b); //< this is allowed
	pass(&mut btr.a, &mut btr.b); //< while this is not.
		//error: cannot borrow `btr.b` as mutable more than once at a time
		//note: previous borrow of `btr.a` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `btr.a` until the borrow ends
		//pass(&mut btr.a, &mut btr.b); //< while this is not. "
		//          ^~~~~
}