This should be written as:

```
Err(...);
//~^ ERROR: cannot determine a type for this expression: unconstrained type
```

The line with the error should go right next to the statement that should fail. If there's a tidy error, you can do what I've done in the example above. That is, put the expected error in the line below and use a `^` to specify that the expected error should be raised by the line above the comment.