An unstable feature was used.
Erroneous code example:
#[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:
#![feature(repr128)] #[repr(u128)] // ok! enum Foo { Bar(u64), }