Understanding Rust Error E0120

Introduction to Rust Error E0120

Rust error E0120 occurs when a programmer attempts to implement the Drop trait on a trait, which is not allowed. The 'Drop' trait is reserved for structs and enums only. This error is typically encountered in code examples where developers have mistakenly implemented the trait on a wrong data type.

Erroneous Code Example

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

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

In the code above, the Drop trait is implemented on MyTrait, resulting in the E0120 error. To fix this error, consider using one of the following workarounds.

Workaround 1: Wrap the Trait in a Struct

You can wrap the trait in a struct and implement the Drop trait for that struct. This approach allows the Drop trait to be utilized as intended.

Code Example for Workaround 1

#![allow(unused)]
fn main() {
trait MyTrait {}
struct MyWrapper<T: MyTrait> { foo: T }

impl <T: MyTrait> Drop for MyWrapper<T> {
    fn drop(&mut self) {}
}

}

In the example above, we created a struct MyWrapper and implemented the Drop trait for it, thus resolving the error.

Workaround 2: Wrap Trait Objects

Another workaround is to wrap trait objects such as references or Box<Trait> types. Wrap the trait in a struct with either a reference or Box, then implement the Drop trait for the new struct.

Code Example for Workaround 2

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

//or Box<MyTrait>, if you wanted an owned trait object
struct MyWrapper<'a> { foo: &'a MyTrait }

impl <'a> Drop for MyWrapper<'a> {
    fn drop(&mut self) {}
}
}

In this case, we created a struct MyWrapper with a reference to MyTrait and implemented the Drop trait for it, avoiding the E0120 error.

Conclusion

To prevent Rust error E0120, do not implement the Drop trait directly on a trait. Use one of the provided workarounds to wrap the trait in a struct or wrap trait objects, then implement the Drop trait for the modified struct. By following these steps, you can continue using the Drop trait without encountering E0120.