1 An unstable feature was used.
3 Erroneous code example:
6 #[repr(u128)] // error: use of unstable library feature 'repr128'
12 If you're using a stable or a beta version of rustc, you won't be able to use
13 any unstable features. In order to do so, please switch to a nightly version of
14 rustc (by using rustup).
16 If you're using a nightly version of rustc, just add the corresponding feature