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Understanding Rust Error E0152

Introduction to Rust Error E0152

Rust Error E0152 occurs when a lang item is redefined. Lang items are fundamental runtime constructs that are part of the standard library. It is not required to provide these items explicitly unless you are writing a free-standing application (e.g., a kernel) or a no_std crate.

Identifying the Issue

Consider this erroneous code example:

#![allow(unused)]
#![feature(lang_items)]

fn main() {
    #[lang = "owned_box"]
    struct Foo<T>(T); // error: duplicate lang item found: `owned_box`
}

In the above code example, the `owned_box` lang item is defined with a struct Foo<T>(T). However, `owned_box` is already part of the standard library and therefore should not be redefined.

How to Fix Rust Error E0152

To fix the error, remove the redefinition of the lang item or create a free-standing crate by adding #![no_std] to the crate attributes if you need to provide a custom lang item.

Here is an example of how to fix the code:

#![no_std]
#![feature(lang_items)]

fn main() {
    #[lang = "owned_box"]
    struct Foo<T>(T); // No error
}

In the fixed example, we added #![no_std] to indicate that it is a free-standing crate, allowing the addition of custom lang items.

Additional Resources

For more information about lang items and free-standing crates, refer to the Rust Programming Language Book and the Rust Unstable Book.

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