src/librustc_error_codes/error_codes/E0658.md

   1 An unstable feature was used.
   2
   3 Erroneous code example:
   4
   5 ```compile_fail,E0658
   6 #[repr(u128)] // error: use of unstable library feature 'repr128'
   7 enum Foo {
   8     Bar(u64),
   9 }
  10 ```
  11
  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).
  15
  16 If you're using a nightly version of rustc, just add the corresponding feature
  17 to be able to use it:
  18
  19 ```
  20 #![feature(repr128)]
  21
  22 #[repr(u128)] // ok!
  23 enum Foo {
  24     Bar(u64),
  25 }
  26 ```