Rust Error E0493: Custom Drop Implementation in const Context

Introduction

Rust Error E0493 occurs when a value with a custom Drop implementation is dropped during const-eval. This error is generated because Rust doesn't allow arbitrary, non-const-checked code to be called within a const context. In this article, we will show you how to resolve this error by providing an example and discussing how const-checked code can be ensured.

Erroneous Code Example

Consider the following erroneous code, which will produce Rust Error E0493:

#![allow(unused)]
fn main() {
    enum DropType {
        A,
    }

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

    struct Foo {
        field1: DropType,
    }

    static FOO: Foo = Foo { field1: (DropType::A, DropType::A).1 }; // error!
}

Explanation of the Error

In the above code, we have an enum called DropType, which has a custom Drop implementation. The struct Foo has a field 'field1' of type DropType. The error is produced when we try to initialize a static variable FOO with a tuple, consequently causing the custom Drop implementation to be called within the constant context. This is not allowed, as it may run arbitrary, non-const-checked code.

Solution to Rust Error E0493

To fix Rust Error E0493, we need to make sure that all values with a custom Drop implementation escape the initializer. We can achieve this by initializing all necessary fields by hand. Here's the corrected version of the code:

#![allow(unused)]
fn main() {
    enum DropType {
        A,
    }

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

    struct Foo {
        field1: DropType,
    }

    static FOO: Foo = Foo { field1: DropType::A }; // We initialize all fields by hand.
}

With these changes, the code will now compile without generating Rust Error E0493.

Conclusion

To fix Rust Error E0493, ensure all values with a custom Drop implementation escape the initializer. By taking this step, you prevent potential issues arising from calling arbitrary, non-const-checked code within a const context. Following the guidelines provided in this article will aid in resolving Rust Error E0493 and keeping your code const-safe.