Impl displaying does not show trait generic args

E.g.:
```rust
trait Foo<T> {
    fn foo(&self) {}
}

impl<T> Foo<()> for T {
    fn foo(&self) {}
}

fn bar() {
    ().foo();
}
```

<img width="587" height="388" alt="Image" src="https://github.com/user-attachments/assets/8235ab29-ba6d-4b80-85a3-aff94a5e1799" />

This is confusing.

We also don't show the impl header when there are no generic parameters, which is wrong IMO but more debatable.