error: compilation successful
  --> $DIR/issue-17263.rs:15:1
   |
LL | / fn main() { #![rustc_error] // rust-lang/rust#49855
LL | |     let mut x: Box<_> = box Foo { a: 1, b: 2 };
LL | |     let (a, b) = (&mut x.a, &mut x.b);
LL | |     //~^ ERROR cannot borrow `x` (via `x.b`) as mutable more than once at a time
...  |
LL | |     //~^ ERROR cannot borrow `foo` (via `foo.b`) as immutable
LL | | }
   | |_^

error: aborting due to previous error