rustfmt deletes comment after arrow in Fn type

Input:
```rust
fn foo(f: &dyn Fn(i32) -> /*result*/ i32)  {
}
```

after `rustfmt` becomes

```
fn foo(f: &dyn Fn(i32) -> i32) {}
```

### Expected

Preserve `/*result*/` comment

### Observed

Comment deleted.