error[E0658]: use of unstable library feature 'ipu_flatten'
  --> $DIR/inference_unstable_forced.rs:11:20
   |
LL |     assert_eq!('x'.ipu_flatten(), 0);
   |                    ^^^^^^^^^^^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/99999
   = help: add #![feature(ipu_flatten)] to the crate attributes to enable

error: aborting due to previous error

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