confusing compile error for second partial borrows of/from Boxes, refers to members rather than box itself

[makoConstruct]

struct Thing{  a: u8,  b: u8  }
fn main(){
  let mut boxed_thing = box Thing{a:2, b:2};
  let btr = &mut boxed_thing;
  let (a, b) = (&mut btr.a, &mut btr.b);
    //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
    //let (a, b) = (&mut btr.a, &mut btr.b);
    //                   ^~~~~
}

[pnkfelix]

see also some follow-up discussion here #43234 (comment)