error[E0658]: use of unstable library feature `ipu_flatten`
  --> $DIR/inference_unstable_forced.rs:11:20
   |
LL |     assert_eq!('x'.ipu_flatten(), 0);
   |                    ^^^^^^^^^^^
   |
   = note: see issue #99999 <https://github.com/rust-lang/rust/issues/99999> for more information
   = help: add `#![feature(ipu_flatten)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0658`.