error: compilation successful
  --> $DIR/issue-17263.rs:15:1
   |
LL | / fn main() { //[nll]~ ERROR compilation successful
LL | |     let mut x: Box<_> = box Foo { a: 1, b: 2 };
LL | |     let (a, b) = (&mut x.a, &mut x.b);
LL | |     //[ast]~^ ERROR cannot borrow `x` (via `x.b`) as mutable more than once at a time
...  |
LL | |     use_mut(a);
LL | | }
   | |_^

error: aborting due to previous error