error[E0658]: cannot cast `dyn A` to `dyn B`, trait upcasting coercion is experimental --> $DIR/upcast-through-struct-tail.rs:10:5 | LL | x | ^ | = note: see issue #65991 for more information = help: add `#![feature(trait_upcasting)]` to the crate attributes to enable = note: required when coercing `Box>` into `Box>` error: aborting due to previous error For more information about this error, try `rustc --explain E0658`.