E0120 - Error codes index
Description: Book listing all Rust error codes

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# 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) {}
    }
    }