src/test/ui/cross/cross-borrow-trait.rs

// Test that cross-borrowing (implicitly converting from `Box<T>` to `&T`) is
// forbidden when `T` is a trait.

struct Foo;
trait Trait { fn foo(&self) {} }
impl Trait for Foo {}

pub fn main() {
    let x: Box<Trait> = Box::new(Foo);
    let _y: &Trait = x; //~ ERROR E0308
                        //~| expected type `&dyn Trait`
                        //~| found type `std::boxed::Box<dyn Trait>`
}