Forgetting to call a variant constructor causes a confusing error message

## STR

``` Rust
struct Foo(u32);

fn test() -> Foo { Foo }

fn main() {
}
```
## Confusing result

```
error[E0308]: mismatched types
 --> <anon>:3:20
  |
3 | fn test() -> Foo { Foo }
  |                    ^^^ expected struct `Foo`, found fn item
  |
  = note: expected type `Foo`
  = note:    found type `fn(u32) -> Foo {Foo::{{constructor}}}`
```
