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) {} } }