Drop was implemented on a trait object or reference, which is not allowed; only structs, enums, and unions can implement Drop.

Erroneous code examples:

trait MyTrait {}

impl Drop for MyTrait {
    fn drop(&mut self) {}
}

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) {}
}