# Error code E0658

An unstable feature was used.

Erroneous code example:

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

If you're using a stable or a beta version of rustc, you won't be able to use any unstable features. In order to do so, please switch to a nightly version of rustc (by using rustup).

If you're using a nightly version of rustc, just add the corresponding feature to be able to use it:

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

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