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+An unstable feature was used.
+
+Erroneous code example:
+
+```compile_fail,E0658
+#[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),
+}
+```
+
+[rustup]: https://rust-lang.github.io/rustup/concepts/channels.html