error[E0658]: cannot cast `dyn Fn()` to `dyn FnMut()`, trait upcasting coercion is experimental --> $DIR/issue-11515.rs:9:33 | LL | let test = box Test { func: closure }; | ^^^^^^^ | = note: see issue #65991 for more information = help: add `#![feature(trait_upcasting)]` to the crate attributes to enable = note: required when coercing `Box<(dyn Fn() + 'static)>` into `Box<(dyn FnMut() + 'static)>` error: aborting due to previous error For more information about this error, try `rustc --explain E0658`.