Error for mut borrow of a immut Box should point to where the Box was bound

[kmcallister]

This code:

fn f(x: &mut u32) { }

fn main() {
    let x = 3;
    f(&mut x);
}

produces a helpful error message:

error: cannot borrow immutable local variable `x` as mutable
 --> <anon>:5:12
  |
4 |     let x = 3;
  |         - use `mut x` here to make mutable
5 |     f(&mut x);
  |            ^ cannot borrow mutably

But the analogous code using a Box:

fn f(x: &mut u32) { }

fn main() {
    let x = Box::new(3);
    f(&mut *x);
}

gives an error which doesn't point at the binding x:

error: cannot borrow immutable `Box` content `*x` as mutable
 --> <anon>:5:12
  |
5 |     f(&mut *x);
  |            ^^

[Mark-Simulacrum]

This is fixed today. I'm going to mark as E-needstest since it feels like the kind of thing that's easy to regress on -- a UI test would be best here.

error[E0596]: cannot borrow immutable `Box` content `*x` as mutable
 --> test.rs:5:12
  |
4 |     let x = Box::new(3);
  |         - consider changing this to `mut x`
5 |     f(&mut *x);
  |            ^^ cannot borrow as mutable

error: aborting due to previous error(s)