Error code E0120

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

Erroneous code examples:

#![allow(unused)] fn main() { trait MyTrait {} impl Drop for MyTrait { fn drop(&mut self) {} } }
#![allow(unused)] fn main() { 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:

#![allow(unused)] fn main() { 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:

#![allow(unused)] fn main() { 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) {} } }