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`Drop` was implemented on a trait object or reference, which is not allowed;
only structs, enums, and unions can implement Drop.
Erroneous code examples:
```compile_fail,E0120
trait MyTrait {}
impl Drop for MyTrait {
fn drop(&mut self) {}
}
```
```compile_fail,E0120
struct Concrete {}
impl Drop for &'_ mut Concrete {
fn drop(&mut self) {}
}
```
A workaround for traits is to create a wrapper struct with a generic type,
add a trait bound to the type, and implement `Drop` on the wrapper:
```
trait MyTrait {}
struct MyWrapper<T: MyTrait> { foo: T }
impl <T: MyTrait> Drop for MyWrapper<T> {
fn drop(&mut self) {}
}
```
Alternatively, the `Drop` wrapper can contain the trait object:
```
trait MyTrait {}
// or Box<dyn MyTrait>, if you wanted an owned trait object
struct MyWrapper<'a> { foo: &'a dyn MyTrait }
impl <'a> Drop for MyWrapper<'a> {
fn drop(&mut self) {}
}
```
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