warning: the feature `return_position_impl_trait_in_trait` is incomplete and may not be safe to use and/or cause compiler crashes
  --> $DIR/box-coerce-span-in-default.rs:5:12
   |
LL | #![feature(return_position_impl_trait_in_trait)]
   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
   = note: `#[warn(incomplete_features)]` on by default

warning: 1 warning emitted