Drop was implemented on a trait, which is not allowed: only structs and
enums can implement Drop.
Erroneous code example:
```compile_fail,E0120
trait MyTrait {}
impl Drop for MyTrait {
fn drop(&mut self) {}
}
```
A workaround for this problem is to wrap the trait up in a struct, and implement
Drop on that:
```
trait MyTrait {}
struct MyWrapper<T: MyTrait> { foo: T }
impl <T: MyTrait> Drop for MyWrapper<T> {
fn drop(&mut self) {}
}
```
Alternatively, wrapping trait objects requires something:
```
trait MyTrait {}
//or Box<MyTrait>, if you wanted an owned trait object
struct MyWrapper<'a> { foo: &'a MyTrait }
impl <'a> Drop for MyWrapper<'a> {
fn drop(&mut self) {}
}
```