Fix borrow so it fails in 2018 edition Fixes #1141

[gurgalex]

Fix #1152

[rust-highfive]

r? @steveklabnik

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[marioidival]

r? @frewsxcv

[frewsxcv]

this causes a different error than the one we want

with rust 2015, the error is:

error[E0505]: cannot move out of `boxed_i32` because it is borrowed
  --> src/main.rs:27:21
   |
23 |         let _ref_to_i32: &i32 = &boxed_i32;
   |                                  --------- borrow of `boxed_i32` occurs here
...
27 |         eat_box_i32(boxed_i32);
   |                     ^^^^^^^^^ move out of `boxed_i32` occurs here

with these changes, the error is now:

error[E0382]: use of moved value: `boxed_i32`
  --> src/main.rs:30:34
   |
27 |         eat_box_i32(boxed_i32);
   |                     --------- value moved here
...
30 |         let _ref_to_i32: &i32 = &boxed_i32;
   |                                  ^^^^^^^^^ value used here after move
   |
   = note: move occurs because `boxed_i32` has type `std::boxed::Box<i32>`, which does not implement the `Copy` trait

[marioidival]

@gurgalex ^

[gurgalex]

Ah, whoops.

I'll think about this again today.

[gurgalex]

@frewsxcv
The error messages are now the same as before.

[marioidival]

@gurgalex thank you!