#![feature(return_position_impl_trait_in_trait)] #![allow(incomplete_features)] use std::fmt::Debug; trait Foo { fn baz(&self) -> impl Debug; } impl Foo for u32 { fn baz(&self) -> u32 { 32 } } fn main() { let i = Box::new(42_u32) as Box; //~^ ERROR the trait `Foo` cannot be made into an object //~| ERROR the trait `Foo` cannot be made into an object let s = i.baz(); //~^ ERROR the trait `Foo` cannot be made into an object }