tests/ui/traits/trait-upcasting/migrate-lint-deny-regions.rs

#![deny(deref_into_dyn_supertrait)]

use std::ops::Deref;

trait Bar<'a> {}
trait Foo<'a>: Bar<'a> {}

impl<'a> Deref for dyn Foo<'a> {
    //~^ ERROR this `Deref` implementation is covered by an implicit supertrait coercion
    //~| WARN this will change its meaning in a future release!
    type Target = dyn Bar<'a>;

    fn deref(&self) -> &Self::Target {
        todo!()
    }
}

fn main() {}