Rust Error E0658: Unstable Feature Usage

Understanding Rust Error E0658

Error E0658 in Rust occurs when a program tries to use unstable features that are only available in nightly builds of the Rust compiler. It is important to understand the implications and consequences of using such features.

If you're experiencing Rust Error E0658, let's dive into the erroneous code and what causes this error.

Erroneous Code Example

#![allow(unused)]
fn main() {
#[repr(u128)] // error: use of unstable library feature 'repr128'
enum Foo {
    Bar(u64),
}
}

The error occurs due to the usage of the unstable library feature 'repr128', which is not available in stable or beta versions of Rust. To use this feature, you must switch to a nightly build of Rust.

How to Fix Rust Error E0658

To fix Rust Error E0658, either remove the unstable feature from your code or switch to a nightly build of Rust. Using a nightly build allows you to access unstable features, but keep in mind that these features may change or be removed in future versions.

Fixed Code Example

#![allow(unused)]
#![feature(repr128)]

fn main() {
#[repr(u128)] // ok!
enum Foo {
    Bar(u64),
}
}

In the fixed code example above, we've added the `#![feature(repr128)]` directive to enable the 'repr128' feature explicitly. This will resolve the error, allowing the code to compile and run using a nightly build of Rust.

Conclusion