Rookie here.

Why printing on line 12 gives the error:

```
println!("x: {}, y: {}", mixed.x, mixed.y);
^^^^^^^ `T` cannot be formatted with the default formatter
```

But the same println works in main?

```
struct Point<T, U> {
x: T,
y: U
}
impl<T, U> Point<T, U> {
fn mixup<V, W>(self, p2: Point<V, W>) -> Point<T, W> {
let mixed = Point {
x: self.x,
y: p2.y
};
println!("x: {}, y: {}", mixed.x, mixed.y); // this is not ok
mixed
}
}
fn main() {
let p1 = Point {x: 1, y: 2.0};
let p2 = Point {x: 2.0, y: "wtf"};
let mixed = p1.mixup(p2);
println!("x: {}, y: {}", mixed.x, mixed.y); // but this is ok
}
```

Thanks for your help