error[E0658]: naming constants with `_` is unstable (see issue #54912)
  --> $DIR/feature-gate-underscore_const_names.rs:8:1
   |
LL | / const _ : () = {
LL | | //~^ ERROR is unstable
LL | |     use std::marker::PhantomData;
LL | |     struct ImplementsTrait<T: Trt>(PhantomData<T>);
LL | |     let _ = ImplementsTrait::<Str>(PhantomData);
LL | |     ()
LL | | };
   | |__^
   |
   = help: add #![feature(underscore_const_names)] to the crate attributes to enable

error: aborting due to previous error

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